diff --git a/src/ast.cpp b/src/ast.cpp
index 2cb1389f6..edd1e9375 100644
--- a/src/ast.cpp
+++ b/src/ast.cpp
@@ -3,12 +3,18 @@
 #include "ast.h"
 
 #include <array>
+#include <algorithm>
+#include <cstdarg>
+#include <cstdlib>
+#include <string>
 
 #include "common.h"
+#include "enum_map.h"
 #include "flog.h"
 #include "parse_constants.h"
 #include "parse_tree.h"
-#include "wutil.h"
+#include "tokenizer.h"
+#include "wutil.h" // IWYU pragma: keep
 
 namespace {
 
diff --git a/src/ast.h b/src/ast.h
index 6cc25b8ca..837a4a966 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -3,16 +3,20 @@
 #ifndef FISH_AST_H
 #define FISH_AST_H
 
-#include <array>
-#include <tuple>
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <iterator>
+#include <memory>
 #include <type_traits>
+#include <utility>
+#include <vector>
 
-#include "flog.h"
+#include "common.h"
+#include "maybe.h"
 #include "parse_constants.h"
-#include "tokenizer.h"
 
 namespace ast {
-
 /**
  * This defines the fish abstract syntax tree.
  * The fish ast is a tree data structure. The nodes of the tree
diff --git a/src/autoload.cpp b/src/autoload.cpp
index 25060ccf7..ade837725 100644
--- a/src/autoload.cpp
+++ b/src/autoload.cpp
@@ -3,11 +3,15 @@
 
 #include "autoload.h"
 
+#include <algorithm>
 #include <chrono>
+#include <functional>
+#include <utility>
+#include <vector>
 
 #include "common.h"
 #include "env.h"
-#include "exec.h"
+#include "io.h"
 #include "lru.h"
 #include "parser.h"
 #include "wutil.h"  // IWYU pragma: keep
diff --git a/src/autoload.h b/src/autoload.h
index a08cfde61..3842571bd 100644
--- a/src/autoload.h
+++ b/src/autoload.h
@@ -4,13 +4,14 @@
 
 #include "config.h"  // IWYU pragma: keep
 
-#include <cassert>
+#include <cstddef>
 #include <memory>
+#include <string>
 #include <unordered_map>
 #include <unordered_set>
 
 #include "common.h"
-#include "env.h"
+#include "maybe.h"
 #include "wutil.h"
 
 class autoload_file_cache_t;
diff --git a/src/builtin.cpp b/src/builtin.cpp
index 0a9836a9b..ed21d9848 100644
--- a/src/builtin.cpp
+++ b/src/builtin.cpp
@@ -22,11 +22,10 @@
 
 #include <unistd.h>
 
-#include <algorithm>
 #include <cerrno>
-#include <cstdlib>
 #include <cstring>
 #include <cwchar>
+#include <deque>
 #include <memory>
 #include <string>
 
@@ -66,12 +65,11 @@
 #include "builtins/type.h"
 #include "builtins/ulimit.h"
 #include "builtins/wait.h"
-#include "common.h"
 #include "complete.h"
-#include "exec.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
 #include "io.h"
+#include "null_terminated_array.h"
 #include "parse_constants.h"
 #include "parse_util.h"
 #include "parser.h"
diff --git a/src/builtin.h b/src/builtin.h
index 4b5e8b094..5668cf059 100644
--- a/src/builtin.h
+++ b/src/builtin.h
@@ -2,13 +2,12 @@
 #ifndef FISH_BUILTIN_H
 #define FISH_BUILTIN_H
 
-#include <stddef.h>
-
 #include <vector>
 
 #include "common.h"
+#include "complete.h"
+#include "maybe.h"
 
-class completion_t;
 class parser_t;
 class proc_status_t;
 class output_stream_t;
diff --git a/src/builtins/argparse.cpp b/src/builtins/argparse.cpp
index eee866a0c..34db468f9 100644
--- a/src/builtins/argparse.cpp
+++ b/src/builtins/argparse.cpp
@@ -5,9 +5,7 @@
 
 #include "argparse.h"
 
-#include <algorithm>
 #include <cerrno>
-#include <cstddef>
 #include <cstdint>
 #include <cwchar>
 #include <memory>
@@ -22,6 +20,7 @@
 #include "../exec.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../wcstringutil.h"
 #include "../wgetopt.h"  // IWYU pragma: keep
diff --git a/src/builtins/bg.cpp b/src/builtins/bg.cpp
index 2d0e84097..4b2b5281e 100644
--- a/src/builtins/bg.cpp
+++ b/src/builtins/bg.cpp
@@ -4,14 +4,16 @@
 #include "bg.h"
 
 #include <cerrno>
-#include <cstdlib>
+#include <deque>
 #include <memory>
+#include <sys/types.h>
 #include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../job_group.h"
 #include "../parser.h"
 #include "../proc.h"
diff --git a/src/builtins/bind.cpp b/src/builtins/bind.cpp
index b93b8c5d4..5ac853c9e 100644
--- a/src/builtins/bind.cpp
+++ b/src/builtins/bind.cpp
@@ -5,18 +5,18 @@
 
 #include <cerrno>
 #include <cstddef>
-#include <memory>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../fallback.h"  // IWYU pragma: keep
-#include "../input.h"     // IWYU pragma: keep
-#include "../io.h"        // IWYU pragma: keep
+#include "../fallback.h" // IWYU pragma: keep
+#include "../input.h"
+#include "../io.h"
+#include "../maybe.h"
 #include "../wgetopt.h"
-#include "../wutil.h"  // IWYU pragma: keep
+#include "../wutil.h" // IWYU pragma: keep
 
 enum { BIND_INSERT, BIND_ERASE, BIND_KEY_NAMES, BIND_FUNCTION_NAMES };
 struct bind_cmd_opts_t {
diff --git a/src/builtins/bind.h b/src/builtins/bind.h
index bbb25df45..6218d2638 100644
--- a/src/builtins/bind.h
+++ b/src/builtins/bind.h
@@ -2,8 +2,6 @@
 #ifndef FISH_BUILTIN_BIND_H
 #define FISH_BUILTIN_BIND_H
 
-#include <cwchar>
-
 #include "../maybe.h"
 
 class parser_t;
diff --git a/src/builtins/block.cpp b/src/builtins/block.cpp
index 907cc5533..665012759 100644
--- a/src/builtins/block.cpp
+++ b/src/builtins/block.cpp
@@ -4,12 +4,13 @@
 #include "block.h"
 
 #include <cstddef>
+#include <deque>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../event.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
diff --git a/src/builtins/builtin.cpp b/src/builtins/builtin.cpp
index 55b7bca18..2ee135b0a 100644
--- a/src/builtins/builtin.cpp
+++ b/src/builtins/builtin.cpp
@@ -4,13 +4,13 @@
 #include "builtin.h"
 
 #include <algorithm>
-#include <cstddef>
 #include <string>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/cd.cpp b/src/builtins/cd.cpp
index 82e0f9cdb..8a2023ed9 100644
--- a/src/builtins/cd.cpp
+++ b/src/builtins/cd.cpp
@@ -4,18 +4,23 @@
 #include "cd.h"
 
 #include <fcntl.h>
-#include <sys/stat.h>
+#include <unistd.h>
 
 #include <cerrno>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
+#include "../fds.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../path.h"
-#include "../proc.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
 /// The cd builtin. Changes the current directory to the one specified or to $HOME if none is
diff --git a/src/builtins/command.cpp b/src/builtins/command.cpp
index 25cf3d203..77a978063 100644
--- a/src/builtins/command.cpp
+++ b/src/builtins/command.cpp
@@ -3,14 +3,14 @@
 
 #include "command.h"
 
-#include <unistd.h>
-
 #include <string>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../path.h"
 #include "../wgetopt.h"
diff --git a/src/builtins/commandline.cpp b/src/builtins/commandline.cpp
index 08b439391..aae6baf8d 100644
--- a/src/builtins/commandline.cpp
+++ b/src/builtins/commandline.cpp
@@ -3,15 +3,19 @@
 
 #include "commandline.h"
 
+#include <algorithm>
 #include <cerrno>
-#include <cstddef>
-#include <cstdlib>
 #include <cwchar>
+#include <string>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
+#include "../io.h"
+#include "../maybe.h"
 #include "../input.h"
+#include "../input_common.h"
+#include "../parse_constants.h"
 #include "../parse_util.h"
 #include "../parser.h"
 #include "../proc.h"
@@ -20,8 +24,6 @@
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
-class parser_t;
-
 /// Which part of the comandbuffer are we operating on.
 enum {
     STRING_MODE = 1,  // operate on entire buffer
diff --git a/src/builtins/commandline.h b/src/builtins/commandline.h
index 45aac048a..1723ed0ee 100644
--- a/src/builtins/commandline.h
+++ b/src/builtins/commandline.h
@@ -2,13 +2,10 @@
 #ifndef FISH_BUILTIN_COMMANDLINE_H
 #define FISH_BUILTIN_COMMANDLINE_H
 
-#include <cstring>
-#include <cwchar>
-
-#include "../io.h"
 #include "../maybe.h"
 
 class parser_t;
+struct io_streams_t;
 
 maybe_t<int> builtin_commandline(parser_t &parser, io_streams_t &streams, const wchar_t **argv);
 #endif
diff --git a/src/builtins/complete.cpp b/src/builtins/complete.cpp
index 8798b20ab..026fab33d 100644
--- a/src/builtins/complete.cpp
+++ b/src/builtins/complete.cpp
@@ -3,19 +3,21 @@
 
 #include "complete.h"
 
-#include <cstddef>
-#include <cwchar>
-#include <memory>
+#include <unistd.h>
+
+#include <functional>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "../builtin.h"
-#include "../color.h"
 #include "../common.h"
 #include "../complete.h"
 #include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../highlight.h"
+#include "../io.h"
+#include "../maybe.h"
 #include "../parse_constants.h"
 #include "../parse_util.h"
 #include "../parser.h"
diff --git a/src/builtins/complete.h b/src/builtins/complete.h
index 8462ffeb6..672778b64 100644
--- a/src/builtins/complete.h
+++ b/src/builtins/complete.h
@@ -2,13 +2,10 @@
 #ifndef FISH_BUILTIN_COMPLETE_H
 #define FISH_BUILTIN_COMPLETE_H
 
-#include <cstring>
-#include <cwchar>
-
-#include "../io.h"
 #include "../maybe.h"
 
 class parser_t;
+struct io_streams_t;
 
 maybe_t<int> builtin_complete(parser_t &parser, io_streams_t &streams, const wchar_t **argv);
 #endif
diff --git a/src/builtins/contains.cpp b/src/builtins/contains.cpp
index 72167e220..1ab398234 100644
--- a/src/builtins/contains.cpp
+++ b/src/builtins/contains.cpp
@@ -3,14 +3,13 @@
 
 #include "contains.h"
 
-#include <unistd.h>
-
 #include <cwchar>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/disown.cpp b/src/builtins/disown.cpp
index 7173c4a38..09176f628 100644
--- a/src/builtins/disown.cpp
+++ b/src/builtins/disown.cpp
@@ -5,12 +5,15 @@
 
 #include <cerrno>
 #include <csignal>
-#include <set>
+#include <deque>
+#include <memory>
+#include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../proc.h"
 #include "../wutil.h"  // IWYU pragma: keep
diff --git a/src/builtins/echo.cpp b/src/builtins/echo.cpp
index 98cd3d07f..0f15e36b8 100644
--- a/src/builtins/echo.cpp
+++ b/src/builtins/echo.cpp
@@ -3,13 +3,13 @@
 
 #include "echo.h"
 
-#include <climits>
 #include <cstddef>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/emit.cpp b/src/builtins/emit.cpp
index d61d11894..b28adb51a 100644
--- a/src/builtins/emit.cpp
+++ b/src/builtins/emit.cpp
@@ -3,11 +3,14 @@
 
 #include "emit.h"
 
+#include <utility>
+
 #include "../builtin.h"
 #include "../common.h"
 #include "../event.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
 /// Implementation of the builtin emit command, used to create events.
diff --git a/src/builtins/eval.cpp b/src/builtins/eval.cpp
index 612126260..19dd5f9b8 100644
--- a/src/builtins/eval.cpp
+++ b/src/builtins/eval.cpp
@@ -1,17 +1,18 @@
 // Functions for executing the eval builtin.
 #include "config.h"  // IWYU pragma: keep
 
-#include <cerrno>
-#include <cstddef>
+#include <unistd.h>
+
+#include <memory>
+#include <utility>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../exec.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../proc.h"
-#include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
 /// Implementation of eval builtin.
diff --git a/src/builtins/exit.cpp b/src/builtins/exit.cpp
index 4df0d2850..ddd5fb9e4 100644
--- a/src/builtins/exit.cpp
+++ b/src/builtins/exit.cpp
@@ -4,15 +4,13 @@
 #include "exit.h"
 
 #include <cerrno>
-#include <cstddef>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
-#include "../proc.h"
-#include "../reader.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/fg.cpp b/src/builtins/fg.cpp
index d8faa8902..455884022 100644
--- a/src/builtins/fg.cpp
+++ b/src/builtins/fg.cpp
@@ -3,16 +3,24 @@
 
 #include "fg.h"
 
+#include <termios.h>
+#include <unistd.h>
+
 #include <cerrno>
 #include <cstdio>
 #include <cstdlib>
 #include <cwchar>
+#include <deque>
+#include <memory>
+#include <utility>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
+#include "../fds.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../job_group.h"
 #include "../parser.h"
 #include "../proc.h"
diff --git a/src/builtins/function.cpp b/src/builtins/function.cpp
index 8841fcb91..d19fbf39c 100644
--- a/src/builtins/function.cpp
+++ b/src/builtins/function.cpp
@@ -6,23 +6,29 @@
 #include <unistd.h>
 
 #include <cerrno>
-#include <cstddef>
-#include <cstdlib>
+#include <cwchar>
+#include <map>
 #include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../complete.h"
+#include "../env.h"
 #include "../event.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../function.h"
 #include "../io.h"
+#include "../maybe.h"
+#include "../null_terminated_array.h"
 #include "../parser.h"
 #include "../parser_keywords.h"
+#include "../parse_tree.h"
 #include "../proc.h"
 #include "../signal.h"
+#include "../wait_handle.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/function.h b/src/builtins/function.h
index 9e32c5342..dc68e0914 100644
--- a/src/builtins/function.h
+++ b/src/builtins/function.h
@@ -3,13 +3,14 @@
 #define FISH_BUILTIN_FUNCTION_H
 
 #include "../common.h"
+#include "../maybe.h"
 #include "../parse_tree.h"
 
 class parser_t;
 struct io_streams_t;
 
 namespace ast {
-struct block_statement_t;
+    struct block_statement_t;
 }
 
 maybe_t<int> builtin_function(parser_t &parser, io_streams_t &streams,
diff --git a/src/builtins/functions.cpp b/src/builtins/functions.cpp
index 49d69f91b..8e8011408 100644
--- a/src/builtins/functions.cpp
+++ b/src/builtins/functions.cpp
@@ -6,29 +6,22 @@
 #include <unistd.h>
 
 #include <algorithm>
-#include <cstddef>
-#include <cwchar>
-#include <map>
 #include <memory>
 #include <string>
-#include <utility>
 #include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../complete.h"
 #include "../env.h"
 #include "../event.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../function.h"
 #include "../highlight.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../parser_keywords.h"
-#include "../proc.h"
-#include "../signal.h"
 #include "../termsize.h"
-#include "../wcstringutil.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/history.cpp b/src/builtins/history.cpp
index d4762f3ba..608b9898e 100644
--- a/src/builtins/history.cpp
+++ b/src/builtins/history.cpp
@@ -6,16 +6,17 @@
 #include <cerrno>
 #include <cstddef>
 #include <cstdint>
-#include <cwchar>
+#include <memory>
 #include <string>
-#include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../env.h"
 #include "../enum_map.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../history.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../reader.h"
 #include "../wgetopt.h"
diff --git a/src/builtins/jobs.cpp b/src/builtins/jobs.cpp
index fb8969c02..a403d2496 100644
--- a/src/builtins/jobs.cpp
+++ b/src/builtins/jobs.cpp
@@ -1,22 +1,20 @@
 // Functions for executing the jobs builtin.
 #include "config.h"  // IWYU pragma: keep
 
-#include <sys/time.h>
-
 #include <cerrno>
-#include <cstddef>
+#include <deque>
+#include <memory>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../fallback.h"  // IWYU pragma: keep
+#include "../fallback.h" // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../proc.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
-class parser_t;
-
 /// Print modes for the jobs builtin.
 enum {
     JOBS_DEFAULT,        // print lots of general info
diff --git a/src/builtins/math.cpp b/src/builtins/math.cpp
index 91c35b6b6..c7d136add 100644
--- a/src/builtins/math.cpp
+++ b/src/builtins/math.cpp
@@ -3,11 +3,9 @@
 
 #include "math.h"
 
-#include <algorithm>
 #include <cerrno>
 #include <cmath>
-#include <cstddef>
-#include <cstring>
+#include <cwchar>
 #include <limits>
 #include <string>
 
@@ -15,6 +13,7 @@
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../tinyexpr.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
diff --git a/src/builtins/path.cpp b/src/builtins/path.cpp
index bca0d44e7..6524ebb2c 100644
--- a/src/builtins/path.cpp
+++ b/src/builtins/path.cpp
@@ -5,17 +5,25 @@
 
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <stdint.h>
 #include <unistd.h>
 
+#include <algorithm>
+#include <climits>
 #include <ctime>
+#include <cstdarg>
+#include <cwchar>
+#include <map>
 #include <string>
+#include <unordered_map>
 #include <utility>
-#include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../path.h"
 #include "../util.h"
@@ -30,7 +38,7 @@
 
 static void path_error(io_streams_t &streams, const wchar_t *fmt, ...) {
     streams.err.append(L"path ");
-    va_list va;
+    std::va_list va;
     va_start(va, fmt);
     streams.err.append_formatv(fmt, va);
     va_end(va);
diff --git a/src/builtins/path.h b/src/builtins/path.h
index 0bfb311f5..885b93de2 100644
--- a/src/builtins/path.h
+++ b/src/builtins/path.h
@@ -1,13 +1,10 @@
 #ifndef FISH_BUILTIN_PATH_H
 #define FISH_BUILTIN_PATH_H
 
-#include <cstring>
-#include <cwchar>
-
-#include "../io.h"
 #include "../maybe.h"
 
 class parser_t;
+struct io_streams_t;
 
 maybe_t<int> builtin_path(parser_t &parser, io_streams_t &streams, const wchar_t **argv);
 #endif
diff --git a/src/builtins/printf.cpp b/src/builtins/printf.cpp
index b85fa43c1..d6ad9bf48 100644
--- a/src/builtins/printf.cpp
+++ b/src/builtins/printf.cpp
@@ -52,21 +52,22 @@
 
 #include "printf.h"
 
-#include <sys/types.h>
-
 #include <cerrno>
 #include <climits>
-#include <clocale>
 #include <cstdarg>
-#include <cstddef>
 #include <cstdint>
-#include <cstdlib>
 #include <cstring>
 #include <cwchar>
 #include <cwctype>
+#include <locale>
+#ifdef HAVE_XLOCALE_H
+#include <xlocale.h>
+#endif
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../io.h"
+#include "../maybe.h"
 #include "../wcstringutil.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/printf.h b/src/builtins/printf.h
index d98d455a5..7f7daebbf 100644
--- a/src/builtins/printf.h
+++ b/src/builtins/printf.h
@@ -2,13 +2,10 @@
 #ifndef FISH_BUILTIN_PRINTF_H
 #define FISH_BUILTIN_PRINTF_H
 
-#include <cstring>
-#include <cwchar>
-
-#include "../io.h"
 #include "../maybe.h"
 
 class parser_t;
+struct io_streams_t;
 
 maybe_t<int> builtin_printf(parser_t &parser, io_streams_t &streams, const wchar_t **argv);
 #endif
diff --git a/src/builtins/pwd.cpp b/src/builtins/pwd.cpp
index 366256a3f..d83811e29 100644
--- a/src/builtins/pwd.cpp
+++ b/src/builtins/pwd.cpp
@@ -3,12 +3,17 @@
 
 #include "pwd.h"
 
+#include <cerrno>
 #include <cstring>
+#include <string>
+#include <utility>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
diff --git a/src/builtins/random.cpp b/src/builtins/random.cpp
index 5393da49b..5f79a2271 100644
--- a/src/builtins/random.cpp
+++ b/src/builtins/random.cpp
@@ -3,7 +3,6 @@
 
 #include "random.h"
 
-#include <algorithm>
 #include <cerrno>
 #include <cstdint>
 #include <cwchar>
@@ -13,6 +12,7 @@
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
 /// \return a random-seeded engine.
diff --git a/src/builtins/read.cpp b/src/builtins/read.cpp
index ad904537e..e432a03c8 100644
--- a/src/builtins/read.cpp
+++ b/src/builtins/read.cpp
@@ -3,34 +3,26 @@
 
 #include "read.h"
 
-#include <termios.h>
 #include <unistd.h>
 
 #include <algorithm>
 #include <cerrno>
-#include <climits>
-#include <cstddef>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
 #include <cwchar>
-#include <memory>
-#include <numeric>
 #include <string>
-#include <vector>
+#include <utility>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../complete.h"
 #include "../env.h"
-#include "../event.h"
 #include "../fallback.h"  // IWYU pragma: keep
-#include "../highlight.h"
-#include "../history.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
-#include "../proc.h"
 #include "../reader.h"
+#include "../tokenizer.h"
 #include "../wcstringutil.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
diff --git a/src/builtins/realpath.cpp b/src/builtins/realpath.cpp
index 5f4872cda..a6d654a5a 100644
--- a/src/builtins/realpath.cpp
+++ b/src/builtins/realpath.cpp
@@ -3,15 +3,15 @@
 
 #include "realpath.h"
 
-#include <cstdio>
-#include <cstdlib>
+#include <cerrno>
 #include <cstring>
-#include <cwchar>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../path.h"
 #include "../wcstringutil.h"
diff --git a/src/builtins/return.cpp b/src/builtins/return.cpp
index 4d0ee3b81..89f9f952b 100644
--- a/src/builtins/return.cpp
+++ b/src/builtins/return.cpp
@@ -4,14 +4,14 @@
 #include "return.h"
 
 #include <cerrno>
-#include <cstddef>
+#include <deque>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
-#include "../proc.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/set.cpp b/src/builtins/set.cpp
index 0f482fcea..119d343dd 100644
--- a/src/builtins/set.cpp
+++ b/src/builtins/set.cpp
@@ -3,31 +3,26 @@
 
 #include "set.h"
 
-#include <sys/stat.h>
-#include <unistd.h>
-
 #include <algorithm>
 #include <cerrno>
-#include <cstddef>
-#include <cstdlib>
-#include <cstring>
 #include <cwchar>
-#include <iterator>
+#include <functional>
+#include <map>
 #include <memory>
-#include <set>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../env.h"
 #include "../expand.h"
+#include "../event.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../history.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
-#include "../proc.h"
-#include "../wcstringutil.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/set.h b/src/builtins/set.h
index 797bdbcb2..4b08f5e42 100644
--- a/src/builtins/set.h
+++ b/src/builtins/set.h
@@ -2,8 +2,6 @@
 #ifndef FISH_BUILTIN_SET_H
 #define FISH_BUILTIN_SET_H
 
-#include <cwchar>
-
 #include "../maybe.h"
 
 class parser_t;
diff --git a/src/builtins/set_color.cpp b/src/builtins/set_color.cpp
index 114fd0880..93afd88ee 100644
--- a/src/builtins/set_color.cpp
+++ b/src/builtins/set_color.cpp
@@ -3,11 +3,12 @@
 
 #include "set_color.h"
 
-#include <cstddef>
+#include <unistd.h>
+
 #include <cstdlib>
 
 #if HAVE_CURSES_H
-#include <curses.h>
+#include <curses.h> // IWYU pragma: keep
 #elif HAVE_NCURSES_H
 #include <ncurses.h>
 #elif HAVE_NCURSES_CURSES_H
@@ -19,7 +20,6 @@
 #include <ncurses/term.h>
 #endif
 
-#include <memory>
 #include <string>
 #include <vector>
 
@@ -27,8 +27,10 @@
 #include "../color.h"
 #include "../common.h"
 #include "../env.h"
+#include "../fallback.h" // IWYU pragma: keep
+#include "../io.h"
+#include "../maybe.h"
 #include "../output.h"
-#include "../parser.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/set_color.h b/src/builtins/set_color.h
index 632df46a6..fec44228a 100644
--- a/src/builtins/set_color.h
+++ b/src/builtins/set_color.h
@@ -2,13 +2,10 @@
 #ifndef FISH_BUILTIN_SET_COLOR_H
 #define FISH_BUILTIN_SET_COLOR_H
 
-#include <cstring>
-#include <cwchar>
-
-#include "../io.h"
 #include "../maybe.h"
 
 class parser_t;
+struct io_streams_t;
 
 maybe_t<int> builtin_set_color(parser_t &parser, io_streams_t &streams, const wchar_t **argv);
 #endif
diff --git a/src/builtins/source.cpp b/src/builtins/source.cpp
index 8ecb937be..75ef81fb6 100644
--- a/src/builtins/source.cpp
+++ b/src/builtins/source.cpp
@@ -8,14 +8,19 @@
 #include <unistd.h>
 
 #include <cwchar>
+#include <memory>
+#include <string>
+#include <utility>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
+#include "../fds.h"
 #include "../io.h"
+#include "../maybe.h"
+#include "../null_terminated_array.h"
 #include "../parser.h"
-#include "../proc.h"
 #include "../reader.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/status.cpp b/src/builtins/status.cpp
index 640c05541..8bc10e769 100644
--- a/src/builtins/status.cpp
+++ b/src/builtins/status.cpp
@@ -3,17 +3,24 @@
 
 #include "status.h"
 
-#include <cstddef>
+#include <unistd.h>
+
+#include <algorithm>
+#include <cerrno>
 #include <cstring>
 #include <cwchar>
+#include <limits>
+#include <memory>
 #include <string>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../enum_map.h"
+#include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../future_feature_flags.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../proc.h"
 #include "../wgetopt.h"
diff --git a/src/builtins/string.cpp b/src/builtins/string.cpp
index eddef0553..c3d88a1ff 100644
--- a/src/builtins/string.cpp
+++ b/src/builtins/string.cpp
@@ -6,15 +6,11 @@
 #include <climits>
 #include <cstdarg>
 #include <cstddef>
-#include <cstdint>
-#include <cstdlib>
 #include <cwchar>
 #include <cwctype>
-#include <functional>
-#include <iterator>
+#include <map>
 #include <memory>
 #include <string>
-#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -23,6 +19,8 @@
 #include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../future_feature_flags.h"
+#include "../io.h"
+#include "../maybe.h"
 #include "../parse_util.h"
 #include "../parser.h"
 #include "../re.h"
diff --git a/src/builtins/test.cpp b/src/builtins/test.cpp
index e22329511..eb682f9cd 100644
--- a/src/builtins/test.cpp
+++ b/src/builtins/test.cpp
@@ -10,19 +10,23 @@
 #include <unistd.h>
 
 #include <cerrno>
+#include <climits>
 #include <cmath>
 #include <cstdarg>
 #include <cstring>
 #include <cwchar>
 #include <cwctype>
+#include <map>
 #include <memory>
 #include <string>
-#include <type_traits>
 #include <utility>
+#include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../fallback.h" // IWYU pragma: keep
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/type.cpp b/src/builtins/type.cpp
index a8a89fb9f..f7642f063 100644
--- a/src/builtins/type.cpp
+++ b/src/builtins/type.cpp
@@ -5,19 +5,20 @@
 
 #include <unistd.h>
 
+#include <memory>
 #include <string>
+#include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
-#include "../complete.h"
+#include "../env.h"
 #include "../fallback.h"  // IWYU pragma: keep
 #include "../function.h"
 #include "../highlight.h"
 #include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../path.h"
-#include "../signal.h"
-#include "../wcstringutil.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/ulimit.cpp b/src/builtins/ulimit.cpp
index 8e13d695e..d6fd2b90e 100644
--- a/src/builtins/ulimit.cpp
+++ b/src/builtins/ulimit.cpp
@@ -5,12 +5,15 @@
 
 #include <sys/resource.h>
 
+#include <algorithm>
 #include <cerrno>
-#include <cstddef>
+#include <cwchar>
 
 #include "../builtin.h"
 #include "../common.h"
 #include "../fallback.h"  // IWYU pragma: keep
+#include "../io.h"
+#include "../maybe.h"
 #include "../wgetopt.h"
 #include "../wutil.h"  // IWYU pragma: keep
 
diff --git a/src/builtins/ulimit.h b/src/builtins/ulimit.h
index cefe1726f..b526910fa 100644
--- a/src/builtins/ulimit.h
+++ b/src/builtins/ulimit.h
@@ -2,12 +2,10 @@
 #ifndef FISH_BUILTIN_ULIMIT_H
 #define FISH_BUILTIN_ULIMIT_H
 
-#include <cstring>
-#include <cwchar>
-
-#include "../io.h"
+#include "../maybe.h"
 
 class parser_t;
+struct io_streams_t;
 
 maybe_t<int> builtin_ulimit(parser_t &parser, io_streams_t &streams, const wchar_t **argv);
 #endif
diff --git a/src/builtins/wait.cpp b/src/builtins/wait.cpp
index 9aea63400..51987d934 100644
--- a/src/builtins/wait.cpp
+++ b/src/builtins/wait.cpp
@@ -3,16 +3,24 @@
 
 #include "wait.h"
 
-#include <sys/wait.h>
-
 #include <algorithm>
+#include <cerrno>
+#include <csignal>
+#include <deque>
+#include <list>
+#include <memory>
+#include <string>
+#include <utility>
 #include <vector>
 
 #include "../builtin.h"
 #include "../common.h"
+#include "../io.h"
+#include "../maybe.h"
 #include "../parser.h"
 #include "../proc.h"
 #include "../signal.h"
+#include "../topic_monitor.h"
 #include "../wait_handle.h"
 #include "../wgetopt.h"
 #include "../wutil.h"
diff --git a/src/color.cpp b/src/color.cpp
index 9b59aac5b..bf326d594 100644
--- a/src/color.cpp
+++ b/src/color.cpp
@@ -4,11 +4,10 @@
 #include "color.h"
 
 #include <algorithm>
-#include <cstddef>
+#include <iterator>
 #include <cstdint>
 #include <cstdlib>
 #include <cwchar>  // IWYU pragma: keep
-#include <cwctype>
 
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
diff --git a/src/color.h b/src/color.h
index 9b0438534..33139a44b 100644
--- a/src/color.h
+++ b/src/color.h
@@ -4,6 +4,7 @@
 
 #include <cstring>
 #include <string>
+#include <cstdint>
 
 #include "common.h"
 
diff --git a/src/common.cpp b/src/common.cpp
index b6029b933..b1b642e56 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -10,21 +10,12 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
-#include <paths.h>
 #include <pthread.h>
 #include <stdarg.h>
-#include <stddef.h>
-#include <stdint.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <sys/stat.h>
 #include <sys/time.h>
 #include <termios.h>
 #include <unistd.h>
-#include <wctype.h>
 
-#include <cstring>
-#include <cwchar>
 #ifdef HAVE_EXECINFO_H
 #include <execinfo.h>
 #endif
@@ -35,21 +26,21 @@
 #endif
 
 #include <algorithm>
-#include <atomic>
-#include <memory>  // IWYU pragma: keep
-#include <type_traits>
+#include <csignal>
+#include <cstring>
+#include <cstdlib>
+#include <cwchar>
+#include <memory>
 
 #include "common.h"
-#include "env.h"
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
 #include "future_feature_flags.h"
 #include "global_safety.h"
 #include "iothread.h"
-#include "parser.h"
-#include "proc.h"
 #include "signal.h"
+#include "topic_monitor.h"
 #include "termsize.h"
 #include "wcstringutil.h"
 #include "wildcard.h"
@@ -57,10 +48,10 @@
 
 // Keep after "common.h"
 #ifdef HAVE_SYS_SYSCTL_H
-#include <sys/sysctl.h>
+#include <sys/sysctl.h> // IWYU pragma: keep
 #endif
 #if defined(__APPLE__)
-#include <mach-o/dyld.h>
+#include <mach-o/dyld.h> // IWYU pragma: keep
 #endif
 
 struct termios shell_modes;
diff --git a/src/common.h b/src/common.h
index feec54c6b..c3efa26a1 100644
--- a/src/common.h
+++ b/src/common.h
@@ -10,14 +10,18 @@
 #ifdef HAVE_SYS_IOCTL_H
 #include <sys/ioctl.h>  // IWYU pragma: keep
 #endif
+#include <sys/types.h>
+#include <termios.h>
 
 #include <algorithm>
-#include <atomic>
-#include <cmath>
+#include <cstdint>
+#include <cwchar>
 #include <functional>
+#include <iterator>
 #include <memory>
 #include <mutex>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "fallback.h"  // IWYU pragma: keep
diff --git a/src/complete.cpp b/src/complete.cpp
index 299c50e64..d218b122f 100644
--- a/src/complete.cpp
+++ b/src/complete.cpp
@@ -7,43 +7,45 @@
 
 #include "complete.h"
 
-#include <pthread.h>
 #include <pwd.h>
-#include <stddef.h>
 #include <wctype.h>
 
 #include <algorithm>
-#include <atomic>
+
 #include <cstddef>
 #include <cwchar>
 #include <forward_list>
 #include <functional>
 #include <iterator>
+#include <map>
 #include <memory>
-#include <numeric>
+#include <mutex>
 #include <set>
 #include <string>
-#include <type_traits>
 #include <unordered_map>
+#include <unordered_set>
 #include <utility>
 
 #include "autoload.h"
 #include "builtin.h"
 #include "common.h"
+#include "enum_set.h"
 #include "env.h"
 #include "exec.h"
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "flog.h"
 #include "function.h"
+#include "global_safety.h"
 #include "history.h"
-#include "iothread.h"
+#include "maybe.h"
+#include "operation_context.h"
 #include "parse_constants.h"
 #include "parse_util.h"
 #include "parser.h"
 #include "parser_keywords.h"
 #include "path.h"
-#include "proc.h"
-#include "reader.h"
+#include "tokenizer.h"
 #include "util.h"
 #include "wcstringutil.h"
 #include "wildcard.h"
diff --git a/src/complete.h b/src/complete.h
index dccbe4f62..3ecc1123b 100644
--- a/src/complete.h
+++ b/src/complete.h
@@ -5,11 +5,15 @@
 #ifndef FISH_COMPLETE_H
 #define FISH_COMPLETE_H
 
-#include <stdint.h>
+#include "config.h"  // IWYU pragma: keep
 
+#include <cstdint>
+#include <cstddef>
 #include <functional>
+#include <utility>
 #include <vector>
 
+//#include "expand.h"
 #include "common.h"
 #include "wcstringutil.h"
 
@@ -25,7 +29,6 @@ struct completion_mode_t {
 /// Character that separates the completion and description on programmable completions.
 #define PROG_COMPLETE_SEP L'\t'
 
-class environment_t;
 class parser_t;
 
 enum {
@@ -124,7 +127,6 @@ struct completion_request_options_t {
     }
 };
 
-class completion_t;
 using completion_list_t = std::vector<completion_t>;
 
 /// A completion receiver accepts completions. It is essentially a wrapper around std::vector with
diff --git a/src/enum_set.h b/src/enum_set.h
index 1c9e67442..2a5a5c26d 100644
--- a/src/enum_set.h
+++ b/src/enum_set.h
@@ -3,7 +3,6 @@
 
 #include <array>
 #include <bitset>
-#include <cassert>
 #include <iterator>
 
 /// A type (to specialize) that provides a count for an enum.
diff --git a/src/env.cpp b/src/env.cpp
index 2e142251d..85ca447fd 100644
--- a/src/env.cpp
+++ b/src/env.cpp
@@ -12,14 +12,12 @@
 #include <unistd.h>
 
 #include <algorithm>
-#include <iterator>
 #include <map>
 #include <mutex>
 #include <set>
 #include <utility>
 #include <vector>
 
-#include "builtins/bind.h"
 #include "common.h"
 #include "env_dispatch.h"
 #include "env_universal_common.h"
@@ -31,6 +29,7 @@
 #include "history.h"
 #include "input.h"
 #include "kill.h"
+#include "null_terminated_array.h"
 #include "path.h"
 #include "proc.h"
 #include "reader.h"
diff --git a/src/env.h b/src/env.h
index d5c106f23..1d44ec8eb 100644
--- a/src/env.h
+++ b/src/env.h
@@ -10,10 +10,12 @@
 #include <string>
 #include <unordered_map>
 #include <vector>
+#include <utility>
 
 #include "common.h"
 #include "maybe.h"
-#include "null_terminated_array.h"
+
+class owning_null_terminated_array_t;
 
 extern size_t read_byte_limit;
 extern bool curses_initialized;
diff --git a/src/env_dispatch.cpp b/src/env_dispatch.cpp
index ce0c9a8d2..472425ed9 100644
--- a/src/env_dispatch.cpp
+++ b/src/env_dispatch.cpp
@@ -4,20 +4,19 @@
 #include <errno.h>
 #include <limits.h>
 #include <locale.h>
-#include <stddef.h>
-#include <stdlib.h>
 #include <time.h>
 #include <unistd.h>
 
+#include <cstdlib>
 #include <cstring>
 #include <cwchar>
 
 #if HAVE_CURSES_H
-#include <curses.h>
+#include <curses.h> // IWYU pragma: keep
 #elif HAVE_NCURSES_H
-#include <ncurses.h>
+#include <ncurses.h> // IWYU pragma: keep
 #elif HAVE_NCURSES_CURSES_H
-#include <ncurses/curses.h>
+#include <ncurses/curses.h> // IWYU pragma: keep
 #endif
 #if HAVE_TERM_H
 #include <term.h>
@@ -25,8 +24,6 @@
 #include <ncurses/term.h>
 #endif
 
-#include <assert.h>
-
 #include <algorithm>
 #include <functional>
 #include <memory>
@@ -38,8 +35,6 @@
 #include "complete.h"
 #include "env.h"
 #include "env_dispatch.h"
-#include "env_universal_common.h"
-#include "event.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
 #include "function.h"
@@ -48,14 +43,13 @@
 #include "input_common.h"
 #include "maybe.h"
 #include "output.h"
-#include "parser.h"
 #include "proc.h"
 #include "reader.h"
 #include "screen.h"
 #include "termsize.h"
 #include "trace.h"
-#include "wutil.h"  // IWYU pragma: keep
-
+#include "wcstringutil.h"
+#include "wutil.h"
 
 // Limit `read` to 100 MiB (bytes not wide chars) by default. This can be overridden by the
 // fish_read_limit variable.
diff --git a/src/env_dispatch.h b/src/env_dispatch.h
index c469e5d9b..7d70eed85 100644
--- a/src/env_dispatch.h
+++ b/src/env_dispatch.h
@@ -4,14 +4,10 @@
 
 #include "config.h"  // IWYU pragma: keep
 
-#include <memory>
-
 #include "common.h"
-#include "env_universal_common.h"
 
 class environment_t;
 class env_stack_t;
-class parser_t;
 
 /// Initialize variable dispatch.
 void env_dispatch_init(const environment_t &vars);
diff --git a/src/env_universal_common.cpp b/src/env_universal_common.cpp
index 32fc979fa..37a5482c9 100644
--- a/src/env_universal_common.cpp
+++ b/src/env_universal_common.cpp
@@ -4,38 +4,33 @@
 #include <arpa/inet.h>  // IWYU pragma: keep
 #include <errno.h>
 #include <fcntl.h>
+#include <unistd.h>
 // We need the sys/file.h for the flock() declaration on Linux but not OS X.
 #include <sys/file.h>  // IWYU pragma: keep
 // We need the ioctl.h header so we can check if SIOCGIFHWADDR is defined by it so we know if we're
 // on a Linux system.
-#include <limits.h>
 #include <netinet/in.h>  // IWYU pragma: keep
 #include <sys/ioctl.h>   // IWYU pragma: keep
-#if !defined(__APPLE__) && !defined(__CYGWIN__)
-#include <pwd.h>
-#endif
-#include <stddef.h>
-#include <stdint.h>
-#include <stdlib.h>
-
-#include <cstring>
 #ifdef __CYGWIN__
 #include <sys/mman.h>
 #endif
 #ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>  // IWYU pragma: keep
 #endif
-#include <signal.h>
+#if !defined(__APPLE__) && !defined(__CYGWIN__)
+#include <pwd.h>
+#endif
 #include <sys/stat.h>
 #include <sys/time.h>   // IWYU pragma: keep
 #include <sys/types.h>  // IWYU pragma: keep
-#include <unistd.h>
 
-#include <atomic>
+#include <algorithm>
+#include <cstring>
+#include <cstdio>
 #include <cwchar>
-#include <map>
+#include <functional>
 #include <string>
-#include <type_traits>
+#include <unordered_map>
 #include <utility>
 
 #include "common.h"
@@ -44,7 +39,6 @@
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
 #include "path.h"
-#include "signal.h"
 #include "utf8.h"
 #include "util.h"  // IWYU pragma: keep
 #include "wcstringutil.h"
diff --git a/src/env_universal_common.h b/src/env_universal_common.h
index 4b6458e30..7ff74998f 100644
--- a/src/env_universal_common.h
+++ b/src/env_universal_common.h
@@ -2,16 +2,17 @@
 #define FISH_ENV_UNIVERSAL_COMMON_H
 #include "config.h"  // IWYU pragma: keep
 
-#include <pthread.h>
-#include <stdio.h>
-
+#include <cstdint>
 #include <memory>
+#include <string>
 #include <unordered_set>
+#include <utility>
 #include <vector>
 
 #include "common.h"
 #include "env.h"
 #include "fds.h"
+#include "maybe.h"
 #include "wutil.h"
 
 /// Callback data, reflecting a change in universal variables.
diff --git a/src/event.cpp b/src/event.cpp
index 306b0b617..0f9781bb5 100644
--- a/src/event.cpp
+++ b/src/event.cpp
@@ -4,21 +4,20 @@
 #include "event.h"
 
 #include <signal.h>
-#include <stddef.h>
 #include <unistd.h>
 
 #include <algorithm>
 #include <array>
 #include <atomic>
-#include <functional>
+#include <bitset>
 #include <memory>
 #include <string>
-#include <type_traits>
 
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
-#include "input_common.h"
+#include "flog.h"
 #include "io.h"
+#include "maybe.h"
 #include "parser.h"
 #include "proc.h"
 #include "signal.h"
diff --git a/src/event.h b/src/event.h
index fe7b37e8a..bbdf7bd30 100644
--- a/src/event.h
+++ b/src/event.h
@@ -8,12 +8,15 @@
 
 #include <unistd.h>
 
-#include <map>
+#include <cstdint>
 #include <memory>
+#include <utility>
 #include <vector>
 
 #include "common.h"
-#include "io.h"
+#include "global_safety.h"
+
+struct io_streams_t;
 
 /// The process id that is used to match any process id.
 #define EVENT_ANY_PID 0
diff --git a/src/exec.cpp b/src/exec.cpp
index b4c1b90bb..a325ffe08 100644
--- a/src/exec.cpp
+++ b/src/exec.cpp
@@ -4,51 +4,52 @@
 // performed have been massive.
 #include "config.h"
 
+#include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
 #ifdef HAVE_SIGINFO_H
 #include <siginfo.h>
 #endif
-#include <signal.h>
-#ifdef HAVE_SPAWN_H
-#include <spawn.h>
-#endif
 #include <paths.h>
 #include <stdio.h>
-#include <sys/wait.h>
 #include <unistd.h>
 
 #include <algorithm>
 #include <cstring>
+#include <cstdint>
+#include <cstdlib>
 #include <functional>
+#include <initializer_list>
 #include <map>
 #include <memory>
-#include <stack>
 #include <string>
-#include <type_traits>
 #include <vector>
+#include <utility>
 
+#include "ast.h"
 #include "builtin.h"
 #include "common.h"
 #include "env.h"
 #include "exec.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "fds.h"
 #include "flog.h"
 #include "function.h"
+#include "global_safety.h"
 #include "io.h"
 #include "iothread.h"
 #include "job_group.h"
+#include "maybe.h"
 #include "null_terminated_array.h"
 #include "parse_tree.h"
 #include "parser.h"
-#include "path.h"
 #include "postfork.h"
 #include "proc.h"
 #include "reader.h"
 #include "redirection.h"
-#include "signal.h"
 #include "timer.h"
 #include "trace.h"
+#include "wait_handle.h"
 #include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
diff --git a/src/exec.h b/src/exec.h
index 873ff3a3c..45d5c88ee 100644
--- a/src/exec.h
+++ b/src/exec.h
@@ -2,13 +2,17 @@
 #ifndef FISH_EXEC_H
 #define FISH_EXEC_H
 
-#include <stddef.h>
+#include "config.h"
 
-#include <vector>
+#include <csignal>
+#include <memory>
 
-#include "common.h"
+#include "flog.h"
+#include "io.h"
 #include "proc.h"
 
+class parser_t;
+
 /// Execute the processes specified by \p j in the parser \p.
 /// On a true return, the job was successfully launched and the parser will take responsibility for
 /// cleaning it up. On a false return, the job could not be launched and the caller must clean it
diff --git a/src/expand.cpp b/src/expand.cpp
index 3bb554325..59b4a4355 100644
--- a/src/expand.cpp
+++ b/src/expand.cpp
@@ -1,12 +1,9 @@
 // String expansion functions. These functions perform several kinds of parameter expansion.
-// IWYU pragma: no_include <cstddef>
-#include "config.h"
+#include "config.h" // IWYU pragma: keep
 
 #include <errno.h>
 #include <pwd.h>
 #include <stdarg.h>
-#include <stddef.h>
-#include <stdlib.h>
 #include <unistd.h>
 #include <wctype.h>
 
@@ -16,20 +13,11 @@
 #ifdef SunOS
 #include <procfs.h>
 #endif
-#ifdef __APPLE__
-#include <sys/time.h>  // Required to build with old SDK versions
-// proc.h needs to be included *after* time.h, this comment stops clang-format from reordering.
-#include <sys/proc.h>
-#else
-#include <dirent.h>
-#include <sys/stat.h>
-#endif
 
 #include <algorithm>
-#include <functional>
 #include <map>
-#include <memory>  // IWYU pragma: keep
-#include <type_traits>
+#include <memory>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -40,13 +28,10 @@
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "history.h"
-#include "iothread.h"
+#include "operation_context.h"
 #include "parse_constants.h"
 #include "parse_util.h"
-#include "parser.h"
 #include "path.h"
-#include "proc.h"
-#include "reader.h"
 #include "util.h"
 #include "wcstringutil.h"
 #include "wildcard.h"
diff --git a/src/expand.h b/src/expand.h
index 6d4f37f27..5649abcb4 100644
--- a/src/expand.h
+++ b/src/expand.h
@@ -7,8 +7,7 @@
 
 #include "config.h"
 
-#include <stddef.h>
-
+#include <initializer_list>
 #include <map>
 #include <string>
 #include <vector>
@@ -18,7 +17,6 @@
 #include "maybe.h"
 #include "parse_constants.h"
 
-class environment_t;
 class env_var_t;
 class environment_t;
 class operation_context_t;
diff --git a/src/fallback.cpp b/src/fallback.cpp
index a776e3345..5e4d2ec61 100644
--- a/src/fallback.cpp
+++ b/src/fallback.cpp
@@ -5,41 +5,35 @@
 #include "config.h"
 
 // IWYU likes to recommend adding term.h when we want ncurses.h.
-// IWYU pragma: no_include term.h
-#include <dirent.h>  // IWYU pragma: keep
+// IWYU pragma: no_include "term.h"
 #include <errno.h>   // IWYU pragma: keep
 #include <fcntl.h>   // IWYU pragma: keep
 #include <limits.h>  // IWYU pragma: keep
-#include <stdarg.h>  // IWYU pragma: keep
-#include <stdio.h>   // IWYU pragma: keep
 #include <stdlib.h>
-#include <sys/stat.h>   // IWYU pragma: keep
-#include <sys/types.h>  // IWYU pragma: keep
 #include <unistd.h>
 #include <wctype.h>
 
-#include <algorithm>
 #include <cstring>
 #include <cwchar>
 #if HAVE_GETTEXT
 #include <libintl.h>
 #endif
+#if defined(TPARM_SOLARIS_KLUDGE)
 #if HAVE_CURSES_H
-#include <curses.h>
+#include <curses.h> // IWYU pragma: keep
 #elif HAVE_NCURSES_H
 #include <ncurses.h>  // IWYU pragma: keep
 #elif HAVE_NCURSES_CURSES_H
-#include <ncurses/curses.h>
+#include <ncurses/curses.h> // IWYU pragma: keep
 #endif
 #if HAVE_TERM_H
 #include <term.h>  // IWYU pragma: keep
 #elif HAVE_NCURSES_TERM_H
 #include <ncurses/term.h>
 #endif
+#endif
 #include <signal.h>  // IWYU pragma: keep
 
-#include <cwchar>  // IWYU pragma: keep
-
 #include "common.h"    // IWYU pragma: keep
 #include "fallback.h"  // IWYU pragma: keep
 
@@ -138,11 +132,11 @@ char *fish_gettext(const char *msgid) { return (char *)msgid; }
 char *fish_bindtextdomain(const char *domainname, const char *dirname) {
     UNUSED(domainname);
     UNUSED(dirname);
-    return NULL;
+    return nullptr;
 }
 char *fish_textdomain(const char *domainname) {
     UNUSED(domainname);
-    return NULL;
+    return nullptr;
 }
 #endif
 
diff --git a/src/fallback.h b/src/fallback.h
index 043eea2d7..377b52a8c 100644
--- a/src/fallback.h
+++ b/src/fallback.h
@@ -3,9 +3,7 @@
 
 #include "config.h"
 
-#include <stddef.h>
 #include <stdint.h>
-#include <unistd.h>
 // The following include must be kept despite what IWYU says. That's because of the interaction
 // between the weak linking of `wcsdup` and `wcscasecmp` via `#define`s below and the declarations
 // in <wchar.h>. At least on OS X if we don't do this we get compilation errors do to the macro
diff --git a/src/fd_monitor.cpp b/src/fd_monitor.cpp
index e5c40f110..de55abd94 100644
--- a/src/fd_monitor.cpp
+++ b/src/fd_monitor.cpp
@@ -3,11 +3,14 @@
 
 #include "fd_monitor.h"
 
-#include <cstring>
+#include <errno.h>
+
+#include <algorithm>
+#include <iterator>
 #include <thread>  //this_thread::sleep_for
+#include <type_traits>
 
 #include "flog.h"
-#include "io.h"
 #include "iothread.h"
 #include "wutil.h"
 
diff --git a/src/fd_monitor.h b/src/fd_monitor.h
index da21f41da..6b4005a68 100644
--- a/src/fd_monitor.h
+++ b/src/fd_monitor.h
@@ -4,6 +4,9 @@
 #include <chrono>
 #include <cstdint>
 #include <functional>
+#include <utility>
+#include <vector>
+
 // Needed for musl
 #include <sys/select.h>  // IWYU pragma: keep
 
@@ -11,8 +14,6 @@
 #include "fds.h"
 #include "maybe.h"
 
-class fd_monitor_t;
-
 /// Each item added to fd_monitor_t is assigned a unique ID, which is not recycled.
 /// Items may have their callback triggered immediately by passing the ID.
 /// Zero is a sentinel.
diff --git a/src/fds.cpp b/src/fds.cpp
index daa74866a..0dbae2eb3 100644
--- a/src/fds.cpp
+++ b/src/fds.cpp
@@ -8,7 +8,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 
-#include <cstring>
+#include <algorithm>
 
 #include "flog.h"
 #include "wutil.h"
diff --git a/src/fds.h b/src/fds.h
index 2ae7101ff..ae4928ead 100644
--- a/src/fds.h
+++ b/src/fds.h
@@ -3,16 +3,16 @@
 #ifndef FISH_FDS_H
 #define FISH_FDS_H
 
-#include "config.h"  // IWYU pragma: keep
+#include "config.h" // IWYU pragma: keep
 
-#include <poll.h>
-#include <sys/select.h>
+#include <poll.h> // IWYU pragma: keep
+#include <sys/select.h> // IWYU pragma: keep
 #include <sys/types.h>
 
-#include <algorithm>
+#include <cstdint>
 #include <limits>
 #include <string>
-#include <vector>
+#include <utility>
 
 #include "common.h"
 #include "maybe.h"
diff --git a/src/fish.cpp b/src/fish.cpp
index b06ca5398..59a6f4c33 100644
--- a/src/fish.cpp
+++ b/src/fish.cpp
@@ -18,37 +18,42 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 */
 #include "config.h"
 
-#include <errno.h>
 #include <fcntl.h>
 #include <getopt.h>
 #include <limits.h>
 #include <locale.h>
-#include <stddef.h>
-#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <strings.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include <algorithm>
 #include <cstring>
 #include <cwchar>
 #include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
-#include "builtin.h"
+#include "ast.h"
 #include "common.h"
 #include "env.h"
 #include "event.h"
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "fds.h"
 #include "fish_version.h"
 #include "flog.h"
 #include "function.h"
 #include "future_feature_flags.h"
+#include "global_safety.h"
 #include "history.h"
 #include "io.h"
+#include "maybe.h"
+#include "parse_constants.h"
+#include "parse_tree.h"
 #include "parse_util.h"
 #include "parser.h"
 #include "path.h"
diff --git a/src/fish_indent.cpp b/src/fish_indent.cpp
index 947725f47..9fb9c1725 100644
--- a/src/fish_indent.cpp
+++ b/src/fish_indent.cpp
@@ -23,18 +23,19 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <wctype.h>
 
+#include <algorithm>
+#include <cstdint>
 #include <cstring>
+#include <cwctype>
 #include <cwchar>
 #include <memory>
-#include <stack>
 #include <string>
-#include <tuple>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "ast.h"
-#include "color.h"
 #include "common.h"
 #include "env.h"
 #include "expand.h"
@@ -42,12 +43,14 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include "fish_version.h"
 #include "flog.h"
 #include "future_feature_flags.h"
+#include "global_safety.h"
 #include "highlight.h"
+#include "maybe.h"
 #include "operation_context.h"
-#include "output.h"
 #include "parse_constants.h"
 #include "parse_util.h"
 #include "print_help.h"
+#include "tokenizer.h"
 #include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
diff --git a/src/fish_key_reader.cpp b/src/fish_key_reader.cpp
index 337c848b3..e3ec92f59 100644
--- a/src/fish_key_reader.cpp
+++ b/src/fish_key_reader.cpp
@@ -8,10 +8,7 @@
 // Type "exit" or "quit" to terminate the program.
 #include "config.h"  // IWYU pragma: keep
 
-#include <errno.h>
 #include <getopt.h>
-#include <signal.h>
-#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <termios.h>
@@ -19,7 +16,6 @@
 
 #include <cstring>
 #include <cwchar>
-#include <memory>
 #include <string>
 #include <vector>
 
@@ -29,6 +25,7 @@
 #include "fish_version.h"
 #include "input.h"
 #include "input_common.h"
+#include "maybe.h"
 #include "parser.h"
 #include "print_help.h"
 #include "proc.h"
diff --git a/src/fish_tests.cpp b/src/fish_tests.cpp
index 42331bcb2..9c8e80fd5 100644
--- a/src/fish_tests.cpp
+++ b/src/fish_tests.cpp
@@ -1,42 +1,44 @@
 // Various bug and feature tests. Compiled and run by make test.
 #include "config.h"  // IWYU pragma: keep
 
-// IWYU pragma: no_include <cstring>
-// IWYU pragma: no_include <cstddef>
-#include <errno.h>
 #include <fcntl.h>
 #include <libgen.h>
 #include <limits.h>
-#include <math.h>
 #include <pthread.h>
 #include <signal.h>
 #include <stdarg.h>
-#include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <sys/select.h>
+#include <sys/resource.h>
 #include <sys/stat.h>
-#include <sys/time.h>
-#include <sys/types.h>
 #include <sys/utsname.h>
-#include <sys/wait.h>
 #include <time.h>
 #include <unistd.h>
-#include <wctype.h>
 
 #include <algorithm>
 #include <array>
 #include <atomic>
 #include <chrono>
+#include <cerrno>
+#include <clocale>
 #include <cmath>
 #include <cstring>
 #include <cwchar>
 #include <functional>
+#include <future>
+#include <initializer_list>
+#include <iterator>
+#include <limits>
+#include <list>
+#include <map>
 #include <memory>
+#include <mutex>
 #include <set>
 #include <string>
 #include <thread>
+#include <type_traits>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -46,16 +48,18 @@
 #include "color.h"
 #include "common.h"
 #include "complete.h"
+#include "enum_set.h"
 #include "env.h"
 #include "env_universal_common.h"
-#include "event.h"
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "fds.h"
 #include "fd_monitor.h"
 #include "function.h"
 #include "future_feature_flags.h"
 #include "highlight.h"
 #include "history.h"
+#include "global_safety.h"
 #include "input.h"
 #include "input_common.h"
 #include "io.h"
@@ -63,6 +67,7 @@
 #include "kill.h"
 #include "lru.h"
 #include "maybe.h"
+#include "null_terminated_array.h"
 #include "operation_context.h"
 #include "pager.h"
 #include "parse_constants.h"
@@ -82,6 +87,7 @@
 #include "topic_monitor.h"
 #include "utf8.h"
 #include "util.h"
+#include "wait_handle.h"
 #include "wcstringutil.h"
 #include "wildcard.h"
 #include "wutil.h"  // IWYU pragma: keep
@@ -6629,8 +6635,8 @@ static void test_fd_event_signaller() {
 static void test_timer_format() {
     say(L"Testing timer format");
     // This test uses numeric output, so we need to set the locale.
-    char *saved_locale = strdup(setlocale(LC_NUMERIC, nullptr));
-    setlocale(LC_NUMERIC, "C");
+    char *saved_locale = strdup(std::setlocale(LC_NUMERIC, nullptr));
+    std::setlocale(LC_NUMERIC, "C");
     auto t1 = timer_snapshot_t::take();
     t1.cpu_fish.ru_utime.tv_usec = 0;
     t1.cpu_fish.ru_stime.tv_usec = 0;
@@ -6657,7 +6663,7 @@ Executed in  500.00 micros    fish         external
         err(L"Failed to format timer snapshot\nExpected: %ls\nActual:%ls\n", expected,
             actual.c_str());
     }
-    setlocale(LC_NUMERIC, saved_locale);
+    std::setlocale(LC_NUMERIC, saved_locale);
     free(saved_locale);
 }
 
@@ -7043,7 +7049,7 @@ void list_tests() {
 
 /// Main test.
 int main(int argc, char **argv) {
-    setlocale(LC_ALL, "");
+    std::setlocale(LC_ALL, "");
 
     if (argc >= 2 && std::strcmp(argv[1], "--list") == 0) {
         list_tests();
diff --git a/src/flog.cpp b/src/flog.cpp
index 3ba2813b5..e7886f87b 100644
--- a/src/flog.cpp
+++ b/src/flog.cpp
@@ -1,12 +1,18 @@
-// fish logging
-#include "config.h"
+/// fish logging
+#include "config.h" // IWYU pragma: keep
 
 #include "flog.h"
 
+#include <unistd.h>
+#include <stdarg.h>
+
+#include <algorithm>
+#include <cstring>
+#include <cwchar>
+#include <memory>
 #include <vector>
 
 #include "common.h"
-#include "enum_set.h"
 #include "global_safety.h"
 #include "parse_util.h"
 #include "wcstringutil.h"
diff --git a/src/flog.h b/src/flog.h
index 4ef54fea8..b1ecfaad5 100644
--- a/src/flog.h
+++ b/src/flog.h
@@ -4,20 +4,20 @@
 
 #include "config.h"  // IWYU pragma: keep
 
+#include <errno.h>
 #include <stdio.h>
 
+#include <cstdint>
 #include <string>
 #include <type_traits>
-#include <utility>
+#include <vector>
 
+#include "common.h"
 #include "global_safety.h"
 
 using wcstring = std::wstring;
 using wcstring_list_t = std::vector<wcstring>;
 
-template <typename T>
-class owning_lock;
-
 namespace flog_details {
 
 class category_list_t;
diff --git a/src/function.cpp b/src/function.cpp
index 22c967a46..d54a43011 100644
--- a/src/function.cpp
+++ b/src/function.cpp
@@ -4,12 +4,9 @@
 //
 #include "config.h"  // IWYU pragma: keep
 
-// IWYU pragma: no_include <type_traits>
-#include <dirent.h>
-#include <pthread.h>
-#include <stddef.h>
-
 #include <algorithm>
+
+#include <cstdint>
 #include <cwchar>
 #include <map>
 #include <memory>
@@ -17,17 +14,20 @@
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>
+#include <vector>
 
+#include "ast.h"
 #include "autoload.h"
 #include "common.h"
+#include "complete.h"
 #include "env.h"
 #include "event.h"
-#include "exec.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "function.h"
+#include "maybe.h"
+#include "parse_constants.h"
 #include "parser.h"
 #include "parser_keywords.h"
-#include "reader.h"
 #include "signal.h"
 #include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
diff --git a/src/function.h b/src/function.h
index 1770c0567..cd1dc1316 100644
--- a/src/function.h
+++ b/src/function.h
@@ -5,11 +5,10 @@
 #define FISH_FUNCTION_H
 
 #include <map>
-#include <vector>
+#include <memory>
+#include <string>
 
 #include "common.h"
-#include "env.h"
-#include "event.h"
 #include "parse_tree.h"
 
 class parser_t;
diff --git a/src/future_feature_flags.cpp b/src/future_feature_flags.cpp
index e30065f0b..ac92b705a 100644
--- a/src/future_feature_flags.cpp
+++ b/src/future_feature_flags.cpp
@@ -3,6 +3,7 @@
 #include "future_feature_flags.h"
 
 #include <cwchar>
+#include <string>
 
 #include "wcstringutil.h"
 
diff --git a/src/future_feature_flags.h b/src/future_feature_flags.h
index 12fd897cb..9e604b724 100644
--- a/src/future_feature_flags.h
+++ b/src/future_feature_flags.h
@@ -2,10 +2,7 @@
 #ifndef FISH_FUTURE_FEATURE_FLAGS_H
 #define FISH_FUTURE_FEATURE_FLAGS_H
 
-#include <assert.h>
-
 #include <atomic>
-#include <unordered_map>
 
 #include "common.h"
 
diff --git a/src/global_safety.h b/src/global_safety.h
index 24b491912..5038a7412 100644
--- a/src/global_safety.h
+++ b/src/global_safety.h
@@ -5,7 +5,6 @@
 #include "config.h"  // IWYU pragma: keep
 
 #include <atomic>
-#include <cassert>
 
 #include "common.h"
 
diff --git a/src/highlight.cpp b/src/highlight.cpp
index 40e20c68e..80abd33af 100644
--- a/src/highlight.cpp
+++ b/src/highlight.cpp
@@ -1,17 +1,16 @@
 // Functions for syntax highlighting.
 #include "config.h"  // IWYU pragma: keep
 
-// IWYU pragma: no_include <cstddef>
 #include <dirent.h>
 #include <errno.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 #include <algorithm>
+#include <climits>
 #include <cwchar>
-#include <memory>
+#include <functional>
 #include <string>
-#include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>
@@ -27,11 +26,13 @@
 #include "future_feature_flags.h"
 #include "highlight.h"
 #include "history.h"
+#include "maybe.h"
 #include "output.h"
+#include "operation_context.h"
 #include "parse_constants.h"
 #include "parse_util.h"
-#include "parser.h"
 #include "path.h"
+#include "redirection.h"
 #include "tokenizer.h"
 #include "wcstringutil.h"
 #include "wildcard.h"
diff --git a/src/highlight.h b/src/highlight.h
index 1f302f9dd..0b9e66c62 100644
--- a/src/highlight.h
+++ b/src/highlight.h
@@ -3,14 +3,18 @@
 #define FISH_HIGHLIGHT_H
 
 #include <stddef.h>
-#include <stdint.h>
 
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <string>
 #include <unordered_map>
 #include <vector>
 
 #include "color.h"
-#include "common.h"
-#include "env.h"
+#include "flog.h"
+
+class environment_t;
 
 /// Describes the role of a span of text.
 enum class highlight_role_t : uint8_t {
diff --git a/src/history.cpp b/src/history.cpp
index 032736d5c..9546bff4c 100644
--- a/src/history.cpp
+++ b/src/history.cpp
@@ -1,50 +1,42 @@
 // History functions, part of the user interface.
 #include "config.h"  // IWYU pragma: keep
 
-#include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
-#include <pthread.h>
-#include <stddef.h>
 #include <stdio.h>
-#include <stdlib.h>
 
-#include <cstdint>
 #include <cstring>
 // We need the sys/file.h for the flock() declaration on Linux but not OS X.
 #include <sys/file.h>  // IWYU pragma: keep
 #include <sys/stat.h>
 #include <unistd.h>
-#include <wctype.h>
 
 #include <algorithm>
-#include <atomic>
 #include <chrono>
 #include <cwchar>
 #include <functional>
 #include <iterator>
 #include <map>
-#include <numeric>
 #include <random>
-#include <type_traits>
 #include <unordered_set>
 
 #include "ast.h"
 #include "common.h"
 #include "env.h"
+#include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "fds.h"
 #include "flog.h"
 #include "global_safety.h"
 #include "history.h"
 #include "history_file.h"
 #include "io.h"
 #include "iothread.h"
+#include "operation_context.h"
 #include "lru.h"
 #include "parse_constants.h"
 #include "parse_util.h"
-#include "parser.h"
 #include "path.h"
-#include "reader.h"
 #include "wcstringutil.h"
 #include "wildcard.h"  // IWYU pragma: keep
 #include "wutil.h"     // IWYU pragma: keep
diff --git a/src/history.h b/src/history.h
index e4b66e9de..75f86c103 100644
--- a/src/history.h
+++ b/src/history.h
@@ -2,14 +2,12 @@
 #ifndef FISH_HISTORY_H
 #define FISH_HISTORY_H
 
-// IWYU pragma: no_include <cstddef>
-#include <pthread.h>
-#include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <time.h>
 #include <wctype.h>
 
+#include <algorithm>
 #include <deque>
 #include <memory>
 #include <string>
@@ -19,6 +17,7 @@
 #include <vector>
 
 #include "common.h"
+#include "maybe.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 struct io_streams_t;
diff --git a/src/history_file.cpp b/src/history_file.cpp
index dd2ce7389..e530a8d96 100644
--- a/src/history_file.cpp
+++ b/src/history_file.cpp
@@ -1,13 +1,23 @@
-#include "config.h"
+#include "config.h" // IWYU pragma: keep
 
 #include "history_file.h"
 
+#include <ctype.h>
+#include <errno.h>
+#include <unistd.h>
+
+#include <algorithm>
+#include <cstdlib>
 #include <cstring>
+#include <cstdint>
+#include <cstdio>
+#include <cwchar>
+#include <utility>
 
 #include "common.h"
-#include "fds.h"
 #include "history.h"
 #include "path.h"
+#include "wutil.h"
 
 // Some forward declarations.
 static history_item_t decode_item_fish_2_0(const char *base, size_t len);
diff --git a/src/history_file.h b/src/history_file.h
index 198ee11f1..3a020d9e2 100644
--- a/src/history_file.h
+++ b/src/history_file.h
@@ -1,19 +1,18 @@
 #ifndef FISH_HISTORY_FILE_H
 #define FISH_HISTORY_FILE_H
 
-#include "config.h"
+#include "config.h" // IWYU pragma: keep
 
 #include <sys/mman.h>
 
-#include <cassert>
 #include <ctime>
 #include <memory>
+#include <string>
 
 #include "common.h"
 #include "maybe.h"
 
 class history_item_t;
-class history_tests_t;
 
 // History file types.
 enum history_file_type_t { history_type_fish_2_0, history_type_fish_1_x };
diff --git a/src/input.cpp b/src/input.cpp
index d5b6a4a24..8dfdb652d 100644
--- a/src/input.cpp
+++ b/src/input.cpp
@@ -2,11 +2,9 @@
 #include "config.h"
 
 #include <errno.h>
-#include <wctype.h>
 
-#include <cwchar>
 #if HAVE_TERM_H
-#include <curses.h>
+#include <curses.h> // IWYU pragma: keep
 #include <term.h>
 #elif HAVE_NCURSES_TERM_H
 #include <ncurses/term.h>
@@ -14,7 +12,6 @@
 #include <termios.h>
 
 #include <algorithm>
-#include <atomic>
 #include <memory>
 #include <string>
 #include <utility>
@@ -24,10 +21,10 @@
 #include "env.h"
 #include "event.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "flog.h"
 #include "global_safety.h"
 #include "input.h"
 #include "input_common.h"
-#include "io.h"
 #include "parser.h"
 #include "proc.h"
 #include "reader.h"
diff --git a/src/input.h b/src/input.h
index c2343baae..b775ad779 100644
--- a/src/input.h
+++ b/src/input.h
@@ -4,11 +4,15 @@
 #define FISH_INPUT_H
 
 #include <stddef.h>
+#include <unistd.h>
 
+#include <functional>
+#include <memory>
 #include <vector>
 
 #include "common.h"
 #include "input_common.h"
+#include "maybe.h"
 
 #define FISH_BIND_MODE_VAR L"fish_bind_mode"
 #define DEFAULT_BIND_MODE L"default"
diff --git a/src/input_common.cpp b/src/input_common.cpp
index a98187b2b..6d4723672 100644
--- a/src/input_common.cpp
+++ b/src/input_common.cpp
@@ -3,31 +3,26 @@
 
 #include <errno.h>
 #include <signal.h>
-#include <unistd.h>
-
-#include <cstring>
+#include <stdio.h>
+#include <sys/types.h>
 #ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>
 #endif
-#include <stdio.h>
-#include <stdlib.h>
-#include <sys/select.h>
-#include <sys/time.h>
-#include <sys/types.h>
 
+#include <algorithm>
+#include <climits>
+#include <cstring>
+#include <cstdlib>
 #include <cwchar>
 #include <deque>
-#include <list>
-#include <memory>
-#include <type_traits>
 #include <utility>
 
 #include "common.h"
 #include "env.h"
 #include "env_universal_common.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "fds.h"
 #include "flog.h"
-#include "global_safety.h"
 #include "input_common.h"
 #include "iothread.h"
 #include "wutil.h"
diff --git a/src/input_common.h b/src/input_common.h
index da7558137..a53c46b5d 100644
--- a/src/input_common.h
+++ b/src/input_common.h
@@ -2,9 +2,12 @@
 #ifndef INPUT_COMMON_H
 #define INPUT_COMMON_H
 
-#include <stddef.h>
+#include <unistd.h>
 
-#include <queue>
+#include <cstdint>
+#include <deque>
+#include <string>
+#include <utility>
 
 #include "common.h"
 #include "maybe.h"
diff --git a/src/io.cpp b/src/io.cpp
index 1dbfc68e9..463311c37 100644
--- a/src/io.cpp
+++ b/src/io.cpp
@@ -5,19 +5,18 @@
 
 #include <errno.h>
 #include <fcntl.h>
-#include <stddef.h>
 #include <stdio.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
-#include <cstring>
 #include <cwchar>
+#include <functional>
 
 #include "common.h"
-#include "exec.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "fd_monitor.h"
-#include "iothread.h"
+#include "flog.h"
+#include "maybe.h"
 #include "path.h"
 #include "redirection.h"
 #include "wutil.h"  // IWYU pragma: keep
diff --git a/src/io.h b/src/io.h
index e362b880f..436eab965 100644
--- a/src/io.h
+++ b/src/io.h
@@ -1,23 +1,20 @@
 #ifndef FISH_IO_H
 #define FISH_IO_H
 
-#include <pthread.h>
 #include <stdarg.h>
-#include <stddef.h>
-#include <stdlib.h>
+#include <unistd.h>
 
-#include <atomic>
+#include <cstdint>
+#include <cwchar>
 #include <future>
 #include <memory>
-#include <mutex>
+#include <string>
+#include <utility>
 #include <vector>
 
 #include "common.h"
-#include "env.h"
 #include "fds.h"
-#include "flog.h"
 #include "global_safety.h"
-#include "maybe.h"
 #include "redirection.h"
 
 using std::shared_ptr;
@@ -238,7 +235,6 @@ class io_pipe_t final : public io_data_t {
 };
 
 class io_buffer_t;
-class io_chain_t;
 
 /// Represents filling an io_buffer_t. Very similar to io_pipe_t.
 class io_bufferfill_t final : public io_data_t {
@@ -275,8 +271,6 @@ class io_bufferfill_t final : public io_data_t {
     static separated_buffer_t finish(std::shared_ptr<io_bufferfill_t> &&filler);
 };
 
-class output_stream_t;
-
 /// An io_buffer_t is a buffer which can populate itself by reading from an fd.
 /// It is not an io_data_t.
 class io_buffer_t {
diff --git a/src/iothread.cpp b/src/iothread.cpp
index fcc7eaf65..ee471f42e 100644
--- a/src/iothread.cpp
+++ b/src/iothread.cpp
@@ -2,28 +2,23 @@
 
 #include "iothread.h"
 
-#include <limits.h>
 #include <pthread.h>
 #include <signal.h>
 #include <stdio.h>
-#include <string.h>
-#include <sys/select.h>
-#include <sys/time.h>
-#include <sys/types.h>
-#include <unistd.h>
 
 #include <atomic>
-#include <condition_variable>
+#include <condition_variable> // IWYU pragma: keep
+#include <chrono>
 #include <functional>
-#include <future>
+#include <mutex>
+#include <vector>
 #include <queue>
-#include <thread>
 
 #include "common.h"
+#include "fallback.h"
 #include "fds.h"
 #include "flog.h"
-#include "global_safety.h"
-#include "wutil.h"
+#include "maybe.h"
 
 /// We just define a thread limit of 1024.
 #define IO_MAX_THREADS 1024
diff --git a/src/iothread.h b/src/iothread.h
index 672dcdebd..c2797a6c4 100644
--- a/src/iothread.h
+++ b/src/iothread.h
@@ -2,14 +2,10 @@
 #ifndef FISH_IOTHREAD_H
 #define FISH_IOTHREAD_H
 
-#include <pthread.h>
-
 #include <cstdint>  // for uint64_t
 #include <functional>
 #include <memory>
-#include <type_traits>
-
-#include "maybe.h"
+#include <utility>
 
 /// \return the fd on which to listen for completion callbacks.
 int iothread_port();
diff --git a/src/job_group.cpp b/src/job_group.cpp
index c2113592c..b210288fb 100644
--- a/src/job_group.cpp
+++ b/src/job_group.cpp
@@ -1,10 +1,13 @@
-#include "config.h"
+#include "config.h" // IWYU pragma: keep
 
 #include "job_group.h"
 
+#include <algorithm>
+#include <utility>
+#include <vector>
+
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
-#include "flog.h"
 #include "proc.h"
 
 // Basic thread safe sorted vector of job IDs in use.
diff --git a/src/job_group.h b/src/job_group.h
index 0b2b9a511..2697f5e33 100644
--- a/src/job_group.h
+++ b/src/job_group.h
@@ -4,10 +4,12 @@
 
 #include <termios.h>
 
+
 #include <memory>
 
 #include "common.h"
 #include "global_safety.h"
+#include "maybe.h"
 
 /// A job ID, corresponding to what is printed in 'jobs'.
 /// 1 is the first valid job ID.
@@ -22,7 +24,6 @@ using job_id_t = int;
 /// they do not consume a job ID, they do not show up in job lists, and they do not have a pgid
 /// because they contain no external procs. Note that job_group_t is intended to eventually be
 /// shared between threads, and so must be thread safe.
-class job_t;
 class job_group_t;
 using job_group_ref_t = std::shared_ptr<job_group_t>;
 
diff --git a/src/kill.cpp b/src/kill.cpp
index 19ec2732c..96f7f4949 100644
--- a/src/kill.cpp
+++ b/src/kill.cpp
@@ -6,12 +6,10 @@
 
 #include "kill.h"
 
-#include <stddef.h>
-
 #include <algorithm>
 #include <list>
-#include <memory>
 #include <string>
+#include <utility>
 
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
diff --git a/src/null_terminated_array.h b/src/null_terminated_array.h
index c8dd93ac4..7a19b4e66 100644
--- a/src/null_terminated_array.h
+++ b/src/null_terminated_array.h
@@ -4,8 +4,9 @@
 
 #include "config.h"  // IWYU pragma: keep
 
-#include <memory>
+#include <cstddef>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "common.h"
diff --git a/src/operation_context.cpp b/src/operation_context.cpp
index f0f4d301f..478f89eec 100644
--- a/src/operation_context.cpp
+++ b/src/operation_context.cpp
@@ -1,8 +1,9 @@
 // Utilities for io redirection.
 #include "config.h"  // IWYU pragma: keep
 
-#include "operation_context.h"
+#include <utility>
 
+#include "operation_context.h"
 #include "env.h"
 
 bool no_cancel() { return false; }
diff --git a/src/operation_context.h b/src/operation_context.h
index 6f89853e6..7cf268bd7 100644
--- a/src/operation_context.h
+++ b/src/operation_context.h
@@ -1,6 +1,8 @@
 #ifndef FISH_OPERATION_CONTEXT_H
 #define FISH_OPERATION_CONTEXT_H
 
+#include <cstddef>
+#include <functional>
 #include <memory>
 
 #include "common.h"
diff --git a/src/output.cpp b/src/output.cpp
index d1a02e61f..c0bda9d09 100644
--- a/src/output.cpp
+++ b/src/output.cpp
@@ -2,11 +2,10 @@
 #include "config.h"
 
 #include <stdio.h>
-#include <stdlib.h>
+#include <unistd.h>
 
-#include <cstring>
 #if HAVE_CURSES_H
-#include <curses.h>
+#include <curses.h>  // IWYU pragma: keep
 #elif HAVE_NCURSES_H
 #include <ncurses.h>
 #elif HAVE_NCURSES_CURSES_H
@@ -17,10 +16,9 @@
 #elif HAVE_NCURSES_TERM_H
 #include <ncurses/term.h>
 #endif
-#include <limits.h>
 
 #include <cwchar>
-#include <memory>
+#include <mutex>
 #include <string>
 #include <vector>
 
@@ -29,6 +27,7 @@
 #include "env.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
+#include "maybe.h"
 #include "output.h"
 #include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
diff --git a/src/output.h b/src/output.h
index 91e5d9a95..2b786c31c 100644
--- a/src/output.h
+++ b/src/output.h
@@ -5,11 +5,14 @@
 #ifndef FISH_OUTPUT_H
 #define FISH_OUTPUT_H
 
-#include <stddef.h>
-
+#include <cstdint>
+#include <cstring>
+#include <cwchar>
+#include <string>
 #include <vector>
 
 #include "color.h"
+#include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
 
 class env_var_t;
diff --git a/src/pager.cpp b/src/pager.cpp
index 72632416a..087536218 100644
--- a/src/pager.cpp
+++ b/src/pager.cpp
@@ -1,11 +1,13 @@
 #include "config.h"  // IWYU pragma: keep
 
-// IWYU pragma: no_include <cstddef>
 #include <stddef.h>
 #include <wctype.h>
 
 #include <algorithm>
+
+#include <cstdlib>
 #include <cwchar>
+#include <functional>
 #include <numeric>
 #include <type_traits>
 #include <unordered_map>
@@ -14,12 +16,14 @@
 #include "common.h"
 #include "complete.h"
 #include "fallback.h"
-#include "flog.h"
 #include "highlight.h"
+#include "maybe.h"
 #include "operation_context.h"
 #include "pager.h"
 #include "reader.h"
 #include "screen.h"
+#include "termsize.h"
+#include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 using comp_t = pager_t::comp_t;
diff --git a/src/pager.h b/src/pager.h
index 8d5e443c6..ff74d055b 100644
--- a/src/pager.h
+++ b/src/pager.h
@@ -4,15 +4,16 @@
 
 #include <stddef.h>
 
-#include <memory>
 #include <string>
 #include <vector>
 
 #include "common.h"
 #include "complete.h"
+#include "highlight.h"
 #include "reader.h"
 #include "screen.h"
-#include "termsize.h"
+
+struct termsize_t;
 
 #define PAGER_SELECTION_NONE static_cast<size_t>(-1)
 
@@ -61,8 +62,6 @@ enum class selection_motion_t {
 // How many rows we will show in the "initial" pager.
 #define PAGER_UNDISCLOSED_MAX_ROWS 4
 
-struct highlight_spec_t;
-
 class pager_t {
     size_t available_term_width{0};
     size_t available_term_height{0};
diff --git a/src/parse_execution.cpp b/src/parse_execution.cpp
index 5dfd0c752..09a7e4757 100644
--- a/src/parse_execution.cpp
+++ b/src/parse_execution.cpp
@@ -9,13 +9,13 @@
 #include <stdlib.h>
 #include <termios.h>
 #include <unistd.h>
-#include <wctype.h>
 
-#include <algorithm>
 #include <cwchar>
+#include <deque>
+#include <functional>
 #include <memory>
 #include <string>
-#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "ast.h"
@@ -32,6 +32,7 @@
 #include "io.h"
 #include "job_group.h"
 #include "maybe.h"
+#include "operation_context.h"
 #include "parse_constants.h"
 #include "parse_util.h"
 #include "parser.h"
@@ -41,7 +42,6 @@
 #include "timer.h"
 #include "tokenizer.h"
 #include "trace.h"
-#include "util.h"
 #include "wildcard.h"
 #include "wutil.h"
 
diff --git a/src/parse_execution.h b/src/parse_execution.h
index c2e2e02da..c3e16ca1b 100644
--- a/src/parse_execution.h
+++ b/src/parse_execution.h
@@ -4,12 +4,17 @@
 
 #include <stddef.h>
 
-#include "ast.h"
+#include <vector>
+
+#include "ast.h" // IWYU pragma: keep
 #include "common.h"
 #include "io.h"
+#include "maybe.h"
 #include "parse_constants.h"
 #include "parse_tree.h"
 #include "proc.h"
+#include "redirection.h"
+
 
 class block_t;
 class operation_context_t;
diff --git a/src/parse_tree.cpp b/src/parse_tree.cpp
index efb6672d6..da11f58f2 100644
--- a/src/parse_tree.cpp
+++ b/src/parse_tree.cpp
@@ -3,23 +3,18 @@
 
 #include "parse_tree.h"
 
-#include <stdarg.h>
 #include <stddef.h>
-#include <stdio.h>
 
-#include <algorithm>
 #include <cwchar>
 #include <string>
-#include <type_traits>
-#include <vector>
+#include <utility>
 
 #include "ast.h"
 #include "common.h"
 #include "fallback.h"
-#include "flog.h"
+#include "enum_map.h"
+#include "maybe.h"
 #include "parse_constants.h"
-#include "parse_tree.h"
-#include "proc.h"
 #include "tokenizer.h"
 #include "wutil.h"  // IWYU pragma: keep
 
diff --git a/src/parse_tree.h b/src/parse_tree.h
index 6382c17d3..95ae4b603 100644
--- a/src/parse_tree.h
+++ b/src/parse_tree.h
@@ -2,17 +2,10 @@
 #ifndef FISH_PARSE_PRODUCTIONS_H
 #define FISH_PARSE_PRODUCTIONS_H
 
-#include <stddef.h>
-#include <stdint.h>
-#include <sys/types.h>
-
-#include <deque>
 #include <memory>
-#include <vector>
 
 #include "ast.h"
 #include "common.h"
-#include "maybe.h"
 #include "parse_constants.h"
 #include "tokenizer.h"
 
@@ -51,10 +44,6 @@ const wchar_t *keyword_description(parse_keyword_t type);
 
 parse_error_code_t parse_error_from_tokenizer_error(tokenizer_error_t err);
 
-namespace ast {
-class ast_t;
-}
-
 /// A type wrapping up a parse tree and the original source behind it.
 struct parsed_source_t : noncopyable_t, nonmovable_t {
     wcstring src;
diff --git a/src/parse_util.cpp b/src/parse_util.cpp
index dd7787cb2..8ac535765 100644
--- a/src/parse_util.cpp
+++ b/src/parse_util.cpp
@@ -7,12 +7,12 @@
 #include "parse_util.h"
 
 #include <stdarg.h>
-#include <stdlib.h>
 
+#include <algorithm>
 #include <cwchar>
 #include <memory>
 #include <string>
-#include <type_traits>
+#include <utility>
 
 #include "ast.h"
 #include "builtin.h"
@@ -20,9 +20,9 @@
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "future_feature_flags.h"
+#include "operation_context.h"
 #include "parse_constants.h"
-#include "parse_util.h"
-#include "parser.h"
+#include "parse_tree.h"
 #include "tokenizer.h"
 #include "wcstringutil.h"
 #include "wildcard.h"
diff --git a/src/parse_util.h b/src/parse_util.h
index 961a1308c..96422baaa 100644
--- a/src/parse_util.h
+++ b/src/parse_util.h
@@ -7,12 +7,14 @@
 #include <vector>
 
 #include "common.h"
-#include "parse_tree.h"
-#include "tokenizer.h"
+#include "maybe.h"
+#include "parse_constants.h"
 
 namespace ast {
-struct argument_t;
+    struct argument_t;
+    class ast_t;
 }
+struct tok_t;
 
 /// Handles slices: the square brackets in an expression like $foo[5..4]
 /// \return the length of the slice starting at \p in, or 0 if there is no slice, or -1 on error.
diff --git a/src/parser.cpp b/src/parser.cpp
index 54fbe5cc6..7b6533ddb 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -8,23 +8,27 @@
 
 #include <algorithm>
 #include <cwchar>
+#include <functional>
+#include <iterator>
 #include <memory>
+#include <string>
+#include <type_traits>
 #include <utility>
 
 #include "ast.h"
 #include "common.h"
+#include "complete.h"
 #include "env.h"
 #include "event.h"
 #include "expand.h"
+#include "fds.h" 
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
 #include "function.h"
 #include "job_group.h"
 #include "parse_constants.h"
 #include "parse_execution.h"
-#include "parse_util.h"
 #include "proc.h"
-#include "reader.h"
 #include "signal.h"
 #include "wutil.h"  // IWYU pragma: keep
 
diff --git a/src/parser.h b/src/parser.h
index f75be1b83..a6cad3b48 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -3,26 +3,30 @@
 #define FISH_PARSER_H
 
 #include <stddef.h>
-#include <unistd.h>
 
 #include <csignal>
+#include <cstdint>
+#include <deque>
 #include <list>
 #include <memory>
-#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "common.h"
+#include "env.h"
 #include "event.h"
 #include "expand.h"
+#include "job_group.h"
+#include "maybe.h"
 #include "operation_context.h"
 #include "parse_constants.h"
-#include "parse_execution.h"
 #include "parse_tree.h"
 #include "proc.h"
 #include "util.h"
 #include "wait_handle.h"
 
 class io_chain_t;
+class autoclose_fd_t;
 
 /// event_blockage_t represents a block on events.
 struct event_blockage_t {};
@@ -135,8 +139,6 @@ struct profile_item_t {
 };
 
 class parse_execution_context_t;
-class completion_t;
-struct event_t;
 
 /// Miscellaneous data used to avoid recursion and others.
 struct library_data_t {
@@ -204,7 +206,7 @@ struct library_data_t {
     filename_ref_t current_filename{};
 
     /// List of events that have been sent but have not yet been delivered because they are blocked.
-    std::vector<shared_ptr<const event_t>> blocked_events{};
+    std::vector<std::shared_ptr<const event_t>> blocked_events{};
 
     /// A stack of fake values to be returned by builtin_commandline. This is used by the completion
     /// machinery when wrapping: e.g. if `tig` wraps `git` then git completions need to see git on
@@ -216,8 +218,6 @@ struct library_data_t {
     std::shared_ptr<const autoclose_fd_t> cwd_fd{};
 };
 
-class operation_context_t;
-
 /// The result of parser_t::eval family.
 struct eval_res_t {
     /// The value for $status.
@@ -282,7 +282,7 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     std::deque<profile_item_t> profile_items;
 
     /// Adds a job to the beginning of the job list.
-    void job_add(shared_ptr<job_t> job);
+    void job_add(std::shared_ptr<job_t> job);
 
     /// \return whether we are currently evaluating a function.
     bool is_function() const;
diff --git a/src/parser_keywords.cpp b/src/parser_keywords.cpp
index c7ae27b46..e5b3ec1c5 100644
--- a/src/parser_keywords.cpp
+++ b/src/parser_keywords.cpp
@@ -3,6 +3,8 @@
 
 #include "parser_keywords.h"
 
+#include <cstddef>
+#include <iterator>
 #include <string>
 #include <unordered_set>
 
diff --git a/src/path.cpp b/src/path.cpp
index 04332c1ba..ed2b88a3e 100644
--- a/src/path.cpp
+++ b/src/path.cpp
@@ -6,7 +6,6 @@
 #include "path.h"
 
 #include <errno.h>
-#include <sys/param.h>
 #include <sys/stat.h>
 #if defined(__linux__)
 #include <sys/statfs.h>
@@ -14,10 +13,8 @@
 #include <unistd.h>
 
 #include <cstring>
-#include <cwchar>
-#include <memory>
 #include <string>
-#include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "common.h"
diff --git a/src/path.h b/src/path.h
index 5e0df6e1f..32b162479 100644
--- a/src/path.h
+++ b/src/path.h
@@ -4,10 +4,10 @@
 #ifndef FISH_PATH_H
 #define FISH_PATH_H
 
-#include <stddef.h>
+#include <string>
 
 #include "common.h"
-#include "env.h"
+#include "maybe.h"
 
 /// Returns the user configuration directory for fish. If the directory or one of its parents
 /// doesn't exist, they are first created.
@@ -39,11 +39,12 @@ dir_remoteness_t path_get_data_remoteness();
 /// Like path_get_data_remoteness but for the config directory.
 dir_remoteness_t path_get_config_remoteness();
 
+class env_stack_t;
 /// Emit any errors if config directories are missing.
 /// Use the given environment stack to ensure this only occurs once.
-class env_stack_t;
 void path_emit_config_directory_messages(env_stack_t &vars);
 
+class environment_t;
 /// Finds the path of an executable named \p cmd, by looking in $PATH taken from \p vars.
 /// \returns the path if found, none if not.
 maybe_t<wcstring> path_get_path(const wcstring &cmd, const environment_t &vars);
diff --git a/src/postfork.cpp b/src/postfork.cpp
index 4e2e7bbfe..151556f79 100644
--- a/src/postfork.cpp
+++ b/src/postfork.cpp
@@ -5,20 +5,20 @@
 #include <fcntl.h>
 #include <paths.h>
 #include <signal.h>
-#include <stdio.h>
 #include <time.h>
+#include <unistd.h>
 
 #include <cstring>
-#include <memory>
 #ifdef HAVE_SPAWN_H
 #include <spawn.h>
 #endif
-#include <cwchar>
+#include <string>
+#include <vector>
 
 #include "common.h"
 #include "exec.h"
+#include "fds.h"
 #include "flog.h"
-#include "io.h"
 #include "iothread.h"
 #include "job_group.h"
 #include "postfork.h"
diff --git a/src/postfork.h b/src/postfork.h
index ed44ae4ee..5c09065c1 100644
--- a/src/postfork.h
+++ b/src/postfork.h
@@ -5,10 +5,6 @@
 
 #include "config.h"
 
-#include <stddef.h>
-#include <unistd.h>
-
-#include "maybe.h"
 #ifdef HAVE_SPAWN_H
 #include <spawn.h>
 #endif
@@ -16,6 +12,9 @@
 #define FISH_USE_POSIX_SPAWN HAVE_SPAWN_H
 #endif
 
+#include "common.h"
+#include "maybe.h"
+
 class dup2_list_t;
 class job_t;
 class process_t;
diff --git a/src/proc.cpp b/src/proc.cpp
index 57298e49f..86019e8f9 100644
--- a/src/proc.cpp
+++ b/src/proc.cpp
@@ -3,21 +3,18 @@
 // library will call proc to create representations of the running jobs as needed.
 //
 // Some of the code in this file is based on code from the Glibc manual.
-// IWYU pragma: no_include <__bit_reference>
 #include "config.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
-#include <stdio.h>
 #include <unistd.h>
-#include <wctype.h>
 
 #include <atomic>
 #include <cwchar>
 
 #if HAVE_TERM_H
-#include <curses.h>
+#include <curses.h> // IWYU pragma: keep
 #include <term.h>
 #elif HAVE_NCURSES_TERM_H
 #include <ncurses/term.h>
@@ -26,32 +23,29 @@
 #ifdef HAVE_SIGINFO_H
 #include <siginfo.h>
 #endif
-#ifdef HAVE_SYS_SELECT_H
-#include <sys/select.h>
-#endif
 #include <sys/time.h>  // IWYU pragma: keep
-#include <sys/types.h>
 #include <sys/wait.h>
 
 #include <algorithm>  // IWYU pragma: keep
+#include <cstring>
+#include <cstdio>
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "common.h"
+#include "env.h"
+#include "fds.h"
 #include "event.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
 #include "global_safety.h"
 #include "io.h"
 #include "job_group.h"
-#include "output.h"
-#include "parse_tree.h"
 #include "parser.h"
 #include "proc.h"
 #include "reader.h"
 #include "signal.h"
-#include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 /// The signals that signify crashes to us.
diff --git a/src/proc.h b/src/proc.h
index 7aa018ea2..331517bea 100644
--- a/src/proc.h
+++ b/src/proc.h
@@ -5,24 +5,28 @@
 #define FISH_PROC_H
 #include "config.h"  // IWYU pragma: keep
 
-#include <signal.h>
-#include <stddef.h>
 #include <sys/time.h>  // IWYU pragma: keep
 #include <sys/wait.h>  // IWYU pragma: keep
-#include <unistd.h>
 
+#include <atomic>
+#include <cstdint>
+#include <cstdlib>
 #include <deque>
 #include <memory>
+#include <string>
+#include <utility>
 #include <vector>
 
 #include "common.h"
-#include "event.h"
-#include "global_safety.h"
-#include "io.h"
+#include "job_group.h"
+#include "maybe.h"
 #include "parse_tree.h"
+#include "redirection.h"
 #include "topic_monitor.h"
 #include "wait_handle.h"
 
+struct statuses_t;
+
 /// Types of processes.
 enum class process_type_t : uint8_t {
     /// A regular external command.
@@ -54,7 +58,6 @@ namespace ast {
 struct statement_t;
 }
 
-class job_group_t;
 using job_group_ref_t = std::shared_ptr<job_group_t>;
 
 /// A proc_status_t is a value type that encapsulates logic around exited vs stopped vs signaled,
@@ -235,7 +238,6 @@ class tty_transfer_t : nonmovable_t, noncopyable_t {
 ///
 /// If the process is of type process_type_t::function, argv is the argument vector, and argv[0] is
 /// the name of the shellscript function.
-class parser_t;
 class process_t : noncopyable_t {
    public:
     process_t();
@@ -342,6 +344,7 @@ class process_t : noncopyable_t {
 
 using process_ptr_t = std::unique_ptr<process_t>;
 using process_list_t = std::vector<process_ptr_t>;
+class parser_t;
 
 /// A struct representing a job. A job is a pipeline of one or more processes.
 class job_t : noncopyable_t {
@@ -548,7 +551,6 @@ void set_job_control_mode(job_control_t mode);
 /// Notify the user about stopped or terminated jobs, and delete completed jobs from the job list.
 /// If \p interactive is set, allow removing interactive jobs; otherwise skip them.
 /// \return whether text was printed to stdout.
-class parser_t;
 bool job_reap(parser_t &parser, bool interactive);
 
 /// \return the list of background jobs which we should warn the user about, if the user attempts to
diff --git a/src/re.cpp b/src/re.cpp
index 03079bd6d..9ca2e2c99 100644
--- a/src/re.cpp
+++ b/src/re.cpp
@@ -1,5 +1,8 @@
 #include "config.h"  // IWYU pragma: keep
 
+#include <algorithm>
+#include <cstdint>
+
 #include "re.h"
 
 #include "flog.h"
diff --git a/src/re.h b/src/re.h
index 5a503099b..197e67122 100644
--- a/src/re.h
+++ b/src/re.h
@@ -2,6 +2,10 @@
 #ifndef FISH_RE_H
 #define FISH_RE_H
 
+#include <cstddef>
+#include <memory>
+#include <utility>
+
 #include "common.h"
 #include "maybe.h"
 
diff --git a/src/reader.cpp b/src/reader.cpp
index 36c757d35..ad065e6fb 100644
--- a/src/reader.cpp
+++ b/src/reader.cpp
@@ -13,37 +13,37 @@
 // end of the list is reached, at which point regular searching will commence.
 #include "config.h"
 
-// IWYU pragma: no_include <type_traits>
 #include <errno.h>
 #include <fcntl.h>
 #include <pthread.h>
 #ifdef HAVE_SIGINFO_H
 #include <siginfo.h>
 #endif
-#include <signal.h>
 #include <stdio.h>
-#include <stdlib.h>
+#include <time.h>
 
 #include <cstring>
-#ifdef HAVE_SYS_SELECT_H
-#include <sys/select.h>
-#endif
-#include <sys/time.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <termios.h>
-#include <time.h>
 #include <unistd.h>
 #include <wctype.h>
 
 #include <algorithm>
 #include <atomic>
 #include <chrono>
+#include <cctype>
+#include <cmath>
 #include <csignal>
+#include <cstdint>
+#include <cstdlib>
 #include <cwchar>
+#include <deque>
 #include <functional>
+#include <limits>
 #include <memory>
 #include <set>
-#include <stack>
+#include <type_traits>
 
 #include "ast.h"
 #include "color.h"
@@ -54,6 +54,7 @@
 #include "exec.h"
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "fds.h"
 #include "flog.h"
 #include "function.h"
 #include "global_safety.h"
@@ -64,9 +65,11 @@
 #include "io.h"
 #include "iothread.h"
 #include "kill.h"
+#include "operation_context.h"
 #include "output.h"
 #include "pager.h"
 #include "parse_constants.h"
+#include "parse_tree.h"
 #include "parse_util.h"
 #include "parser.h"
 #include "proc.h"
@@ -76,6 +79,7 @@
 #include "termsize.h"
 #include "tokenizer.h"
 #include "wildcard.h"
+#include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 // Name of the variable that tells how long it took, in milliseconds, for the previous
@@ -3903,7 +3907,7 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
                 if (make_uppercase) {
                     chr = towupper(chr);
                 } else {
-                    chr = tolower(chr);
+                    chr = std::tolower(chr);
                 }
 
                 replacement.push_back(chr);
@@ -3935,7 +3939,7 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
                     if (make_uppercase) {
                         chr = towupper(chr);
                     } else {
-                        chr = tolower(chr);
+                        chr = std::tolower(chr);
                     }
 
                     replacement.push_back(chr);
diff --git a/src/reader.h b/src/reader.h
index e8ef270b4..0401bd598 100644
--- a/src/reader.h
+++ b/src/reader.h
@@ -5,19 +5,22 @@
 #define FISH_READER_H
 
 #include <stddef.h>
+#include <stdint.h>
 
 #include <string>
+#include <memory>
 #include <vector>
+#include <utility>
 
 #include "common.h"
 #include "complete.h"
-#include "highlight.h"
+#include "maybe.h"
 #include "parse_constants.h"
 
+class env_stack_t;
 class environment_t;
 class history_t;
 class io_chain_t;
-class operation_context_t;
 class parser_t;
 
 /// An edit action that can be undone.
diff --git a/src/redirection.cpp b/src/redirection.cpp
index 92a01d04a..59098f02e 100644
--- a/src/redirection.cpp
+++ b/src/redirection.cpp
@@ -2,7 +2,9 @@
 
 #include "redirection.h"
 
+#include <errno.h>
 #include <fcntl.h>
+#include <memory>
 
 #include "io.h"
 #include "wutil.h"
diff --git a/src/redirection.h b/src/redirection.h
index fe1f9bb66..c00082960 100644
--- a/src/redirection.h
+++ b/src/redirection.h
@@ -1,6 +1,8 @@
 #ifndef FISH_REDIRECTION_H
 #define FISH_REDIRECTION_H
 
+#include <string>
+#include <utility>
 #include <vector>
 
 #include "common.h"
diff --git a/src/screen.cpp b/src/screen.cpp
index 5bb5d0a82..d106cc0a0 100644
--- a/src/screen.cpp
+++ b/src/screen.cpp
@@ -4,21 +4,20 @@
 // keeping an internal representation of the current screen contents and trying to find the most
 // efficient way for transforming that to the desired screen content.
 //
-// IWYU pragma: no_include <cstddef>
 #include "config.h"
 
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <termios.h>
-#include <time.h>
 #include <unistd.h>
 
+#include <cstdint>
 #include <cstring>
 #include <cwchar>
 
 #if HAVE_CURSES_H
-#include <curses.h>
+#include <curses.h> // IWYU pragma: keep
 #elif HAVE_NCURSES_H
 #include <ncurses.h>
 #elif HAVE_NCURSES_CURSES_H
@@ -38,10 +37,12 @@
 #include "env.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "flog.h"
+#include "global_safety.h"
 #include "highlight.h"
 #include "output.h"
 #include "pager.h"
 #include "screen.h"
+#include "termsize.h"
 
 /// The number of characters to indent new blocks.
 #define INDENT_STEP 4u
diff --git a/src/screen.h b/src/screen.h
index 8bfb0117f..f5737640c 100644
--- a/src/screen.h
+++ b/src/screen.h
@@ -15,18 +15,18 @@
 
 #include <algorithm>
 #include <cstddef>
-#include <cwchar>
+#include <limits>
 #include <list>
-#include <memory>
-#include <unordered_map>
-#include <unordered_set>
+#include <string>
 #include <utility>
 #include <vector>
 
 #include "common.h"
 #include "highlight.h"
+#include "maybe.h"
 #include "wcstringutil.h"
 
+class environment_t;
 class pager_t;
 class page_rendering_t;
 
diff --git a/src/signal.cpp b/src/signal.cpp
index 2873928ab..1e71d4d06 100644
--- a/src/signal.cpp
+++ b/src/signal.cpp
@@ -2,19 +2,19 @@
 #include "config.h"  // IWYU pragma: keep
 
 #include <errno.h>
-#include <stdio.h>
 #ifdef HAVE_SIGINFO_H
 #include <siginfo.h>
 #endif
-#include <pthread.h>
+#include <unistd.h>
 
 #include <csignal>
+#include <cwchar>
+#include <mutex>
 
 #include "common.h"
 #include "event.h"
 #include "fallback.h"  // IWYU pragma: keep
-#include "parser.h"
-#include "proc.h"
+#include "global_safety.h"
 #include "reader.h"
 #include "signal.h"
 #include "termsize.h"
diff --git a/src/termsize.cpp b/src/termsize.cpp
index 8a1b231d9..f9269a7da 100644
--- a/src/termsize.cpp
+++ b/src/termsize.cpp
@@ -2,12 +2,20 @@
 
 #include "termsize.h"
 
+#include <unistd.h>
+
+#include "env.h"
+#include "flog.h"
 #include "maybe.h"
 #include "parser.h"
 #include "wcstringutil.h"
 #include "wutil.h"
 
+#include <cerrno>
+#include <climits>
+
 #ifdef HAVE_WINSIZE
+#include <sys/ioctl.h>
 #include <termios.h>
 #endif
 
diff --git a/src/termsize.h b/src/termsize.h
index 46416429b..c3e46a0e5 100644
--- a/src/termsize.h
+++ b/src/termsize.h
@@ -7,6 +7,7 @@
 #include <stdint.h>
 
 #include "common.h"
+#include "maybe.h"
 #include "global_safety.h"
 
 class environment_t;
diff --git a/src/timer.cpp b/src/timer.cpp
index c424b759a..22b9b6ea4 100644
--- a/src/timer.cpp
+++ b/src/timer.cpp
@@ -4,21 +4,18 @@
 #include "timer.h"
 
 #include <string.h>
+#include <stdint.h>
+#include <stdio.h>
 
 #include <algorithm>
-#include <cerrno>
 #include <chrono>
-#include <cstddef>
-#include <ctime>
+#include <cwchar>
+#include <functional>
+#include <string>
+#include <vector>
 
-#include "builtin.h"
 #include "common.h"
-#include "exec.h"
 #include "fallback.h"  // IWYU pragma: keep
-#include "io.h"
-#include "parser.h"
-#include "proc.h"
-#include "wgetopt.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 // Measuring time is always complicated with many caveats. Quite apart from the typical
diff --git a/src/timer.h b/src/timer.h
index e43ead5d4..ab41a2374 100644
--- a/src/timer.h
+++ b/src/timer.h
@@ -3,16 +3,11 @@
 #define FISH_TIMER_H
 
 #include <sys/resource.h>
-#include <sys/time.h>
-#include <sys/types.h>
 
 #include <chrono>
 
 #include "common.h"
 
-class parser_t;
-struct io_streams_t;
-
 cleanup_t push_timer(bool enabled);
 
 struct timer_snapshot_t {
diff --git a/src/tokenizer.cpp b/src/tokenizer.cpp
index c96d4a846..c44dfb275 100644
--- a/src/tokenizer.cpp
+++ b/src/tokenizer.cpp
@@ -10,13 +10,11 @@
 #include <wctype.h>
 
 #include <cwchar>
-#include <string>
-#include <type_traits>
+#include <vector>
 
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "future_feature_flags.h"
-#include "tokenizer.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 // _(s) is already wgettext(s).c_str(), so let's not convert back to wcstring
diff --git a/src/tokenizer.h b/src/tokenizer.h
index a86ef1849..35a6661c8 100644
--- a/src/tokenizer.h
+++ b/src/tokenizer.h
@@ -4,6 +4,7 @@
 #define FISH_TOKENIZER_H
 
 #include <stddef.h>
+#include <stdint.h>
 
 #include "common.h"
 #include "maybe.h"
diff --git a/src/topic_monitor.cpp b/src/topic_monitor.cpp
index 41ef6885e..626d3eec6 100644
--- a/src/topic_monitor.cpp
+++ b/src/topic_monitor.cpp
@@ -2,11 +2,13 @@
 
 #include "topic_monitor.h"
 
-#include <limits.h>
 #include <unistd.h>
 
+#include <cerrno>
+
 #include "flog.h"
 #include "iothread.h"
+#include "maybe.h"
 #include "wcstringutil.h"
 #include "wutil.h"
 
diff --git a/src/topic_monitor.h b/src/topic_monitor.h
index cacb8564f..f589f13ea 100644
--- a/src/topic_monitor.h
+++ b/src/topic_monitor.h
@@ -5,13 +5,13 @@
 
 #include <array>
 #include <atomic>
-#include <bitset>
-#include <condition_variable>
+#include <condition_variable> // IWYU pragma: keep
+#include <cstdint>
 #include <limits>
-#include <numeric>
+#include <mutex>
 
 #include "common.h"
-#include "io.h"
+#include "fds.h"
 
 /** Topic monitoring support. Topics are conceptually "a thing that can happen." For example,
  delivery of a SIGINT, a child process exits, etc. It is possible to post to a topic, which means
diff --git a/src/trace.cpp b/src/trace.cpp
index 446436078..4d352848c 100644
--- a/src/trace.cpp
+++ b/src/trace.cpp
@@ -1,7 +1,10 @@
-#include "config.h"
+#include "config.h" // IWYU pragma: keep
 
 #include "trace.h"
 
+#include <deque>
+#include <string>
+
 #include "common.h"
 #include "flog.h"
 #include "parser.h"
diff --git a/src/trace.h b/src/trace.h
index f5a0ff7e1..e891fdbb3 100644
--- a/src/trace.h
+++ b/src/trace.h
@@ -7,7 +7,6 @@
 #include "common.h"
 
 class parser_t;
-class process_t;
 
 /// Trace an "argv": a list of arguments. Each argument is escaped.
 /// If \p command is not null, it is traced first (and not escaped)
diff --git a/src/utf8.cpp b/src/utf8.cpp
index 751ed8f29..dfd031708 100644
--- a/src/utf8.cpp
+++ b/src/utf8.cpp
@@ -18,7 +18,6 @@
 #include "utf8.h"
 
 #include <stdint.h>  // IWYU pragma: keep
-#include <sys/types.h>
 
 #include <limits>
 #include <string>
diff --git a/src/util.cpp b/src/util.cpp
index dfc70839e..fae9d9e15 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -3,7 +3,6 @@
 
 #include "util.h"
 
-#include <errno.h>
 #include <stddef.h>
 #include <sys/time.h>
 #include <wctype.h>
diff --git a/src/wait_handle.cpp b/src/wait_handle.cpp
index 2e01ad4d6..43076f801 100644
--- a/src/wait_handle.cpp
+++ b/src/wait_handle.cpp
@@ -1,8 +1,8 @@
-#include "config.h"
+#include "config.h" // IWYU pragma: keep
 
 #include "wait_handle.h"
 
-#include "flog.h"
+#include <iterator>
 
 wait_handle_store_t::wait_handle_store_t(size_t limit) : limit_(limit) {}
 
diff --git a/src/wait_handle.h b/src/wait_handle.h
index 0ae474b36..a040330dd 100644
--- a/src/wait_handle.h
+++ b/src/wait_handle.h
@@ -9,8 +9,9 @@
 #include <unistd.h>
 
 #include <list>
+#include <memory>
 #include <unordered_map>
-#include <vector>
+#include <utility>
 
 #include "common.h"
 
diff --git a/src/wcstringutil.cpp b/src/wcstringutil.cpp
index 2bdba17f8..6d8c9e801 100644
--- a/src/wcstringutil.cpp
+++ b/src/wcstringutil.cpp
@@ -3,11 +3,14 @@
 
 #include "wcstringutil.h"
 
+#include <stddef.h>
 #include <wctype.h>
 
 #include <locale>
+#include <utility>
 
 #include "common.h"
+#include "fallback.h" // IWYU pragma: keep
 #include "flog.h"
 
 wcstring truncate(const wcstring &input, int max_len, ellipsis_type etype) {
diff --git a/src/wcstringutil.h b/src/wcstringutil.h
index d3a8b5bd6..4a768c0bd 100644
--- a/src/wcstringutil.h
+++ b/src/wcstringutil.h
@@ -2,13 +2,20 @@
 #ifndef FISH_WCSTRINGUTIL_H
 #define FISH_WCSTRINGUTIL_H
 
+#include <limits.h>
+#include <stdint.h>
+
 #include <algorithm>
+#include <cstdlib>
 #include <cstring>
+#include <cwchar>
+#include <iterator>
+#include <limits>
 #include <string>
-#include <utility>
 
 #include "common.h"
 #include "expand.h"
+#include "maybe.h"
 
 /// Test if a string prefixes another. Returns true if a is a prefix of b.
 bool string_prefixes_string(const wcstring &proposed_prefix, const wcstring &value);
diff --git a/src/wgetopt.cpp b/src/wgetopt.cpp
index 2f052b0ff..cd0de046c 100644
--- a/src/wgetopt.cpp
+++ b/src/wgetopt.cpp
@@ -39,8 +39,8 @@
 #include "config.h"  // IWYU pragma: keep
 
 #include <stdio.h>
+#include <stddef.h>
 
-#include <cstring>
 #include <cwchar>
 
 // This version of `getopt' appears to the caller like standard Unix `getopt' but it behaves
diff --git a/src/wgetopt.h b/src/wgetopt.h
index dcfe8abb2..a12674491 100644
--- a/src/wgetopt.h
+++ b/src/wgetopt.h
@@ -41,8 +41,6 @@ Cambridge, MA 02139, USA.  */
 #ifndef FISH_WGETOPT_H
 #define FISH_WGETOPT_H
 
-#include <stddef.h>
-
 /// Instanced getopt() wrapper.
 class wgetopter_t {
    public:
diff --git a/src/wildcard.cpp b/src/wildcard.cpp
index f468d0e35..61d20f4fe 100644
--- a/src/wildcard.cpp
+++ b/src/wildcard.cpp
@@ -6,26 +6,28 @@
 
 #include <dirent.h>
 #include <errno.h>
-#include <stddef.h>
+#include <stdint.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
+#include <algorithm>
 #include <cwchar>
-#include <memory>
+#include <functional>
 #include <string>
 #include <unordered_set>
 #include <utility>
+#include <vector>
 
 #include "common.h"
 #include "complete.h"
+#include "enum_set.h"
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "future_feature_flags.h"
+#include "maybe.h"
 #include "path.h"
-#include "reader.h"
 #include "wcstringutil.h"
-#include "wildcard.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 /// Finds an internal (ANY_STRING, etc.) style wildcard, or wcstring::npos.
diff --git a/src/wildcard.h b/src/wildcard.h
index 4e4babc8b..55a76fb96 100644
--- a/src/wildcard.h
+++ b/src/wildcard.h
@@ -3,7 +3,7 @@
 #ifndef FISH_WILDCARD_H
 #define FISH_WILDCARD_H
 
-#include <vector>
+#include <stddef.h>
 
 #include "common.h"
 #include "complete.h"
diff --git a/src/wutil.cpp b/src/wutil.cpp
index 0b58bf4cf..c08a1a3ed 100644
--- a/src/wutil.cpp
+++ b/src/wutil.cpp
@@ -7,23 +7,28 @@
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
-#include <libgen.h>
 #include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/stat.h>
-#include <sys/statvfs.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <wctype.h>
 
+#ifdef HAVE_XLOCALE_H
+#include <xlocale.h>
+#endif
+
 #include <algorithm>
-#include <atomic>
 #include <cstring>
 #include <cwchar>
 #include <iterator>
+#include <locale>
+#include <mutex>
 #include <string>
 #include <unordered_map>
+#include <utility>
+#include <vector>
 
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
diff --git a/src/wutil.h b/src/wutil.h
index cf05fc2df..caa69b35f 100644
--- a/src/wutil.h
+++ b/src/wutil.h
@@ -2,20 +2,20 @@
 #ifndef FISH_WUTIL_H
 #define FISH_WUTIL_H
 
+#include "config.h" // IWYU pragma: keep
+
 #include <dirent.h>
-#include <locale.h>
 #include <stddef.h>
-#include <stdio.h>
+#include <stdint.h>
 #include <sys/types.h>
-#include <time.h>
-#include <wctype.h>
 
+#include <ctime>
+#include <functional>
+#include <limits>
+#include <locale>
+#include <memory>
 #include <string>
 
-#ifdef HAVE_XLOCALE_H
-#include <xlocale.h>
-#endif
-
 #include "common.h"
 #include "maybe.h"