cmake: install fish_{indent,key_reader} as hardlinks to fish

As mentioned in
https://github.com/fish-shell/fish-shell/issues/11921#issuecomment-3540587001,
binaries duplicate a lot of information unnecessarily.

	$ cargo b --release
	$ du -h target/release/fish{,_indent,_key_reader}
	15M	target/release/fish
	15M	target/release/fish_indent
	4.1M	target/release/fish_key_reader
	34M	total

Remove the duplication in CMake-installed builds by creating hard
links. I'm not sure how to do that for Cargo binaries yet (can we
write a portable wrapper script)?

This is still a bit weird because hardlinks are rarely used like
this; but symlinks may cause issues on MSYS2.  Maybe we should write
a /bin/sh wrapper script instead.

CHANGELOG.rst

@@ -13,6 +13,7 @@ Interactive improvements
 
 For distributors and developers
 -------------------------------
+- ``fish_key_reader`` and ``fish_indent`` are now hardlinks to ``fish``.
 
 fish 4.2.1 (released November 13, 2025)
 =======================================

CMakeLists.txt

@@ -58,37 +58,40 @@ if(NOT "${CMAKE_BUILD_TYPE}" IN_LIST build_types)
   message(WARNING "Unsupported build type ${CMAKE_BUILD_TYPE}. If this doesn't build, try one of Release, RelWithDebInfo or Debug")
 endif()
 
-# Define a function to build and link dependencies.
-function(CREATE_TARGET target)
+add_custom_target(
+  fish ALL
+  COMMAND
+    "${CMAKE_COMMAND}" -E
+      env ${VARS_FOR_CARGO}
+        ${Rust_CARGO}
+          build --bin fish
+          $<$<CONFIG:Release>:--release>
+          $<$<CONFIG:RelWithDebInfo>:--profile=release-with-debug>
+          --target ${Rust_CARGO_TARGET}
+          --no-default-features
+          --features=${FISH_CARGO_FEATURES}
+          ${CARGO_FLAGS}
+    &&
+    "${CMAKE_COMMAND}" -E
+      copy "${rust_target_dir}/${rust_profile}/fish" "${CMAKE_CURRENT_BINARY_DIR}"
+  WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+  USES_TERMINAL
+)
+
+function(CREATE_LINK target)
   add_custom_target(
     ${target} ALL
-    COMMAND
-      "${CMAKE_COMMAND}" -E
-        env ${VARS_FOR_CARGO}
-          ${Rust_CARGO}
-            build --bin ${target}
-            $<$<CONFIG:Release>:--release>
-            $<$<CONFIG:RelWithDebInfo>:--profile=release-with-debug>
-            --target ${Rust_CARGO_TARGET}
-            --no-default-features
-            --features=${FISH_CARGO_FEATURES}
-            ${CARGO_FLAGS}
-      &&
-      "${CMAKE_COMMAND}" -E
-        copy "${rust_target_dir}/${rust_profile}/${target}" "${CMAKE_CURRENT_BINARY_DIR}"
-    WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
-    USES_TERMINAL
+    DEPENDS fish
+    COMMAND ln -f fish ${target}
+    WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
   )
-endfunction(CREATE_TARGET)
-
-# Define fish.
-create_target(fish)
+endfunction(CREATE_LINK)
 
 # Define fish_indent.
-create_target(fish_indent)
+create_link(fish_indent)
 
 # Define fish_key_reader.
-create_target(fish_key_reader)
+create_link(fish_key_reader)
 
 # Set up tests.
 include(cmake/Tests.cmake)

cmake/Install.cmake

@@ -1,7 +1,5 @@
 set(CMAKE_INSTALL_MESSAGE NEVER)
 
-set(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/fish ${CMAKE_CURRENT_BINARY_DIR}/fish_indent ${CMAKE_CURRENT_BINARY_DIR}/fish_key_reader)
-
 set(prefix ${CMAKE_INSTALL_PREFIX})
 set(bindir ${CMAKE_INSTALL_BINDIR})
 set(sysconfdir ${CMAKE_INSTALL_SYSCONFDIR})
@@ -75,11 +73,19 @@ function(FISH_TRY_CREATE_DIRS)
   endforeach()
 endfunction(FISH_TRY_CREATE_DIRS)
 
-install(PROGRAMS ${PROGRAMS}
+install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/fish
         PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE GROUP_READ
                     GROUP_EXECUTE WORLD_READ WORLD_EXECUTE
         DESTINATION ${bindir})
 
+if(NOT IS_ABSOLUTE ${bindir})
+  set(abs_bindir "\$ENV{DESTDIR}\${CMAKE_INSTALL_PREFIX}/${bindir}")
+else()
+  set(abs_bindir "\$ENV{DESTDIR}${bindir}")
+endif()
+install(CODE "file(CREATE_LINK ${abs_bindir}/fish ${abs_bindir}/fish_indent)")
+install(CODE "file(CREATE_LINK ${abs_bindir}/fish ${abs_bindir}/fish_key_reader)")
+
 fish_create_dirs(${sysconfdir}/fish/conf.d ${sysconfdir}/fish/completions
     ${sysconfdir}/fish/functions)
 install(FILES etc/config.fish DESTINATION ${sysconfdir}/fish/)