Add string-replace-fewer-backslashes feature

This disables an extra round of escaping in the `string replace -r`
replacement string.

Currently, to add a backslash to an a or b (to "escape" it):

    string replace -ra '([ab])' '\\\\\\\$1' a

7 backslashes!

This removes one of the layers, so now 3 or 4 works (each one escaped
for the single-quotes, so pcre receives two, which it reads as one literal):

    string replace -ra '([ab])' '\\\\$1' a

This is backwards-incompatible as replacement strings will change
meaning, so we put it behind a feature flag.

The name is kinda crappy, though.

Fixes #5474.

src/builtin_string.cpp

@@ -26,6 +26,7 @@
 #include "builtin.h"
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "future_feature_flags.h"
 #include "io.h"
 #include "parse_util.h"
 #include "pcre2.h"
@@ -933,8 +934,13 @@ class regex_replacer_t : public string_replacer_t {
     regex_replacer_t(const wchar_t *argv0, const wcstring &pattern, const wcstring &replacement_,
                      const options_t &opts, io_streams_t &streams)
         : string_replacer_t(argv0, opts, streams),
-          regex(argv0, pattern, opts.ignore_case, streams),
-          replacement(interpret_escapes(replacement_)) {}
+          regex(argv0, pattern, opts.ignore_case, streams) {
+        if (feature_test(features_t::string_replace_backslash)) {
+            replacement = replacement_;
+        } else {
+            replacement = interpret_escapes(replacement_);
+        }
+    }
 
     bool replace_matches(const wcstring &arg) override;
 };

src/future_feature_flags.cpp

@@ -13,6 +13,7 @@ features_t &mutable_fish_features() { return global_features; }
 const features_t::metadata_t features_t::metadata[features_t::flag_count] = {
     {stderr_nocaret, L"stderr-nocaret", L"3.0", L"^ no longer redirects stderr"},
     {qmark_noglob, L"qmark-noglob", L"3.0", L"? no longer globs"},
+    {string_replace_backslash, L"string-replace-fewer-backslashes", L"3.1", L"string replace -r needs fewer backslashes in the replacement"},
 };
 
 const struct features_t::metadata_t *features_t::metadata_for(const wchar_t *name) {

src/future_feature_flags.h

@@ -17,6 +17,9 @@ class features_t {
      /// Whether ? is supported as a glob.
      qmark_noglob,
 
+     /// Whether string replace -r double-unescapes the replacement.
+     string_replace_backslash,
+
      /// The number of flags.
      flag_count
  };

tests/invocation/features-string-backslashes-off.invoke

@@ -0,0 +1 @@
+--features 'no-string-replace-fewer-backslashes' -C 'string replace -ra "\\\\" "\\\\\\\\" -- "a\b\c"'

tests/invocation/features-string-backslashes-off.out

@@ -0,0 +1 @@
+a\b\c

tests/invocation/features-string-backslashes.invoke

@@ -0,0 +1 @@
+--features 'string-replace-fewer-backslashes' -C 'string replace -ra "\\\\" "\\\\\\\\" -- "a\b\c"'

tests/invocation/features-string-backslashes.out

@@ -0,0 +1 @@
+a\\b\\c

tests/status.out

@@ -6,5 +6,6 @@ test_function
 # Future Feature Flags
 stderr-nocaret	off	3.0	^ no longer redirects stderr
 qmark-noglob	off	3.0	? no longer globs
+string-replace-fewer-backslashes	off	3.1	string replace -r needs fewer backslashes in the replacement
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