all: use a faster vendored regexp/syntax/Regexp.String

internal/syntaxutil/README.md

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+# vendored std regexp/syntax
+
+This package contains a vendored copy of std regexp/syntax. However, it only
+contains the code for converting syntax.Regexp into a String. It is the
+version of the code at a recent go commit, but with a commit which introduces
+a significant performance regression reverted.
+
+At the time of writing regexp.String on go1.22 is taking 40% of CPU at
+Sourcegraph. This should return to ~0% with this vendored code.
+
+https://github.com/sourcegraph/sourcegraph/issues/61462
+
+## Vendored commit
+
+```
+commit 2e1003e2f7e42efc5771812b9ee6ed264803796c
+Author: Daniel Martí <[email protected]>
+Date:   Tue Mar 26 22:59:41 2024 +0200
+
+    cmd/go: replace reflect.DeepEqual with slices.Equal and maps.Equal
+
+    All of these maps and slices are made up of comparable types,
+    so we can avoid the overhead of reflection entirely.
+
+    Change-Id: If77dbe648a336ba729c171e84c9ff3f7e160297d
+    Reviewed-on: https://go-review.googlesource.com/c/go/+/574597
+    Reviewed-by: Than McIntosh <[email protected]>
+    LUCI-TryBot-Result: Go LUCI <[email protected]>
+    Reviewed-by: Ian Lance Taylor <[email protected]>
+```
+
+## Reverted commit
+
+```
+commit 98c9f271d67b501ecf2ce995539abd2cdc81d505
+Author: Russ Cox <[email protected]>
+Date:   Wed Jun 28 17:45:26 2023 -0400
+
+    regexp/syntax: use more compact Regexp.String output
+
+    Compact the Regexp.String output. It was only ever intended for debugging,
+    but there are at least some uses in the wild where regexps are built up
+    using regexp/syntax and then formatted using the String method.
+    Compact the output to help that use case. Specifically:
+
+     - Compact 2-element character class ranges: [a-b] -> [ab].
+     - Aggregate flags: (?i:A)(?i:B)*(?i:C)|(?i:D)?(?i:E) -> (?i:AB*C|D?E).
+
+    Fixes #57950.
+
+    Change-Id: I1161d0e3aa6c3ae5a302677032bb7cd55caae5fb
+    Reviewed-on: https://go-review.googlesource.com/c/go/+/507015
+    TryBot-Result: Gopher Robot <[email protected]>
+    Reviewed-by: Than McIntosh <[email protected]>
+    Run-TryBot: Russ Cox <[email protected]>
+    Reviewed-by: Rob Pike <[email protected]>
+    Auto-Submit: Russ Cox <[email protected]>
+```

internal/syntaxutil/alias_test.go

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+package syntaxutil
+
+import "regexp/syntax"
+
+// A bunch of aliases to avoid needing to modify parse_test.go too much.
+
+type Regexp = syntax.Regexp
+
+type Op = syntax.Op
+
+const (
+	OpNoMatch        = syntax.OpNoMatch
+	OpEmptyMatch     = syntax.OpEmptyMatch
+	OpLiteral        = syntax.OpLiteral
+	OpCharClass      = syntax.OpCharClass
+	OpAnyCharNotNL   = syntax.OpAnyCharNotNL
+	OpAnyChar        = syntax.OpAnyChar
+	OpBeginLine      = syntax.OpBeginLine
+	OpEndLine        = syntax.OpEndLine
+	OpBeginText      = syntax.OpBeginText
+	OpEndText        = syntax.OpEndText
+	OpWordBoundary   = syntax.OpWordBoundary
+	OpNoWordBoundary = syntax.OpNoWordBoundary
+	OpCapture        = syntax.OpCapture
+	OpStar           = syntax.OpStar
+	OpPlus           = syntax.OpPlus
+	OpQuest          = syntax.OpQuest
+	OpRepeat         = syntax.OpRepeat
+	OpConcat         = syntax.OpConcat
+	OpAlternate      = syntax.OpAlternate
+)
+
+type Flags = syntax.Flags
+
+const (
+	FoldCase      = syntax.FoldCase
+	Literal       = syntax.Literal
+	ClassNL       = syntax.ClassNL
+	DotNL         = syntax.DotNL
+	OneLine       = syntax.OneLine
+	NonGreedy     = syntax.NonGreedy
+	PerlX         = syntax.PerlX
+	UnicodeGroups = syntax.UnicodeGroups
+	WasDollar     = syntax.WasDollar
+	Simple        = syntax.Simple
+	MatchNL       = syntax.MatchNL
+	Perl          = syntax.Perl
+	POSIX         = syntax.POSIX
+)
+
+var Parse = syntax.Parse