From 0ea65e0bcd98403bddf49ed4641ec47c32624ad5 Mon Sep 17 00:00:00 2001
From: Cameron Steffen <cam.steffen94@gmail.com>
Date: Sat, 4 Oct 2025 10:37:22 -0500
Subject: [PATCH] Break out some parsing cold functions

This recovers some instruction count regressions after changing most Pat
parsing functions return a non-boxed Pat. It seems to be beneficial to
break out a #[cold] parsing recovery function when the function includes
more parsing, presumably because this requires more stack space and/or
mem copies when LLVM optimizes the wrong path.
---
 .../rustc_parse/src/parser/diagnostics.rs     | 26 ++++++++++++-------
 compiler/rustc_parse/src/parser/pat.rs        | 21 +++++++--------
 2 files changed, 26 insertions(+), 21 deletions(-)

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 9e9a31d80474..c36cd610c6a5 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -1845,15 +1845,17 @@ impl<'a> Parser<'a> {
         &mut self,
         base: T,
     ) -> PResult<'a, T> {
-        if !self.may_recover() {
-            return Ok(base);
-        }
-
         // Do not add `::` to expected tokens.
-        if self.token == token::PathSep {
-            if let Some(ty) = base.to_ty() {
-                return self.maybe_recover_from_bad_qpath_stage_2(ty.span, ty);
-            }
+        if self.may_recover() && self.token == token::PathSep {
+            return self.recover_from_bad_qpath(base);
+        }
+        Ok(base)
+    }
+
+    #[cold]
+    fn recover_from_bad_qpath<T: RecoverQPath>(&mut self, base: T) -> PResult<'a, T> {
+        if let Some(ty) = base.to_ty() {
+            return self.maybe_recover_from_bad_qpath_stage_2(ty.span, ty);
         }
         Ok(base)
     }
@@ -2370,6 +2372,7 @@ impl<'a> Parser<'a> {
         None
     }
 
+    #[cold]
     pub(super) fn recover_arg_parse(&mut self) -> PResult<'a, (Box<ast::Pat>, Box<ast::Ty>)> {
         let pat = self.parse_pat_no_top_alt(Some(Expected::ArgumentName), None)?;
         self.expect(exp!(Colon))?;
@@ -2750,7 +2753,8 @@ impl<'a> Parser<'a> {
 
     /// Some special error handling for the "top-level" patterns in a match arm,
     /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
-    pub(crate) fn maybe_recover_colon_colon_in_pat_typo(
+    #[cold]
+    pub(crate) fn recover_colon_colon_in_pat_typo(
         &mut self,
         mut first_pat: Pat,
         expected: Option<Expected>,
@@ -2935,7 +2939,11 @@ impl<'a> Parser<'a> {
         if self.token != token::Comma {
             return Ok(());
         }
+        self.recover_unexpected_comma(lo, rt)
+    }
 
+    #[cold]
+    fn recover_unexpected_comma(&mut self, lo: Span, rt: CommaRecoveryMode) -> PResult<'a, ()> {
         // An unexpected comma after a top-level pattern is a clue that the
         // user (perhaps more accustomed to some other language) forgot the
         // parentheses in what should have been a tuple pattern; return a
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index 3ef772af97dd..4f522d57e414 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -199,8 +199,8 @@ impl<'a> Parser<'a> {
             // This complicated procedure is done purely for diagnostics UX.
 
             // Check if the user wrote `foo:bar` instead of `foo::bar`.
-            if ra == RecoverColon::Yes {
-                first_pat = self.maybe_recover_colon_colon_in_pat_typo(first_pat, expected);
+            if ra == RecoverColon::Yes && token::Colon == self.token.kind {
+                first_pat = self.recover_colon_colon_in_pat_typo(first_pat, expected);
             }
 
             if let Some(leading_vert_span) = leading_vert_span {
@@ -874,9 +874,12 @@ impl<'a> Parser<'a> {
             }
         };
 
-        let pat = self.mk_pat(lo.to(self.prev_token.span), pat);
-        let pat = self.maybe_recover_from_bad_qpath(pat)?;
-        let pat = self.recover_intersection_pat(pat)?;
+        let mut pat = self.mk_pat(lo.to(self.prev_token.span), pat);
+
+        pat = self.maybe_recover_from_bad_qpath(pat)?;
+        if self.eat_noexpect(&token::At) {
+            pat = self.recover_intersection_pat(pat)?;
+        }
 
         if !allow_range_pat {
             self.ban_pat_range_if_ambiguous(&pat)
@@ -922,14 +925,8 @@ impl<'a> Parser<'a> {
     /// e.g. [F#][and] where they are called AND-patterns.
     ///
     /// [and]: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/pattern-matching
+    #[cold]
     fn recover_intersection_pat(&mut self, lhs: Pat) -> PResult<'a, Pat> {
-        if self.token != token::At {
-            // Next token is not `@` so it's not going to be an intersection pattern.
-            return Ok(lhs);
-        }
-
-        // At this point we attempt to parse `@ $pat_rhs` and emit an error.
-        self.bump(); // `@`
         let mut rhs = self.parse_pat_no_top_alt(None, None)?;
         let whole_span = lhs.span.to(rhs.span);