From 6141e639022d2f21f9cf553e5cb7a1725c498a02 Mon Sep 17 00:00:00 2001
From: xmakro <xmakro@users.noreply.github.com>
Date: Tue, 8 Sep 2026 13:47:11 -0700
Subject: [PATCH] Avoid small macro matching and transcription allocations

[skip ci]
---
 compiler/rustc_expand/src/mbe/macro_parser.rs | 7 +++++--
 compiler/rustc_expand/src/mbe/transcribe.rs   | 5 +++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/compiler/rustc_expand/src/mbe/macro_parser.rs b/compiler/rustc_expand/src/mbe/macro_parser.rs
index 3e94e0ca34773..d096ccf1e5933 100644
--- a/compiler/rustc_expand/src/mbe/macro_parser.rs
+++ b/compiler/rustc_expand/src/mbe/macro_parser.rs
@@ -722,14 +722,17 @@ impl TtParser {
         ErrorReported(guarantee)
     }
 
-    fn nameize<I: Iterator<Item = NamedMatch>>(
+    fn nameize<I: ExactSizeIterator<Item = NamedMatch>>(
         &self,
         matcher: &[MatcherLoc],
         mut res: I,
     ) -> NamedMatches {
         // Make that each metavar has _exactly one_ binding. If so, insert the binding into the
         // `NamedParseResult`. Otherwise, it's an error.
-        let mut ret_val = FxHashMap::default();
+        if res.len() == 0 {
+            return FxHashMap::default();
+        }
+        let mut ret_val = FxHashMap::with_capacity_and_hasher(res.len(), Default::default());
         for loc in matcher {
             if let &MatcherLoc::MetaVarDecl { bind, .. } = loc
                 && ret_val
diff --git a/compiler/rustc_expand/src/mbe/transcribe.rs b/compiler/rustc_expand/src/mbe/transcribe.rs
index eabec05cd66c6..fc6adbf1458f6 100644
--- a/compiler/rustc_expand/src/mbe/transcribe.rs
+++ b/compiler/rustc_expand/src/mbe/transcribe.rs
@@ -186,7 +186,7 @@ pub(super) fn transcribe<'a>(
             src_span,
             DelimSpacing::new(Spacing::Alone, Spacing::Alone)
         )],
-        result: Vec::new(),
+        result: Vec::with_capacity(src.tts.len()),
         result_stack: Vec::new(),
     };
 
@@ -271,7 +271,8 @@ pub(super) fn transcribe<'a>(
                 tscx.marker.mark_span(&mut span.open);
                 tscx.marker.mark_span(&mut span.close);
                 tscx.stack.push(Frame::new_delimited(delimited, span, *spacing));
-                tscx.result_stack.push(mem::take(&mut tscx.result));
+                tscx.result_stack
+                    .push(mem::replace(&mut tscx.result, Vec::with_capacity(delimited.tts.len())));
             }
 
             // Nothing much to do here. Just push the token to the result, being careful to