perf(codegen): avoid allocations for single comment anchors (#25598)

Cargo.lock

@@ -1905,6 +1905,7 @@ dependencies = [
  "oxc_syntax",
  "pico-args",
  "rustc-hash",
+ "smallvec",
 ]
 
 [[package]]

crates/oxc_codegen/Cargo.toml

@@ -34,6 +34,7 @@ oxc_syntax = { workspace = true }
 bitflags = { workspace = true }
 cow-utils = { workspace = true }
 rustc-hash = { workspace = true }
+smallvec = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }

crates/oxc_codegen/src/comment.rs

@@ -1,6 +1,7 @@
 use std::borrow::Cow;
 
 use rustc_hash::{FxHashMap, FxHashSet};
+use smallvec::SmallVec;
 
 use oxc_ast::{
     Comment, CommentKind,
@@ -11,7 +12,9 @@ use oxc_syntax::line_terminator::LineTerminatorSplitter;
 
 use crate::{Codegen, LegalComment, options::CommentOptions};
 
-pub type CommentsMap = FxHashMap</* attached_to */ u32, Vec<Comment>>;
+type CommentList = SmallVec<[Comment; 1]>;
+
+pub type CommentsMap = FxHashMap</* attached_to */ u32, CommentList>;
 
 /// Whether a comment remains meaningful if its original AST anchor is removed.
 fn preserve_when_orphaned(comment: Comment) -> bool {
@@ -72,6 +75,9 @@ impl Codegen<'_> {
         if self.options.comments == CommentOptions::disabled() {
             return;
         }
+        // Each retained comment can create at most one map entry. Reserving
+        // this upper bound avoids incremental map growth while preprocessing.
+        self.comments.reserve(comments.len());
         for comment in comments {
             // Stash pure / no-side-effects comments by `attached_to` so the
             // emission site can recover the verbatim source text instead of
@@ -158,7 +164,7 @@ impl Codegen<'_> {
         }
     }
 
-    pub(crate) fn get_comments(&mut self, start: u32) -> Option<Vec<Comment>> {
+    pub(crate) fn get_comments(&mut self, start: u32) -> Option<CommentList> {
         if self.comments.is_empty() {
             return None;
         }
@@ -283,7 +289,14 @@ impl Codegen<'_> {
         for k in self.orphan_comment_keys.drain(..idx) {
             let Some(entry) = comments.get_mut(&k) else { continue };
             debug_assert!(entry.iter().any(|c| preserve_when_orphaned(*c)));
-            orphans.extend(entry.extract_if(.., |c| preserve_when_orphaned(*c)));
+            entry.retain(|comment| {
+                if preserve_when_orphaned(*comment) {
+                    orphans.push(*comment);
+                    false
+                } else {
+                    true
+                }
+            });
             if entry.is_empty() {
                 comments.remove(&k);
             }