perf(concat): hoist per-token chunk state out of the per-iteration closure

src/concat_sourcemap_builder.rs

@@ -107,35 +107,71 @@ impl<'a> ConcatSourceMapBuilder<'a> {
 
         // Extend `tokens`, skipping the first token if it duplicates the last existing one.
         //
-        // Compute the offset-adjusted source/name ids BEFORE building the token,
-        // and only commit them to `self.token_chunk_prev_*` once we've decided
-        // to actually push the token. Updating the prev-id state for a token
-        // that gets skipped by the dedup `continue` below would pollute the
-        // next sourcemap's TokenChunk header.
+        // Two optimizations vs. the original loop:
+        //
+        //  1. Track `last_source_id` / `last_name_id` in plain locals instead
+        //     of writing back to `self.token_chunk_prev_*` on every iteration.
+        //     The closure-based version captured `&mut self`, forcing a store
+        //     to memory per-token; with locals the optimizer can keep them in
+        //     registers and write back once at the end.
+        //
+        //  2. Hoist the `i == 0` dedup check out of the hot loop by handling
+        //     the first token separately.
+        //
+        // Correctness: only commit the per-token id updates to `last_*` when
+        // the token is actually pushed. Updating the prev-id state for a
+        // token that gets dropped by dedup would pollute the next
+        // sourcemap's TokenChunk header.
+        let mut last_source_id = self.token_chunk_prev_source_id;
+        let mut last_name_id = self.token_chunk_prev_name_id;
+
         self.tokens.reserve(sourcemap.tokens.len());
-        for (i, token) in sourcemap.get_tokens().enumerate() {
-            let source_id_offset = token.get_source_id().map(|x| x + source_offset);
-            let name_id_offset = token.get_name_id().map(|x| x + name_offset);
+
+        let mut iter = sourcemap.get_tokens();
+        if let Some(first) = iter.next() {
+            let source_id_offset = first.get_source_id().map(|x| x + source_offset);
+            let name_id_offset = first.get_name_id().map(|x| x + name_offset);
             let new_token = Token::new(
-                token.get_dst_line() + line_offset,
-                token.get_dst_col(),
-                token.get_src_line(),
-                token.get_src_col(),
+                first.get_dst_line() + line_offset,
+                first.get_dst_col(),
+                first.get_src_line(),
+                first.get_src_col(),
                 source_id_offset,
                 name_id_offset,
             );
-            if i == 0 && self.tokens.last() == Some(&new_token) {
-                continue;
+            if self.tokens.last() != Some(&new_token) {
+                if let Some(s) = source_id_offset {
+                    last_source_id = s;
+                }
+                if let Some(n) = name_id_offset {
+                    last_name_id = n;
+                }
+                self.tokens.push(new_token);
             }
+        }
+
+        for token in iter {
+            let source_id_offset = token.get_source_id().map(|x| x + source_offset);
+            let name_id_offset = token.get_name_id().map(|x| x + name_offset);
             if let Some(s) = source_id_offset {
-                self.token_chunk_prev_source_id = s;
+                last_source_id = s;
             }
             if let Some(n) = name_id_offset {
-                self.token_chunk_prev_name_id = n;
+                last_name_id = n;
             }
-            self.tokens.push(new_token);
+            self.tokens.push(Token::new(
+                token.get_dst_line() + line_offset,
+                token.get_dst_col(),
+                token.get_src_line(),
+                token.get_src_col(),
+                source_id_offset,
+                name_id_offset,
+            ));
         }
 
+        self.token_chunk_prev_source_id = last_source_id;
+        self.token_chunk_prev_name_id = last_name_id;
+
         // Add `token_chunks` after tokens are added so we know the actual end index
         let end_token_idx = self.tokens.len() as u32;