perf(codegen): further reduce memory allocations in generate_line_offset_tables

Author: overlookmotel

crates/oxc_codegen/src/sourcemap_builder.rs

@@ -312,6 +312,9 @@ impl<'a> SourcemapBuilder<'a> {
         let mut lines = vec![];
         let mut column_offsets = IndexVec::new();
 
+        // Used as a buffer to reduce memory reallocations
+        let mut columns = vec![];
+
         // Process content line-by-line.
         // For each line, start by assuming line will be entirely ASCII, and read byte-by-byte.
         // If line is all ASCII, UTF-8 columns and UTF-16 columns are the same,
@@ -325,7 +328,6 @@ impl<'a> SourcemapBuilder<'a> {
                 column_offsets_id: None,
             });
 
-            let mut columns = vec![]; // Used as a buffer to reduce memory reallocations.
             let remaining = &content.as_bytes()[line_byte_offset as usize..];
             for (byte_offset_from_line_start, b) in remaining.iter().enumerate() {
                 #[expect(clippy::cast_possible_truncation)]
@@ -351,8 +353,6 @@ impl<'a> SourcemapBuilder<'a> {
                         line.column_offsets_id =
                             Some(ColumnOffsetsId::from_usize(column_offsets.len()));
 
-                        columns.clear();
-
                         // Loop through rest of line char-by-char.
                         // `chunk_byte_offset` in this loop is byte offset from start of this 1st
                         // Unicode char.
@@ -396,8 +396,9 @@ impl<'a> SourcemapBuilder<'a> {
                             // Record column offsets
                             column_offsets.push(ColumnOffsets {
                                 byte_offset_to_first: byte_offset_from_line_start,
-                                columns: columns.into_iter().collect(),
+                                columns: columns.clone().into_boxed_slice(),
                             });
+                            columns.clear();
 
                             // Revert back to outer loop for next line
                             continue 'lines;
@@ -410,7 +411,7 @@ impl<'a> SourcemapBuilder<'a> {
                         // Record column offsets
                         column_offsets.push(ColumnOffsets {
                             byte_offset_to_first: byte_offset_from_line_start,
-                            columns: columns.into_iter().collect(),
+                            columns: columns.into_boxed_slice(),
                         });
 
                         break 'lines;