perf(packages/codegen): eagerly initialize mapNames (#25979)

Previously `mapNames` property of state was lazily initialized - it started as `null`, and an array is only created when a named mapping is written.

Often lazy initialization is a good strategy, but in this case IMO it's a poor trade-off - it adds a check for every named mapping that's written in return for saving a single allocation per file, and only in files which produce a source map with no names. Allocating 1 empty array is very cheap.

packages/codegen/src-js/print/source_map.ts

@@ -39,15 +39,13 @@ const ASCII_DECODER = /* @__PURE__ */ new TextDecoder();
  */
 export function generateSourceMap(state: State, options: Options): SourceMap {
   debugAssert(
-    state.mapPositions !== null,
-    "Source map positions should exist when sourcemap generation is enabled",
+    state.mapPositions !== null && state.mapNames !== null && state.sourceText !== null,
+    "`mapPositions`, `mapNames` and `sourceText` should be defined when source maps are enabled",
   );
 
   const { output, mapPositions, mapNames, sourceText } = state;
   const mappingCount = mapPositions.length >> 1;
 
-  debugAssert(sourceText !== null, "`sourceText` should be defined when producing a source map");
-
   if (mappingCount === 0) {
     return {
       version: 3,
@@ -65,7 +63,7 @@ export function generateSourceMap(state: State, options: Options): SourceMap {
   const names: string[] = [];
   let nameIds: Map<string, number> | undefined;
   let mapNameEntryIndex = 0;
-  let nextNamedMappingIndex = (mapNames?.[0] as number | undefined) ?? Infinity;
+  let nextNamedMappingIndex = (mapNames[0] as number | undefined) ?? Infinity;
 
   let sourceLineStarts: number[] | undefined;
   let sourceScanOffset = 0;
@@ -244,7 +242,7 @@ export function generateSourceMap(state: State, options: Options): SourceMap {
     if (index === nextNamedMappingIndex) {
       nameIds ??= new Map<string, number>();
 
-      const name = mapNames![mapNameEntryIndex + 1] as string;
+      const name = mapNames[mapNameEntryIndex + 1] as string;
       let nameId = nameIds.get(name);
       if (nameId === undefined) {
         nameId = names.length;
@@ -255,7 +253,7 @@ export function generateSourceMap(state: State, options: Options): SourceMap {
       mappingLength = writeVlq(mappingBuffer, mappingLength, nameId - previousNameId);
       previousNameId = nameId;
       mapNameEntryIndex += 2;
-      nextNamedMappingIndex = (mapNames![mapNameEntryIndex] as number | undefined) ?? Infinity;
+      nextNamedMappingIndex = (mapNames[mapNameEntryIndex] as number | undefined) ?? Infinity;
     }
 
     hasSegmentOnLine = true;

packages/codegen/src-js/print/write.ts

@@ -490,8 +490,8 @@ function markMapNamed(state: State, node: NamedMappableNode): void {
   if (!SOURCEMAPS || !hasMappableSpan(node)) return;
 
   debugAssert(
-    state.mapPositions !== null && state.sourceText !== null,
-    "`mapPositions` and `sourceText` should be defined when source maps are enabled",
+    state.mapPositions !== null && state.mapNames !== null && state.sourceText !== null,
+    "`mapPositions`, `mapNames` and `sourceText` should be defined when source maps are enabled",
   );
 
   const { start, end } = node;
@@ -541,7 +541,7 @@ function markMapNamed(state: State, node: NamedMappableNode): void {
     // A transformed or hand-authored AST can carry ranges unrelated to `sourceText`.
     // Preserve the existing fallback in that case instead of recording an arbitrary source substring.
     if (originalName === undefined || !isSameToken(originalName, printedName, hashLength)) {
-      (state.mapNames ??= []).push(
+      state.mapNames.push(
         mapPositions.length >> 1,
         originalName === undefined ? printedName : originalName,
       );

packages/codegen/src-js/state.ts

@@ -146,7 +146,7 @@ export class State {
       debugAssert(options.sourceText != null);
       this.sourceText = options.sourceText;
       this.mapPositions = [];
-      this.mapNames = null;
+      this.mapNames = [];
     }
   }
 }