Match transformForLint masking to ember-estree's toPlaceholderJS

typescript-eslint hashes the code the parser passes in against what the
watch program last read from disk (via the patched ts.sys.readFile). The
two texts came from different masking strategies: toPlaceholderJS keeps
template content and blanks only backticks/dollars, while transformForLint
blanked every non-newline character. Any .gts/.gjs file with a non-empty
template therefore hashed differently, which fired the file-changed watch
callback and silently rebuilt the entire TS program inside getProgram() —
once per file linted. On large codebases that is hour-long lint runs and
tens of GB of memory (#229).

Blank only backticks/dollars, byte-matching ember-estree 0.6.10, and add a
parity regression test so the two implementations cannot drift again. This
masking is still overflow-safe for the #226 case: replacement is 1:1, so
the placeholder never outgrows the original region.

Fixes #229

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: NullVoxPopuli

src/parser/transforms.js

@@ -279,8 +279,11 @@ export const replaceRange = function replaceRange(s, start, end, substitute) {
 };
 
 function maskTemplateContentForLint(content) {
-  // Preserve line endings and UTF-16 length, but remove template syntax from the JS placeholder.
-  return content.replace(/[^\r\n]/g, ' ');
+  // Must produce byte-identical output to ember-estree's toPlaceholderJS
+  // masking, or typescript-eslint sees two different contents for the same
+  // .gts file (disk read vs lint parse) and invalidates + rebuilds the
+  // program on every file.
+  return content.replace(/[`$]/g, ' ');
 }
 
 const processor = new Preprocessor();

tests/placeholder-parity.test.js

@@ -0,0 +1,59 @@
+import { describe, expect, it } from 'vitest';
+import { toTree } from 'ember-estree';
+import { transformForLint } from '../src/parser/transforms.js';
+
+/**
+ * transformForLint (used by the patched ts.sys.readFile for type-aware
+ * linting) must produce byte-identical output to the placeholder JS that
+ * ember-estree's toTree hands to the JS/TS parser at lint time.
+ *
+ * typescript-eslint hashes the code passed to parseForESLint and compares it
+ * against what the watch program last read off disk; any difference marks the
+ * file as changed and silently rebuilds the whole program inside
+ * getProgram() — once per .gts/.gjs file linted (#229).
+ */
+
+function toTreePlaceholder(code) {
+  let placeholder;
+  try {
+    toTree(code, {
+      filePath: 'x.gts',
+      parser: (js) => {
+        placeholder = js;
+        // Only the placeholder is needed; abort before JS parsing.
+        throw new Error('stop');
+      },
+    });
+  } catch {
+    // expected
+  }
+  return placeholder;
+}
+
+const cases = {
+  'backtick-heavy template comments (#226)': `import Component from '@glimmer/component';
+
+export default class MyComponent extends Component {
+  <template>
+    {{!  \`asd\` \`qwe\` \`zxc\` \`undefined\` \`asd\` }}
+    {{! \`@foo\` }}
+  </template>
+}
+`,
+  'expression template with dollar signs': `export const x = <template>costs \${{amount}} \`really\` $$$</template>;
+`,
+  'multibyte content (emoji, CJK)': `export const y = <template>🎉 日本語 \` $ 🚀</template>;
+`,
+  'CRLF line endings': `export const z = <template>\r\n  hi \`there\`\r\n</template>;\r\n`,
+  'multiple templates in one module': `export const a = <template>one \`x\`</template>;
+export const b = <template>two $y</template>;
+`,
+};
+
+describe('transformForLint matches toTree placeholder byte-for-byte', () => {
+  for (const [name, code] of Object.entries(cases)) {
+    it(name, () => {
+      expect(transformForLint(code).output).toBe(toTreePlaceholder(code));
+    });
+  }
+});