[BUGFIX] Ensure legacy path.parts matches existing semantics

packages/@glimmer/syntax/lib/v1/legacy-interop.ts

@@ -61,12 +61,35 @@ export function buildLegacyPath({ head, tail, loc }: PathExpressionParams): ASTv
   Object.defineProperty(node, 'parts', {
     enumerable: false,
     get(this: { original: string }): readonly string[] {
-      deprecate(`The parts property on path nodes is deprecated, use trusting instead`);
-      return Object.freeze(this.original.split('.'));
+      deprecate(`The parts property on path nodes is deprecated, use head and tail instead`);
+      let parts = asPresentArray(this.original.split('.'));
+
+      if (parts[0] === 'this') {
+        // parts does not include `this`
+        parts.shift();
+      } else if (parts[0].startsWith('@')) {
+        // parts does not include leading `@`
+        parts[0] = parts[0].slice(1);
+      }
+
+      return Object.freeze(parts);
     },
-    set(this: { original: string }, value: PresentArray<string>) {
-      deprecate(`The parts property on mustache nodes is deprecated, use trusting instead`);
-      this.original = value.join('.');
+    set(this: { head: ASTv1.PathHead; original: string }, values: PresentArray<string>) {
+      deprecate(`The parts property on mustache nodes is deprecated, use head and tail instead`);
+
+      let parts = [...values];
+
+      // you are not supposed to already have `this` or `@` in the parts, but since this is
+      // deprecated anyway, we will infer what you meant and allow it
+      if (parts[0] !== 'this' && !parts[0]?.startsWith('@')) {
+        if (this.head.type === 'ThisHead') {
+          parts.unshift('this');
+        } else if (this.head.type === 'AtHead') {
+          parts[0] = `@${parts[0]}`;
+        }
+      }
+
+      this.original = parts.join('.');
     },
   });
 

packages/@glimmer/syntax/test/legacy-interop-test.ts

@@ -0,0 +1,51 @@
+import { builders as b } from '@glimmer/syntax';
+
+QUnit.module('[glimmer-syntax] AST nodes legacy interop');
+
+QUnit.test('path.parts does not include this', (assert) => {
+  let path = b.path('this.foo.bar');
+
+  assert.deepEqual(path.original, 'this.foo.bar', 'path.original should include this');
+  assert.deepEqual(path.head.type, 'ThisHead', 'path.head should be a ThisHead');
+  assert.deepEqual(path.parts, ['foo', 'bar'], 'path.parts should not include this');
+
+  path.parts = ['bar', 'baz'];
+
+  assert.deepEqual(path.original, 'this.bar.baz', 'path.original should include this');
+  assert.deepEqual(path.head.type, 'ThisHead', 'path.head should be a ThisHead');
+  assert.deepEqual(path.parts, ['bar', 'baz'], 'path.parts should not include this');
+
+  path.head = b.head('@foo');
+  assert.deepEqual(path.head.type, 'AtHead', 'path.head should be a AtHead');
+
+  // Inconsistent, but we will allow it
+  path.parts = ['this', 'foo', 'bar', 'baz'];
+
+  assert.deepEqual(path.original, 'this.foo.bar.baz', 'path.original should include this');
+  assert.deepEqual(path.head.type, 'ThisHead', 'path.head should be a ThisHead');
+  assert.deepEqual(path.parts, ['foo', 'bar', 'baz'], 'path.parts should not include this');
+});
+
+QUnit.test('path.parts does not include @', (assert) => {
+  let path = b.path('@foo.bar');
+
+  assert.deepEqual(path.original, '@foo.bar', 'path.original should include @');
+  assert.deepEqual(path.head.type, 'AtHead', 'path.head should be a AtHead');
+  assert.deepEqual(path.parts, ['foo', 'bar'], 'path.parts should not include @');
+
+  path.parts = ['bar', 'baz'];
+
+  assert.deepEqual(path.original, '@bar.baz', 'path.original should include @');
+  assert.deepEqual(path.head.type, 'AtHead', 'path.head should be a AtHead');
+  assert.deepEqual(path.parts, ['bar', 'baz'], 'path.parts should not include @');
+
+  path.head = b.head('this');
+  assert.deepEqual(path.head.type, 'ThisHead', 'path.head should be a ThisHead');
+
+  // Inconsistent, but we will allow it
+  path.parts = ['@foo', 'bar', 'baz'];
+
+  assert.deepEqual(path.original, '@foo.bar.baz', 'path.original should include @');
+  assert.deepEqual(path.head.type, 'AtHead', 'path.head should be a AtHead');
+  assert.deepEqual(path.parts, ['foo', 'bar', 'baz'], 'path.parts should not include this');
+});