Lexical "this" capture

Author: ef4

packages/@glimmer-workspace/integration-tests/test/compiler/compile-options-test.ts

@@ -210,4 +210,21 @@ module('[glimmer-compiler] precompile', ({ test }) => {
     let elementName = openElementExpr[1];
     assert.strictEqual(elementName, 'rental', 'element name is correct');
   });
+
+  test('when "this" in in locals, it compiles to GetLexicalSymbol', (assert) => {
+    let target = { message: 'hello' };
+    let _wire: ReturnType<typeof compile>;
+    (function() {
+      _wire = compile(`{{this.message}}`, ['this'], (source) => eval(source));
+    }).call(target)
+    let wire = _wire!;
+    assert.deepEqual(wire.scope?.(), [target]);
+    assert.deepEqual(wire.block[0], [[SexpOpcodes.Append,[SexpOpcodes.GetLexicalSymbol,0,["message"]]]])
+  });
+
+  test('when "this" is not in locals, it compiles to GetSymbol', (assert) => {
+    let wire = compile(`{{this.message}}`, [], (source) => eval(source));
+    assert.strictEqual(wire.scope, undefined);
+    assert.deepEqual(wire.block[0], [[SexpOpcodes.Append,[SexpOpcodes.GetSymbol,0,["message"]]]])
+  });
 });

packages/@glimmer/syntax/lib/v2/builders.ts

@@ -208,7 +208,6 @@ export class Builder {
     isTemplateLocal: boolean,
     loc: SourceSpan
   ): ASTv2.VariableReference {
-    assert(name !== 'this', `You called builders.var() with 'this'. Call builders.this instead`);
     assert(
       name[0] !== '@',
       `You called builders.var() with '${name}'. Call builders.at('${name}') instead`

packages/@glimmer/syntax/lib/v2/normalize.ts

@@ -323,6 +323,10 @@ class ExpressionNormalizer {
 
     switch (head.type) {
       case 'ThisHead':
+        if (block.hasBinding('this')) {
+          let [symbol, isRoot] = table.get('this');
+          return block.builder.localVar('this', symbol, isRoot, offsets);
+        }
         return builder.self(offsets);
       case 'AtHead': {
         let symbol = table.allocateNamed(head.name);