fix(transformer): legacy decorator on computed property key leaves variable unassigned (#20430)

This PR has two fixes. The second fix exposed a pre-existing bug that
needed to be fixed first.

- 24a72d7bc: `create_read_write_target` was used for a simple `=`
assignment when creating a temp var for decorated computed property
keys. Should be `create_write_target`.

- f0affc6f4: `transform_class_on_exit` was removing all no-initializer
properties unconditionally. But the legacy decorator embeds an
assignment in the computed key (`[FIELD_NAME]` → `[_a =
FIELD_NAME]`), so removing the property also removes the assignment,
leaving `_a` undefined in `_decorate()`. Keep properties whose computed
keys have side effects, matching TypeScript's
  behavior.

  Fixes #20418

---------

Co-authored-by: Dunqing <dengqing0821@gmail.com>

crates/oxc_transformer/src/decorator/legacy/mod.rs

@@ -1115,7 +1115,7 @@ impl<'a> LegacyDecorator<'a> {
                 // Create a unique binding for the computed property key, and insert it outside of the class
                 let binding = VarDeclarationsStore::create_uid_var_based_on_node(key, ctx);
                 let operator = AssignmentOperator::Assign;
-                let left = binding.create_read_write_target(ctx);
+                let left = binding.create_write_target(ctx);
                 let right = key.to_expression_mut().take_in(ctx.ast);
                 let key_expr = ctx.ast.expression_assignment(SPAN, operator, left, right);
                 *key = PropertyKey::from(key_expr);

crates/oxc_transformer/src/typescript/class.rs

@@ -206,11 +206,7 @@ impl<'a> TypeScript<'a> {
         }
     }
 
-    pub(super) fn transform_class_on_exit(
-        &self,
-        class: &mut Class<'a>,
-        _ctx: &mut TraverseCtx<'a>,
-    ) {
+    pub(super) fn transform_class_on_exit(&self, class: &mut Class<'a>, ctx: &TraverseCtx<'a>) {
         if !self.remove_class_fields_without_initializer {
             return;
         }
@@ -220,6 +216,9 @@ impl<'a> TypeScript<'a> {
                 && prop.value.is_none()
                 && !prop.key.is_private_identifier()
             {
+                if let Some(key) = prop.key.as_expression() {
+                    return key_needs_temp_var(key, ctx);
+                }
                 return false;
             }
             true

tasks/coverage/snapshots/semantic_typescript.snap

@@ -22822,9 +22822,6 @@ semantic Error: tasks/coverage/typescript/tests/cases/conformance/esDecorators/c
 Bindings mismatch:
 after transform: ScopeId(0): ["C", "_decorate", "_decorateMetadata", "_field", "dec", "field3"]
 rebuilt        : ScopeId(0): ["C", "_decorate", "_decorateMetadata", "_field", "field3"]
-Reference flags mismatch for "_field":
-after transform: ReferenceId(15): ReferenceFlags(Read | Write)
-rebuilt        : ReferenceId(2): ReferenceFlags(Write)
 Reference symbol mismatch for "dec":
 after transform: SymbolId(0) "dec"
 rebuilt        : <None>
@@ -22864,9 +22861,6 @@ semantic Error: tasks/coverage/typescript/tests/cases/conformance/esDecorators/c
 Bindings mismatch:
 after transform: ScopeId(0): ["C", "_decorate", "_decorateMetadata", "_defineProperty", "_field", "_field2", "dec", "field3"]
 rebuilt        : ScopeId(0): ["C", "_decorate", "_decorateMetadata", "_defineProperty", "_field", "_field2", "field3"]
-Reference flags mismatch for "_field":
-after transform: ReferenceId(15): ReferenceFlags(Read | Write)
-rebuilt        : ReferenceId(4): ReferenceFlags(Write)
 Reference symbol mismatch for "dec":
 after transform: SymbolId(0) "dec"
 rebuilt        : <None>
@@ -22896,9 +22890,6 @@ rebuilt        : SymbolId(7): Span { start: 199, end: 200 }
 Symbol reference IDs mismatch for "D":
 after transform: SymbolId(7): [ReferenceId(24)]
 rebuilt        : SymbolId(7): []
-Reference flags mismatch for "_field":
-after transform: ReferenceId(16): ReferenceFlags(Read | Write)
-rebuilt        : ReferenceId(2): ReferenceFlags(Write)
 Reference symbol mismatch for "dec":
 after transform: SymbolId(0) "dec"
 rebuilt        : <None>

tasks/transform_conformance/snapshots/oxc.snap.md

@@ -1,6 +1,6 @@
 commit: 87a048db
 
-Passed: 204/337
+Passed: 206/339
 
 # All Passed:
 * babel-plugin-transform-class-static-block
@@ -555,7 +555,7 @@ x Output mismatch
 x Output mismatch
 
 
-# legacy-decorators (6/88)
+# legacy-decorators (8/90)
 * oxc/class-without-name-with-decorated_class/input.ts
 Bindings mismatch:
 after transform: ScopeId(0): ["dec"]

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/computed-key-property-decorator-with-initializer/input.ts

@@ -0,0 +1,8 @@
+const FIELD_NAME = "myField";
+
+function dec(target: any, key: string) {}
+
+class MyModel {
+	@dec
+	[FIELD_NAME] = "value";
+}

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/computed-key-property-decorator-with-initializer/output.js

@@ -0,0 +1,7 @@
+var _FIELD_NAME;
+const FIELD_NAME = "myField";
+function dec(target, key) {}
+class MyModel {
+  [_FIELD_NAME = FIELD_NAME] = "value";
+}
+babelHelpers.decorate([dec], MyModel.prototype, _FIELD_NAME, void 0);

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/computed-key-property-decorator/input.ts

@@ -0,0 +1,8 @@
+const FIELD_NAME = "myField";
+
+function dec(target: any, key: string) {}
+
+class MyModel {
+	@dec
+	[FIELD_NAME]: string;
+}

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/computed-key-property-decorator/options.json

@@ -0,0 +1,11 @@
+{
+  "plugins": [
+    "transform-legacy-decorator",
+    [
+      "transform-typescript",
+      {
+        "removeClassFieldsWithoutInitializer": true
+      }
+    ]
+  ]
+}

tasks/transform_conformance/tests/legacy-decorators/test/fixtures/oxc/computed-key-property-decorator/output.js

@@ -0,0 +1,7 @@
+var _FIELD_NAME;
+const FIELD_NAME = "myField";
+function dec(target, key) {}
+class MyModel {
+  [_FIELD_NAME = FIELD_NAME];
+}
+babelHelpers.decorate([dec], MyModel.prototype, _FIELD_NAME, void 0);