fix(transformer/object-rest-spread): arrow function expressions with object rest patterns (#12185)

close: #12184

I couldn't figure out where **exactly** to put tests regarding object
rest spread and arrow function transformation.

Probably somewhere underneath tasks/transform_conformance

Author: cod1r

crates/oxc_transformer/src/es2018/object_rest_spread.rs

@@ -561,22 +561,33 @@ impl<'a> ObjectRestSpread<'a, '_> {
     // Transform `(...x) => {}`.
     fn transform_arrow(arrow: &mut ArrowFunctionExpression<'a>, ctx: &mut TraverseCtx<'a>) {
         let scope_id = arrow.scope_id();
-        let mut replaced = false;
         for param in &mut arrow.params.items {
             if Self::has_nested_object_rest(&param.pattern) {
+                // `({ ...args }) => { args }`
+                if arrow.expression {
+                    arrow.expression = false;
+
+                    debug_assert!(arrow.body.statements.len() == 1);
+
+                    let Statement::ExpressionStatement(stmt) = arrow.body.statements.pop().unwrap()
+                    else {
+                        unreachable!(
+                            "`arrow.expression` is true, which means it has only one ExpressionStatement."
+                        );
+                    };
+                    let return_stmt =
+                        ctx.ast.statement_return(stmt.span, Some(stmt.unbox().expression));
+                    arrow.body.statements.push(return_stmt);
+                }
                 Self::replace_rest_element(
                     VariableDeclarationKind::Var,
                     &mut param.pattern,
                     &mut arrow.body.statements,
                     scope_id,
                     ctx,
                 );
-                replaced = true;
             }
         }
-        if replaced && arrow.expression {
-            arrow.expression = false;
-        }
     }
 
     // Transform `try {} catch ({...x}) {}`.

tasks/transform_conformance/snapshots/oxc.snap.md

@@ -1,6 +1,6 @@
 commit: 1d4546bc
 
-Passed: 178/296
+Passed: 179/297
 
 # All Passed:
 * babel-plugin-transform-class-static-block

tasks/transform_conformance/tests/babel-plugin-transform-object-rest-spread/test/fixtures/object-rest/with-arrow-function-expression/input.js

@@ -0,0 +1 @@
+const fn = ({ ...args }) => args;

tasks/transform_conformance/tests/babel-plugin-transform-object-rest-spread/test/fixtures/object-rest/with-arrow-function-expression/output.js

@@ -0,0 +1,7 @@
+const fn = (_ref) => {
+  let args = babelHelpers.extends(
+    {},
+    (babelHelpers.objectDestructuringEmpty(_ref), _ref),
+  );
+  return args;
+};