fix(es/compat): Prepend generated _loop to the correct scope (#6604)

crates/swc_ecma_transforms_compat/src/es2015/block_scoping/mod.rs

@@ -361,6 +361,44 @@ impl BlockScoping {
             *body = Box::new(call.take().into_stmt());
         }
     }
+
+    /// This method will turn stmt like
+    /// ```js
+    /// for (let i in [1, 2])
+    ///   for (let j in [1, 2])
+    ///     console.log(i, j)
+    /// ```
+    /// into
+    /// ```js
+    /// for (let i in [1, 2]) {
+    ///   for (let j in [1, 2]) {
+    ///     console.log(i, j)
+    ///   }
+    /// }
+    /// ```
+    /// which fixes https://github.com/swc-project/swc/issues/6573
+    fn blockify_for_stmt_body(&self, body: &mut Box<Stmt>) -> bool {
+        if !body.is_block() {
+            *body = Box::new(Stmt::Block(BlockStmt {
+                span: Default::default(),
+                stmts: vec![*body.take()],
+            }));
+            true
+        } else {
+            false
+        }
+    }
+
+    fn undo_blockify_for_stmt_body(&self, body: &mut Box<Stmt>, blockifyed: bool) {
+        if blockifyed {
+            let stmt = body
+                .as_mut_block()
+                .and_then(|block| (block.stmts.len() == 1).then(|| block.stmts[0].take()));
+            if let Some(stmt) = stmt {
+                *body = Box::new(stmt)
+            }
+        }
+    }
 }
 
 #[swc_trace]
@@ -400,6 +438,7 @@ impl VisitMut for BlockScoping {
     }
 
     fn visit_mut_for_in_stmt(&mut self, node: &mut ForInStmt) {
+        let blockifyed = self.blockify_for_stmt_body(&mut node.body);
         let lexical_var = if let VarDeclOrPat::VarDecl(decl) = &node.left {
             find_lexical_vars(decl)
         } else {
@@ -420,9 +459,11 @@ impl VisitMut for BlockScoping {
 
         self.visit_mut_with_scope(kind, &mut node.body);
         self.handle_capture_of_vars(&mut node.body);
+        self.undo_blockify_for_stmt_body(&mut node.body, blockifyed);
     }
 
     fn visit_mut_for_of_stmt(&mut self, node: &mut ForOfStmt) {
+        let blockifyed = self.blockify_for_stmt_body(&mut node.body);
         let vars = if let VarDeclOrPat::VarDecl(decl) = &node.left {
             find_lexical_vars(decl)
         } else {
@@ -444,9 +485,11 @@ impl VisitMut for BlockScoping {
 
         self.visit_mut_with_scope(kind, &mut node.body);
         self.handle_capture_of_vars(&mut node.body);
+        self.undo_blockify_for_stmt_body(&mut node.body, blockifyed);
     }
 
     fn visit_mut_for_stmt(&mut self, node: &mut ForStmt) {
+        let blockifyed = self.blockify_for_stmt_body(&mut node.body);
         let lexical_var = if let Some(VarDeclOrExpr::VarDecl(decl)) = &node.init {
             find_lexical_vars(decl)
         } else {
@@ -467,6 +510,7 @@ impl VisitMut for BlockScoping {
         };
         self.visit_mut_with_scope(kind, &mut node.body);
         self.handle_capture_of_vars(&mut node.body);
+        self.undo_blockify_for_stmt_body(&mut node.body, blockifyed);
     }
 
     fn visit_mut_function(&mut self, f: &mut Function) {

crates/swc_ecma_transforms_compat/tests/block-scoping/issue-6573/1/input.js

@@ -0,0 +1,6 @@
+let funcs = [];
+for (let i = 0; i < 2; i++)
+    for (let j = 0; j < 2; j++)
+        funcs.push(() => console.log(i, j));
+
+funcs.forEach((f) => f());

crates/swc_ecma_transforms_compat/tests/block-scoping/issue-6573/1/output.js

@@ -0,0 +1,9 @@
+var _loop__7 = function(i__2) {
+    var _loop__6 = function(j__3) {
+        funcs__1.push(()=>console.log(i__2, j__3));
+    };
+    for(var j__3 = 0; j__3 < 2; j__3++)_loop__6(j__3);
+};
+var funcs__1 = [];
+for(var i__2 = 0; i__2 < 2; i__2++)_loop__7(i__2);
+funcs__1.forEach((f__5)=>f__5());

crates/swc_ecma_transforms_compat/tests/block-scoping/issue-6573/2/input.js

@@ -0,0 +1,6 @@
+let funcs = [];
+for (let i in [1, 2])
+    for (let j in [1, 2])
+        funcs.push(() => console.log(i, j));
+
+funcs.forEach((f) => f());

crates/swc_ecma_transforms_compat/tests/block-scoping/issue-6573/2/output.js

@@ -0,0 +1,15 @@
+var _loop__7 = function(i__2) {
+    var _loop__6 = function(j__3) {
+        funcs__1.push(()=>console.log(i__2, j__3));
+    };
+    for(var j__3 in [
+        1,
+        2
+    ])_loop__6(j__3);
+};
+var funcs__1 = [];
+for(var i__2 in [
+    1,
+    2
+])_loop__7(i__2);
+funcs__1.forEach((f__5)=>f__5());

crates/swc_ecma_transforms_compat/tests/block-scoping/issue-6573/3/input.js

@@ -0,0 +1,6 @@
+let funcs = [];
+for (let i of [1, 2])
+    for (let j of [1, 2])
+        funcs.push(() => console.log(i, j));
+
+funcs.forEach((f) => f());

crates/swc_ecma_transforms_compat/tests/block-scoping/issue-6573/3/output.js

@@ -0,0 +1,15 @@
+var _loop__7 = function(i__2) {
+    var _loop__6 = function(j__3) {
+        funcs__1.push(()=>console.log(i__2, j__3));
+    };
+    for (var j__3 of [
+        1,
+        2
+    ])_loop__6(j__3);
+};
+var funcs__1 = [];
+for (var i__2 of [
+    1,
+    2
+])_loop__7(i__2);
+funcs__1.forEach((f__5)=>f__5());