Allow usage of this and arguments in nested function expression (#…

…74179)

In #73059 we added build-time checks for the forbidden usage of `this`
and `arguments` in server functions. A nested function expression is
however allowed to use these expressions.

fixes #74181

To add a bit more context: `this` is forbidden in server actions because
the Next.js compiler hoists inline server actions out of their original
location into the module scope, so that they can be imported and invoked
by the action handler. Due to this hoisting, the `this` context gets
lost and is not available to the server action.

To prevent surprising runtime errors for such cases, we emit a build
error to provide feedback to developers as early as possible.

However, nested function declarations or function expressions create a
new `this` context, and in those it's allowed to access `this` and
`arguments`.

For consistency, and to prevent surprises when refactoring server
actions, we apply the same rules for module-level server actions.

crates/next-custom-transforms/src/transforms/server_actions.rs

@@ -932,11 +932,13 @@ impl<C: Comments> VisitMut for ServerActions<C> {
     }
 
     fn visit_mut_fn_expr(&mut self, f: &mut FnExpr) {
+        let old_this_status = replace(&mut self.this_status, ThisStatus::Allowed);
         let old_arrow_or_fn_expr_ident = self.arrow_or_fn_expr_ident.clone();
         if let Some(ident) = &f.ident {
             self.arrow_or_fn_expr_ident = Some(ident.clone());
         }
         f.visit_mut_children_with(self);
+        self.this_status = old_this_status;
         self.arrow_or_fn_expr_ident = old_arrow_or_fn_expr_ident;
     }
 
@@ -1264,6 +1266,12 @@ impl<C: Comments> VisitMut for ServerActions<C> {
         self.in_exported_expr = old_in_exported_expr;
     }
 
+    fn visit_mut_class(&mut self, n: &mut Class) {
+        let old_this_status = replace(&mut self.this_status, ThisStatus::Allowed);
+        n.visit_mut_children_with(self);
+        self.this_status = old_this_status;
+    }
+
     fn visit_mut_class_member(&mut self, n: &mut ClassMember) {
         if let ClassMember::Method(ClassMethod {
             is_abstract: false,

crates/next-custom-transforms/tests/fixture/server-actions/server/59/input.js

@@ -0,0 +1,41 @@
+'use server'
+
+import { db } from './database'
+
+export const createItem = async (title) => {
+  return new Promise((resolve, reject) => {
+    db.serialize(() => {
+      db.run(
+        `INSERT INTO items (title) VALUES ($title)`,
+        { $title: title },
+        function () {
+          // arguments is allowed here
+          const [err] = arguments
+
+          if (err) {
+            reject(err)
+          }
+
+          // this is allowed here
+          resolve(this.lastID)
+        }
+      )
+    })
+  })
+}
+
+export async function test() {
+  const MyClass = class {
+    x = 1
+    foo() {
+      // this is allowed here
+      return this.x
+    }
+    bar = () => {
+      // this is allowed here
+      return this.x
+    }
+  }
+  const myObj = new MyClass()
+  return myObj.foo() + myObj.bar()
+}