add test for closure precedence in TokenStreams

tests/ui/proc-macro/identity-closure-preserving.rs

@@ -0,0 +1,22 @@
+//! Make sure that the closure still gets the correct precedence when round-tripping
+//! through a proc macro.
+//! The correct precendence is `(|| ()) as fn()`, even though these parentheses are not
+//! directly part of the code.
+//! If it would get lost, the code would be `|| () as fn()`, get parsed as
+//! `|| (() as fn())` and fail to compile.
+//! Notably, this will also fail to compile if we use `recollect` instead of `identity`.
+//! Regression test for https://github.com/rust-lang/rust/pull/151830#issuecomment-4010899019.
+//@ proc-macro: test-macros.rs
+//@ check-pass
+
+macro_rules! operator_impl {
+    ($target_expr:expr) => {
+        test_macros::identity! {
+            $target_expr as fn()
+        };
+    };
+}
+
+fn main() {
+    operator_impl!(|| ());
+}