Fix UI test

fmt::Formatter is still not Send/Sync, but the UI test emitted two errors, for
the dyn Write and the Void inside Formatter. As of this PR, the Void is now
gone, but the dyn Write remains.

Author: Mark-Simulacrum

src/test/ui/async-await/async-fn-nonsend.rs

@@ -2,15 +2,15 @@
 // edition:2018
 // compile-flags: --crate-type lib
 
-use std::{
-    cell::RefCell,
-    fmt::Debug,
-    rc::Rc,
-};
+use std::{cell::RefCell, fmt::Debug, rc::Rc};
 
-fn non_sync() -> impl Debug { RefCell::new(()) }
+fn non_sync() -> impl Debug {
+    RefCell::new(())
+}
 
-fn non_send() -> impl Debug { Rc::new(()) }
+fn non_send() -> impl Debug {
+    Rc::new(())
+}
 
 fn take_ref<T>(_: &T) {}
 
@@ -53,5 +53,4 @@ pub fn pass_assert() {
     //~^ ERROR future cannot be sent between threads safely
     assert_send(non_sync_with_method_call());
     //~^ ERROR future cannot be sent between threads safely
-    //~^^ ERROR future cannot be sent between threads safely
 }

src/test/ui/async-await/async-fn-nonsend.stderr

@@ -62,27 +62,5 @@ LL |     }
 LL | }
    | - `f` is later dropped here
 
-error: future cannot be sent between threads safely
-  --> $DIR/async-fn-nonsend.rs:54:5
-   |
-LL | fn assert_send(_: impl Send) {}
-   |    -----------         ---- required by this bound in `assert_send`
-...
-LL |     assert_send(non_sync_with_method_call());
-   |     ^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
-   |
-   = help: within `std::fmt::ArgumentV1<'_>`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
-note: future is not `Send` as this value is used across an await
-  --> $DIR/async-fn-nonsend.rs:43:9
-   |
-LL |     let f: &mut std::fmt::Formatter = panic!();
-   |         - has type `&mut std::fmt::Formatter<'_>`
-LL |     if non_sync().fmt(f).unwrap() == () {
-LL |         fut().await;
-   |         ^^^^^^^^^^^ await occurs here, with `f` maybe used later
-LL |     }
-LL | }
-   | - `f` is later dropped here
-
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors