Add ui test to reproduce non-Send panic temporary

Author: dtolnay

tests/ui/macros/panic-temporaries.rs

@@ -0,0 +1,19 @@
+// check-fail
+// edition:2021
+
+#![allow(unreachable_code)]
+
+async fn f(_: u8) {}
+
+async fn g() {
+    // Todo returns `!`, so the await is never reached, and in particular the
+    // temporaries inside the formatting machinery are not still alive at the
+    // await point.
+    f(todo!("...")).await;
+}
+
+fn require_send(_: impl Send) {}
+
+fn main() {
+    require_send(g()); //~ future cannot be sent between threads safely
+}

tests/ui/macros/panic-temporaries.stderr

@@ -0,0 +1,27 @@
+error: future cannot be sent between threads safely
+  --> $DIR/panic-temporaries.rs:18:18
+   |
+LL |     require_send(g());
+   |                  ^^^ future returned by `g` is not `Send`
+   |
+   = help: the trait `Sync` is not implemented for `core::fmt::Opaque`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/panic-temporaries.rs:12:20
+   |
+LL |     f(todo!("...")).await;
+   |       ------------ ^^^^^^ await occurs here, with `$crate::format_args!($($arg)+)` maybe used later
+   |       |
+   |       has type `ArgumentV1<'_>` which is not `Send`
+note: `$crate::format_args!($($arg)+)` is later dropped here
+  --> $DIR/panic-temporaries.rs:12:26
+   |
+LL |     f(todo!("...")).await;
+   |                          ^
+note: required by a bound in `require_send`
+  --> $DIR/panic-temporaries.rs:15:25
+   |
+LL | fn require_send(_: impl Send) {}
+   |                         ^^^^ required by this bound in `require_send`
+
+error: aborting due to previous error
+