Test async fn

Author: Nadrieril

tests/ui/rfcs/rfc-0000-never_patterns/120240-async-fn-never-arg.rs

@@ -0,0 +1,16 @@
+// edition: 2018
+// known-bug: #120240
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+fn main() {}
+
+enum Void {}
+
+// Divergence is not detected.
+async fn async_never(!: Void) -> ! {} // gives an error
+
+// Divergence is detected
+async fn async_let(x: Void) -> ! {
+    let ! = x;
+}

tests/ui/rfcs/rfc-0000-never_patterns/120240-async-fn-never-arg.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/120240-async-fn-never-arg.rs:11:36
+   |
+LL | async fn async_never(!: Void) -> ! {} // gives an error
+   |                                    ^^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/rfcs/rfc-0000-never_patterns/diverges.rs

@@ -1,4 +1,5 @@
 // check-pass
+// edition: 2018
 #![feature(never_patterns)]
 #![allow(incomplete_features)]
 #![deny(unreachable_patterns)]
@@ -30,3 +31,8 @@ fn never_match() -> ! {
     // Ensures this typechecks because of divergence and not the type of the match expression.
     println!();
 }
+
+// Note: divergence is not detected for async fns when the `!` is in the argument (#120240).
+async fn async_let(x: Void) -> ! {
+    let ! = x;
+}