add test for rust-lang#123420

Author: matthiaskrgr

tests/ui/async-await/lifetime-name-annotation-2021-ice-123420.rs

@@ -0,0 +1,18 @@
+// issue rust-lang/rust#123420
+// ICE assertion failed: i32 != &'{erased} mut i32
+//@ edition:2021
+#![feature(async_closure)]
+use std::future::Future;
+
+pub fn async_closure_explicit_return_type(x: &mut i32) -> impl Future {
+    (async move || -> &i32 {
+    //~^ ERROR lifetime may not live long enough
+    //~| ERROR hidden type for `impl Future` captures lifetime that does not appear in bounds
+        let y = &*x;
+        *x += 1;
+        //~^ ERROR cannot assign to `*x` because it is borrowed
+        y
+    })()
+}
+
+pub fn main() {}

tests/ui/async-await/lifetime-name-annotation-2021-ice-123420.stderr

@@ -0,0 +1,56 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/lifetime-name-annotation-2021-ice-123420.rs:12:9
+   |
+LL |         let y = &*x;
+   |                 --- `*x` is borrowed here
+LL |         *x += 1;
+   |         ^^^^^^^ `*x` is assigned to here but it was already borrowed
+LL |
+LL |         y
+   |         - returning this value requires that `*x` is borrowed for `'1`
+LL |     })()
+   |     - return type of async closure is &'1 i32
+
+error: lifetime may not live long enough
+  --> $DIR/lifetime-name-annotation-2021-ice-123420.rs:8:28
+   |
+LL |       (async move || -> &i32 {
+   |  ______---------------------_^
+   | |      |                |
+   | |      |                return type of async closure `{async closure body@$DIR/lifetime-name-annotation-2021-ice-123420.rs:8:28: 15:6}` contains a lifetime `'2`
+   | |      lifetime `'1` represents this closure's body
+LL | |
+LL | |
+LL | |         let y = &*x;
+...  |
+LL | |         y
+LL | |     })()
+   | |_____^ returning this value requires that `'1` must outlive `'2`
+   |
+   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+
+error[E0700]: hidden type for `impl Future` captures lifetime that does not appear in bounds
+  --> $DIR/lifetime-name-annotation-2021-ice-123420.rs:8:5
+   |
+LL |   pub fn async_closure_explicit_return_type(x: &mut i32) -> impl Future {
+   |                                                --------     ----------- opaque type defined here
+   |                                                |
+   |                                                hidden type `{async closure body@$DIR/lifetime-name-annotation-2021-ice-123420.rs:8:28: 15:6}` captures the anonymous lifetime defined here
+LL | /     (async move || -> &i32 {
+LL | |
+LL | |
+LL | |         let y = &*x;
+...  |
+LL | |         y
+LL | |     })()
+   | |________^
+   |
+help: to declare that `impl Future` captures `'_`, you can add an explicit `'_` lifetime bound
+   |
+LL | pub fn async_closure_explicit_return_type(x: &mut i32) -> impl Future + '_ {
+   |                                                                       ++++
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0506, E0700.
+For more information about an error, try `rustc --explain E0506`.