add test for issue 89960

Author: cormacrelf

src/test/ui/let-else/issue-89960.rs

@@ -0,0 +1,9 @@
+// This had an ICE, see issue #89960
+
+#![feature(let_else)]
+#![deny(unused_variables)]
+
+fn main() {
+    let Some(ref mut meow) = Some(()) else { return };
+    //~^ ERROR unused variable: `meow`
+}

src/test/ui/let-else/issue-89960.stderr

@@ -0,0 +1,14 @@
+error: unused variable: `meow`
+  --> $DIR/issue-89960.rs:7:22
+   |
+LL |     let Some(ref mut meow) = Some(()) else { return };
+   |                      ^^^^ help: if this is intentional, prefix it with an underscore: `_meow`
+   |
+note: the lint level is defined here
+  --> $DIR/issue-89960.rs:4:9
+   |
+LL | #![deny(unused_variables)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+