improve test for issue 89960

Author: cormacrelf

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

@@ -1,7 +1,9 @@
+// This had an ICE, see issue #89960
+
 #![feature(let_else)]
+#![deny(unused_variables)]
 
 fn main() {
-    // FIXME: more precise diagnostics
     let Some(ref mut meow) = Some(()) else { return };
-    //~^ ERROR: cannot borrow value as mutable, as `val` is not declared as mutable
+    //~^ ERROR unused variable: `meow`
 }

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

@@ -1,12 +1,14 @@
-error[E0596]: cannot borrow value as mutable, as `val` is not declared as mutable
-  --> $DIR/issue-89960.rs:5:14
+error: unused variable: `meow`
+  --> $DIR/issue-89960.rs:7:22
    |
 LL |     let Some(ref mut meow) = Some(()) else { return };
-   |     ---------^^^^^^^^^^^^-----------------------------
-   |     |        |
-   |     |        cannot borrow as mutable
-   |     help: consider changing this to be mutable: `mut val`
+   |                      ^^^^ 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
 
-For more information about this error, try `rustc --explain E0596`.