Better test unsized_fn_params

Author: spastorino

src/test/ui/unsized-locals/unsized-index.rs

@@ -1,20 +1,16 @@
-// build-pass (FIXME(62277): could be check-pass?)
+// run-pass
 
-// `std::ops::Index` has an `: ?Sized` bound on the `Idx` type param. This is
-// an accidental left-over from the times when it `Index` was by-reference.
-// Tightening the bound now could be a breaking change. Although no crater
-// regression were observed (https://github.com/rust-lang/rust/pull/59527),
-// let's be conservative and just add a test for this.
 #![feature(unsized_fn_params)]
 
 use std::ops;
+use std::ops::Index;
 
 pub struct A;
 
 impl ops::Index<str> for A {
     type Output = ();
     fn index(&self, _: str) -> &Self::Output {
-        panic!()
+        &()
     }
 }
 
@@ -24,4 +20,8 @@ impl ops::IndexMut<str> for A {
     }
 }
 
-fn main() {}
+fn main() {
+    let a = A {};
+    let s = String::new().into_boxed_str();
+    assert_eq!(&(), a.index(*s));
+}