Auto merge of #115696 - RalfJung:closure-ty-print, r=oli-obk

adjust how closure/generator types are printed

I saw `&[closure@$DIR/issue-20862.rs:2:5]` and I thought it is a slice type, because that's usually what `&[_]` is... it took me a while to realize that this is just a confusing printer and actually there's no slice. Let's use something that cannot be mistaken for a regular type.

tests/ui/box_default.fixed

@@ -36,7 +36,7 @@ fn main() {
     issue_10381();
 
     // `Box::<Option<_>>::default()` would be valid here, but not `Box::default()` or
-    // `Box::<Option<[closure@...]>::default()`
+    // `Box::<Option<{closure@...}>::default()`
     //
     // Would have a suggestion after https://github.com/rust-lang/rust/blob/fdd030127cc68afec44a8d3f6341525dd34e50ae/compiler/rustc_middle/src/ty/diagnostics.rs#L554-L563
     let mut unnameable = Box::new(Option::default());

tests/ui/box_default.rs

@@ -36,7 +36,7 @@ fn main() {
     issue_10381();
 
     // `Box::<Option<_>>::default()` would be valid here, but not `Box::default()` or
-    // `Box::<Option<[closure@...]>::default()`
+    // `Box::<Option<{closure@...}>::default()`
     //
     // Would have a suggestion after https://github.com/rust-lang/rust/blob/fdd030127cc68afec44a8d3f6341525dd34e50ae/compiler/rustc_middle/src/ty/diagnostics.rs#L554-L563
     let mut unnameable = Box::new(Option::default());

tests/ui/crashes/ice-6251.stderr

@@ -27,7 +27,7 @@ LL | fn bug<T>() -> impl Iterator<Item = [(); { |x: [u8]| x }]> {
    |                                            ^^^^^^^^^^^ expected `usize`, found closure
    |
    = note: expected type `usize`
-           found closure `[closure@$DIR/ice-6251.rs:4:44: 4:53]`
+           found closure `{closure@$DIR/ice-6251.rs:4:44: 4:53}`
 
 error: aborting due to 3 previous errors