give better error messages when a cycle arises

src/test/ui/span/coerce-suggestions.stderr

@@ -43,9 +43,6 @@ error[E0308]: mismatched types
    |
 41 |     f = box f;
    |         ^^^^^ cyclic type of infinite size
-   |
-   = note: expected type `_`
-              found type `std::boxed::Box<_>`
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:48:9

src/test/ui/unboxed-closure-no-cyclic-sig.rs

@@ -8,8 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Test that unboxed closures cannot capture their own type.
+//
+// Also regression test for issue #21410.
+
 fn g<F>(_: F) where F: FnOnce(Option<F>) {}
 
 fn main() {
-    g(|_| {  }); //~ ERROR mismatched types
+    g(|_| {  });
 }

src/test/ui/unboxed-closure-no-cyclic-sig.stderr

@@ -0,0 +1,12 @@
+error[E0644]: closure/generator type that references itself
+  --> $DIR/unboxed-closure-no-cyclic-sig.rs:18:7
+   |
+18 |     g(|_| {  });
+   |       ^^^^^^^^ cyclic type of infinite size
+   |
+   = note: closures cannot capture themselves or take themselves as argument;
+           this error may be the result of a recent compiler bug-fix,
+           see https://github.com/rust-lang/rust/issues/46062 for more details
+
+error: aborting due to previous error
+