Note closure captures when reporting deferred cast to fn ptr failed

compiler/rustc_hir_typeck/src/cast.rs

@@ -495,6 +495,7 @@ fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
                     err.span_label(self.span, "invalid cast");
                 }
 
+                fcx.suggest_no_capture_closure(&mut err, self.cast_ty, self.expr_ty);
                 self.try_suggest_collection_to_bool(fcx, &mut err);
 
                 err.emit();

tests/ui/closures/closure-no-fn-3.stderr

@@ -3,6 +3,12 @@ error[E0605]: non-primitive cast: `{closure@$DIR/closure-no-fn-3.rs:6:28: 6:30}`
    |
 LL |     let baz: fn() -> u8 = (|| { b }) as fn() -> u8;
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^ invalid cast
+   |
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> $DIR/closure-no-fn-3.rs:6:33
+   |
+LL |     let baz: fn() -> u8 = (|| { b }) as fn() -> u8;
+   |                                 ^ `b` captured here
 
 error: aborting due to 1 previous error