compiler: do not write const _: () backwards

Co-authored-by: Christopher Serr <christopher.serr@gmail.com>

Author: workingjubilee

compiler/rustc_parse/src/parser/item.rs

@@ -88,7 +88,7 @@ impl<'a> Parser<'a> {
                 } else if self.parse_const_block(span, false).is_ok() {
                     err.span_label(
                         span,
-                        "if you meant to create an anonymous const, use `const (): _ = {};` instead"
+                        "if you meant to create an anonymous const, use `const _: () = {};` instead"
                     );
                 } else {
                     err.span_label(span, "expected item")

tests/ui/inline-const/expr-in-item-context.rs

@@ -8,7 +8,7 @@ const _: () = {};
 
 const { assert!(size_of::<u32>() <= size_of::<usize>()) };
 //~^ expected item, found keyword
-//~| if you meant to create an anonymous const
+//~| if you meant to create an anonymous const, use `const _: () =
 
 fn main() {
     const { assert!(size_of::<u32>() <= size_of::<usize>()) };

tests/ui/inline-const/expr-in-item-context.stderr

@@ -2,7 +2,7 @@ error: expected item, found keyword `const`
   --> $DIR/expr-in-item-context.rs:9:1
    |
 LL | const { assert!(size_of::<u32>() <= size_of::<usize>()) };
-   | ^^^^^ if you meant to create an anonymous const, use `const (): _ = {};` instead
+   | ^^^^^ if you meant to create an anonymous const, use `const _: () = {};` instead
 
 error: aborting due to 1 previous error