compiler: diagnose anon const items written as exprs

Author: workingjubilee

compiler/rustc_parse/src/parser/item.rs

@@ -85,6 +85,11 @@ impl<'a> Parser<'a> {
                         span,
                         "consider using `const` or `static` instead of `let` for global variables",
                     );
+                } 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"
+                    );
                 } else {
                     err.span_label(span, "expected item")
                         .note("for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>");

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

@@ -7,8 +7,8 @@ const _: () = {};
 // const _ are often used as compile-time assertions that don't conflict with other const items
 
 const { assert!(size_of::<u32>() <= size_of::<usize>()) };
-//~^ error: expected item
-//~| for a full list of items
+//~^ expected item, found keyword
+//~| if you meant to create an anonymous const
 
 fn main() {
     const { assert!(size_of::<u32>() <= size_of::<usize>()) };

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

@@ -2,9 +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>()) };
-   | ^^^^^ expected item
-   |
-   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+   | ^^^^^ if you meant to create an anonymous const, use `const (): _ = {};` instead
 
 error: aborting due to 1 previous error