tests: overly-terse inline-const-as-item error

Author: workingjubilee

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

@@ -0,0 +1,15 @@
+// reported in https://github.com/rust-lang/rust/issues/128338
+
+// This had an overly-terse diagnostic, because we recognized this is where an item should be, but
+// but didn't recognize an inline const in this place is probably trying to be an "anon const item".
+// Those are written like this:
+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
+
+fn main() {
+    const { assert!(size_of::<u32>() <= size_of::<usize>()) };
+}

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

@@ -0,0 +1,10 @@
+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>
+
+error: aborting due to 1 previous error
+