compiler: suggest const _ for a misplaced const {}

compiler/rustc_parse/src/parser/item.rs

@@ -86,9 +86,24 @@ impl<'a> Parser<'a> {
                         "consider using `const` or `static` instead of `let` for global variables",
                     );
                 } 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>");
-                };
+                    let snapshot = self.create_snapshot_for_diagnostic();
+                    match self.parse_const_block(span, false) {
+                        Ok(_) => {
+                            err.span_suggestion_verbose(
+                                span.shrink_to_lo(),
+                                "to evaluate a const expression, use an anonymous const",
+                                "const _: () = ",
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                        Err(diag) => {
+                            diag.cancel();
+                            self.restore_snapshot(snapshot);
+                            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>");
+                        }
+                    }
+                }
                 return Err(err);
             }
         }

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>()) };
+//~^ expected item, found keyword
+//~| to evaluate a const expression, use an anonymous const
+
+fn main() {
+    const { assert!(size_of::<u32>() <= size_of::<usize>()) };
+}

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

@@ -0,0 +1,13 @@
+error: expected item, found keyword `const`
+  --> $DIR/expr-in-item-context.rs:9:1
+   |
+LL | const { assert!(size_of::<u32>() <= size_of::<usize>()) };
+   | ^^^^^
+   |
+help: to evaluate a const expression, use an anonymous const
+   |
+LL | const _: () = const { assert!(size_of::<u32>() <= size_of::<usize>()) };
+   | +++++++++++++
+
+error: aborting due to 1 previous error
+