Add note about wrapping in braces

Previously it was not clear why this errored or if it was even supported, as there was no
diagnostic that suggested wrapping it in braces.

Thus, add a simple diagnostic that suggests wrapping enum variants in braces.

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1199,17 +1199,26 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     (_, _) => {
                         let got = if let Some(_) = term.ty() { "type" } else { "constant" };
                         let expected = def_kind.descr(assoc_item_def_id);
-                        let reported = tcx
-                            .sess
-                            .struct_span_err(
+                        let mut err = tcx.sess.struct_span_err(
+                            binding.span,
+                            &format!("expected {expected} bound, found {got}"),
+                        );
+                        err.span_note(
+                            tcx.def_span(assoc_item_def_id),
+                            &format!("{expected} defined here"),
+                        );
+
+                        if let hir::def::DefKind::AssocConst = def_kind
+                          && let Some(t) = term.ty() && (t.is_enum() || t.references_error())
+                          && tcx.features().associated_const_equality {
+                            err.span_suggestion(
                                 binding.span,
-                                &format!("expected {expected} bound, found {got}"),
-                            )
-                            .span_note(
-                                tcx.def_span(assoc_item_def_id),
-                                &format!("{expected} defined here"),
-                            )
-                            .emit();
+                                "if equating a const, try wrapping with braces",
+                                format!("{} = {{ const }}", binding.item_name),
+                                Applicability::HasPlaceholders,
+                            );
+                        }
+                        let reported = err.emit();
                         term = match def_kind {
                             hir::def::DefKind::AssocTy => {
                                 tcx.ty_error_with_guaranteed(reported).into()

src/test/ui/const-generics/assoc_const_eq_diagnostic.rs

@@ -0,0 +1,18 @@
+#![feature(associated_const_equality)]
+
+pub enum Mode {
+    Cool,
+}
+
+pub trait Parse {
+    const MODE: Mode;
+}
+
+pub trait CoolStuff: Parse<MODE = Mode::Cool> {}
+//~^ ERROR expected associated constant bound
+//~| ERROR expected type
+
+fn no_help() -> Mode::Cool {}
+//~^ ERROR expected type, found variant
+
+fn main() {}

src/test/ui/const-generics/assoc_const_eq_diagnostic.stderr

@@ -0,0 +1,33 @@
+error[E0573]: expected type, found variant `Mode::Cool`
+  --> $DIR/assoc_const_eq_diagnostic.rs:11:35
+   |
+LL | pub trait CoolStuff: Parse<MODE = Mode::Cool> {}
+   |                                   ^^^^^^^^^^
+   |                                   |
+   |                                   not a type
+   |                                   help: try using the variant's enum: `Mode`
+
+error[E0573]: expected type, found variant `Mode::Cool`
+  --> $DIR/assoc_const_eq_diagnostic.rs:15:17
+   |
+LL | fn no_help() -> Mode::Cool {}
+   |                 ^^^^^^^^^^
+   |                 |
+   |                 not a type
+   |                 help: try using the variant's enum: `Mode`
+
+error: expected associated constant bound, found type
+  --> $DIR/assoc_const_eq_diagnostic.rs:11:28
+   |
+LL | pub trait CoolStuff: Parse<MODE = Mode::Cool> {}
+   |                            ^^^^^^^^^^^^^^^^^ help: if equating a const, try wrapping with braces: `MODE = { const }`
+   |
+note: associated constant defined here
+  --> $DIR/assoc_const_eq_diagnostic.rs:8:5
+   |
+LL |     const MODE: Mode;
+   |     ^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0573`.