Add feature gate note and test

Author: camelid

compiler/rustc_ast_passes/messages.ftl

@@ -218,9 +218,6 @@ ast_passes_impl_fn_const =
 ast_passes_incompatible_features = `{$f1}` and `{$f2}` are incompatible, using them at the same time is not allowed
     .help = remove one of these features
 
-ast_passes_missing_dependent_features = `{$parent}` requires {$missing} to be enabled
-    .help = enable all of these features
-
 ast_passes_item_invalid_safety = items outside of `unsafe extern {"{ }"}` cannot be declared with `safe` safety qualifier
     .suggestion = remove safe from this item
 
@@ -231,6 +228,9 @@ ast_passes_match_arm_with_no_body =
     `match` arm with no body
     .suggestion = add a body after the pattern
 
+ast_passes_missing_dependent_features = `{$parent}` requires {$missing} to be enabled
+    .help = enable all of these features
+
 ast_passes_missing_unsafe_on_extern = extern blocks must be unsafe
     .suggestion = needs `unsafe` before the extern keyword
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -433,6 +433,9 @@ impl<'tcx> ForbidMCGParamUsesFolder<'tcx> {
                 diag.span_note(impl_.self_ty.span, "not a concrete type");
             }
         }
+        if !self.tcx.features().opaque_generic_const_args() {
+            diag.help("add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items");
+        }
         diag.emit()
     }
 }

tests/ui/feature-gates/feature-gate-opaque-generic-const-args.rs

@@ -0,0 +1,9 @@
+#![feature(generic_const_items, min_generic_const_args)]
+#![expect(incomplete_features)]
+
+#[type_const]
+const INC<const N: usize>: usize = const { N + 1 };
+//~^ ERROR generic parameters may not be used in const operations
+//~| HELP add `#![feature(opaque_generic_const_args)]`
+
+fn main() {}

tests/ui/feature-gates/feature-gate-opaque-generic-const-args.stderr

@@ -0,0 +1,10 @@
+error: generic parameters may not be used in const operations
+  --> $DIR/feature-gate-opaque-generic-const-args.rs:5:44
+   |
+LL | const INC<const N: usize>: usize = const { N + 1 };
+   |                                            ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
+
+error: aborting due to 1 previous error
+