Add feature gate note and test

camelid · camelid · commit 7dc3a0c8c1bf · 2026-01-23T21:15:27.000-08:00
diff --git a/compiler/rustc_ast_passes/messages.ftl b/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
 
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -405,6 +405,11 @@ impl<'tcx> ForbidMCGParamUsesFolder<'tcx> {
                 diag.span_note(impl_.self_ty.span, "not a concrete type");
             }
         }
+        if self.tcx.features().min_generic_const_args()
+            && !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()
     }
 }
diff --git a/tests/ui/const-generics/associated-const-bindings/ambiguity.stderr b/tests/ui/const-generics/associated-const-bindings/ambiguity.stderr
@@ -1,3 +1,11 @@
+error: `unsized_const_params` requires `adt_const_params` to be enabled
+  --> $DIR/ambiguity.rs:4:36
+   |
+LL | #![feature(min_generic_const_args, unsized_const_params)]
+   |                                    ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: enable all of these features
+
 error[E0222]: ambiguous associated constant `K` in bounds of `Trait0`
   --> $DIR/ambiguity.rs:13:25
    |
@@ -34,6 +42,6 @@ LL | fn take1(_: impl Trait1<C = "?">) {}
                    T: Parent2::C = "?",
                    T: Parent1::C = "?"
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0222`.
diff --git a/tests/ui/const-generics/associated-const-bindings/esc-bound-var-in-ty.stderr b/tests/ui/const-generics/associated-const-bindings/esc-bound-var-in-ty.stderr
@@ -1,3 +1,11 @@
+error: `unsized_const_params` requires `adt_const_params` to be enabled
+  --> $DIR/esc-bound-var-in-ty.rs:5:5
+   |
+LL |     unsized_const_params,
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: enable all of these features
+
 error: the type of the associated constant `K` cannot capture late-bound generic parameters
   --> $DIR/esc-bound-var-in-ty.rs:15:35
    |
@@ -8,5 +16,5 @@ LL | fn take(_: impl for<'r> Trait<'r, K = const { &() }>) {}
    |
    = note: `K` has type `&'r ()`
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/mgca/adt_expr_arg_simple.stderr b/tests/ui/const-generics/mgca/adt_expr_arg_simple.stderr
@@ -9,6 +9,8 @@ error: generic parameters may not be used in const operations
    |
 LL |     foo::<{ Some::<u32> { 0: const { N + 1 } } }>();
    |                                      ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/mgca/early-bound-param-lt-bad.stderr b/tests/ui/const-generics/mgca/early-bound-param-lt-bad.stderr
@@ -3,6 +3,8 @@ error: generic parameters may not be used in const operations
    |
 LL |     T: Trait<const { let a: &'a (); 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
 
diff --git a/tests/ui/const-generics/mgca/explicit_anon_consts.stderr b/tests/ui/const-generics/mgca/explicit_anon_consts.stderr
@@ -39,42 +39,56 @@ error: generic parameters may not be used in const operations
    |
 LL | const ITEM3<const N: usize>: usize = const { N };
    |                                              ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/explicit_anon_consts.rs:60:31
    |
 LL |     T3: Trait<ASSOC = const { N }>,
    |                               ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/explicit_anon_consts.rs:69:58
    |
 LL | struct Default3<const N: usize, const M: usize = const { N }>;
    |                                                          ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/explicit_anon_consts.rs:28:27
    |
 LL |     let _3 = [(); const { N }];
    |                           ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/explicit_anon_consts.rs:33:26
    |
 LL |     let _6: [(); const { N }] = todo!();
    |                          ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/explicit_anon_consts.rs:11:41
    |
 LL | type Adt3<const N: usize> = Foo<const { N }>;
    |                                         ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/explicit_anon_consts.rs:19:42
    |
 LL | type Arr3<const N: usize> = [(); const { N }];
    |                                          ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 13 previous errors
 
diff --git a/tests/ui/const-generics/mgca/selftyalias-containing-param.stderr b/tests/ui/const-generics/mgca/selftyalias-containing-param.stderr
@@ -9,6 +9,7 @@ note: not a concrete type
    |
 LL | impl<const N: usize> S<N> {
    |                      ^^^^
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/const-generics/mgca/selftyparam.stderr b/tests/ui/const-generics/mgca/selftyparam.stderr
@@ -3,6 +3,8 @@ error: generic parameters may not be used in const operations
    |
 LL |     fn foo() -> [(); const { let _: Self; 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
 
diff --git a/tests/ui/const-generics/mgca/tuple_ctor_complex_args.stderr b/tests/ui/const-generics/mgca/tuple_ctor_complex_args.stderr
@@ -9,6 +9,8 @@ error: generic parameters may not be used in const operations
    |
 LL |     with_point::<{ Point(const { N + 1 }, N) }>();
    |                                  ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/mgca/tuple_expr_arg_complex.stderr b/tests/ui/const-generics/mgca/tuple_expr_arg_complex.stderr
@@ -21,6 +21,8 @@ error: generic parameters may not be used in const operations
    |
 LL |     takes_nested_tuple::<{ (N, (N, const { N + 1 })) }>();
    |                                            ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 4 previous errors
 
diff --git a/tests/ui/const-generics/mgca/type_const-on-generic-expr.stderr b/tests/ui/const-generics/mgca/type_const-on-generic-expr.stderr
@@ -3,12 +3,16 @@ error: generic parameters may not be used in const operations
    |
 LL | const FREE1<T>: usize = const { std::mem::size_of::<T>() };
    |                                                     ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/type_const-on-generic-expr.rs:8:46
    |
 LL | const FREE2<const I: usize>: usize = const { I + 1 };
    |                                              ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/const-generics/mgca/type_const-on-generic_expr-2.stderr b/tests/ui/const-generics/mgca/type_const-on-generic_expr-2.stderr
@@ -3,18 +3,24 @@ error: generic parameters may not be used in const operations
    |
 LL |     const N1<T>: usize = const { std::mem::size_of::<T>() };
    |                                                      ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/type_const-on-generic_expr-2.rs:20:47
    |
 LL |     const N2<const I: usize>: usize = const { I + 1 };
    |                                               ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: generic parameters may not be used in const operations
   --> $DIR/type_const-on-generic_expr-2.rs:23:35
    |
 LL |     const N3: usize = const { 2 & X };
    |                                   ^
+   |
+   = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/feature-gates/feature-gate-opaque-generic-const-args.rs b/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() {}
diff --git a/tests/ui/feature-gates/feature-gate-opaque-generic-const-args.stderr b/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
+
diff --git a/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.stderr b/tests/ui/generic-const-items/assoc-const-no-infer-ice-115806.stderr
@@ -1,3 +1,11 @@
+error: `unsized_const_params` requires `adt_const_params` to be enabled
+  --> $DIR/assoc-const-no-infer-ice-115806.rs:3:36
+   |
+LL | #![feature(min_generic_const_args, unsized_const_params)]
+   |                                    ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: enable all of these features
+
 error[E0119]: conflicting implementations of trait `Pins<_>` for type `NoPin`
   --> $DIR/assoc-const-no-infer-ice-115806.rs:17:1
    |
@@ -9,6 +17,6 @@ LL | impl<USART, T> Pins<USART> for T where T: PinA<USART, A = const { &() }> {}
    |
    = note: downstream crates may implement trait `PinA<_>` for type `NoPin`
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0119`.

Original file line number	Diff line number	Diff line change
`@@ -405,6 +405,11 @@ impl<'tcx> ForbidMCGParamUsesFolder<'tcx> {`
`405`	`405`	`diag.span_note(impl_.self_ty.span, "not a concrete type");`
`406`	`406`	`}`
`407`	`407`	`}`
	`408`	`+ if self.tcx.features().min_generic_const_args()`
	`409`	`+ && !self.tcx.features().opaque_generic_const_args()`
	`410`	`+ {`
	`411`	+ diag.help("add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items");
	`412`	`+ }`
`408`	`413`	`diag.emit()`
`409`	`414`	`}`
`410`	`415`	`}`
Original file line number	Diff line number	Diff line change
`@@ -9,6 +9,8 @@ error: generic parameters may not be used in const operations`
`9`	`9`	`\|`
`10`	`10`	`LL \| foo::<{ Some::<u32> { 0: const { N + 1 } } }>();`
`11`	`11`	`\| ^`
	`12`	`+ \|`
	`13`	+ = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
`12`	`14`
`13`	`15`	`error: aborting due to 2 previous errors`
`14`	`16`
Original file line number	Diff line number	Diff line change
`@@ -3,6 +3,8 @@ error: generic parameters may not be used in const operations`
`3`	`3`	`\|`
`4`	`4`	`LL \| T: Trait<const { let a: &'a (); 1 }>`
`5`	`5`	`\| ^^`
	`6`	`+ \|`
	`7`	+ = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
`6`	`8`
`7`	`9`	`error: aborting due to 1 previous error`
`8`	`10`
Original file line number	Diff line number	Diff line change
`@@ -9,6 +9,7 @@ note: not a concrete type`
`9`	`9`	`\|`
`10`	`10`	`LL \| impl<const N: usize> S<N> {`
`11`	`11`	`\| ^^^^`
	`12`	+ = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
`12`	`13`
`13`	`14`	`error: aborting due to 1 previous error`
`14`	`15`
Original file line number	Diff line number	Diff line change
`@@ -3,6 +3,8 @@ error: generic parameters may not be used in const operations`
`3`	`3`	`\|`
`4`	`4`	`LL \| fn foo() -> [(); const { let _: Self; 1 }];`
`5`	`5`	`\| ^^^^`
	`6`	`+ \|`
	`7`	+ = help: add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items
`6`	`8`
`7`	`9`	`error: aborting due to 1 previous error`
`8`	`10`