Do not suggest generic const items unless enabled

compiler/rustc_resolve/src/late.rs

@@ -2450,7 +2450,11 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             ItemKind::Const(box ast::ConstItem { ref generics, ref ty, ref expr, .. }) => {
                 self.with_generic_param_rib(
                     &generics.params,
-                    RibKind::Item(HasGenericParams::Yes(generics.span)),
+                    RibKind::Item(if self.r.tcx.features().generic_const_items {
+                        HasGenericParams::Yes(generics.span)
+                    } else {
+                        HasGenericParams::No
+                    }),
                     LifetimeRibKind::Generics {
                         binder: item.id,
                         kind: LifetimeBinderKind::ConstItem,

tests/ui/resolve/generic-params-from-outer-item-in-const-item.default.stderr

@@ -0,0 +1,28 @@
+error[E0401]: can't use generic parameters from outer item
+  --> $DIR/generic-params-from-outer-item-in-const-item.rs:12:20
+   |
+LL | fn outer<T: Tr>() { // outer function
+   |          - type parameter from outer item
+LL |     const K: u32 = T::C;
+   |                    ^^^^ use of generic parameter from outer item
+
+error[E0401]: can't use generic parameters from outer item
+  --> $DIR/generic-params-from-outer-item-in-const-item.rs:19:24
+   |
+LL | impl<T> Tr for T { // outer impl block
+   |      - type parameter from outer item
+LL |     const C: u32 = {
+LL |         const I: u32 = T::C;
+   |                        ^^^^ use of generic parameter from outer item
+
+error[E0401]: can't use generic parameters from outer item
+  --> $DIR/generic-params-from-outer-item-in-const-item.rs:27:20
+   |
+LL | struct S<T: Tr>(U32<{ // outer struct
+   |          - type parameter from outer item
+LL |     const _: u32 = T::C;
+   |                    ^^^^ use of generic parameter from outer item
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0401`.

tests/ui/resolve/generic-params-from-outer-item-in-const-item.generic_const_items.stderr

@@ -0,0 +1,34 @@
+error[E0401]: can't use generic parameters from outer item
+  --> $DIR/generic-params-from-outer-item-in-const-item.rs:12:20
+   |
+LL | fn outer<T: Tr>() { // outer function
+   |          - type parameter from outer item
+LL |     const K: u32 = T::C;
+   |            -       ^^^^ use of generic parameter from outer item
+   |            |
+   |            help: try introducing a local generic parameter here: `<T>`
+
+error[E0401]: can't use generic parameters from outer item
+  --> $DIR/generic-params-from-outer-item-in-const-item.rs:19:24
+   |
+LL | impl<T> Tr for T { // outer impl block
+   |      - type parameter from outer item
+LL |     const C: u32 = {
+LL |         const I: u32 = T::C;
+   |                -       ^^^^ use of generic parameter from outer item
+   |                |
+   |                help: try introducing a local generic parameter here: `<T>`
+
+error[E0401]: can't use generic parameters from outer item
+  --> $DIR/generic-params-from-outer-item-in-const-item.rs:27:20
+   |
+LL | struct S<T: Tr>(U32<{ // outer struct
+   |          - type parameter from outer item
+LL |     const _: u32 = T::C;
+   |            -       ^^^^ use of generic parameter from outer item
+   |            |
+   |            help: try introducing a local generic parameter here: `<T>`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0401`.

tests/ui/resolve/generic-params-from-outer-item-in-const-item.rs

@@ -0,0 +1,39 @@
+// Regression test for issue #115720.
+// If a const item contains generic params from an outer items, only suggest
+// turning the const item generic if the feature `generic_const_items` is enabled.
+
+// revisions: default generic_const_items
+
+#![cfg_attr(generic_const_items, feature(generic_const_items))]
+#![feature(generic_const_exprs)] // only used for the test case "outer struct"
+#![allow(incomplete_features)]
+
+fn outer<T: Tr>() { // outer function
+    const K: u32 = T::C;
+    //~^ ERROR can't use generic parameters from outer item
+    //[generic_const_items]~| HELP try introducing a local generic parameter here
+}
+
+impl<T> Tr for T { // outer impl block
+    const C: u32 = {
+        const I: u32 = T::C;
+        //~^ ERROR can't use generic parameters from outer item
+        //[generic_const_items]~| HELP try introducing a local generic parameter here
+        I
+    };
+}
+
+struct S<T: Tr>(U32<{ // outer struct
+    const _: u32 = T::C;
+    //~^ ERROR can't use generic parameters from outer item
+    //[generic_const_items]~| HELP try introducing a local generic parameter here
+    0
+}>);
+
+trait Tr {
+    const C: u32;
+}
+
+struct U32<const N: u32>;
+
+fn main() {}