fix: Error on illegal [const]s inside blocks within legal positions

compiler/rustc_ast_passes/messages.ftl

@@ -241,6 +241,10 @@ ast_passes_tilde_const_disallowed = `[const]` is not allowed here
     .trait_assoc_ty = associated types in non-`const` traits cannot have `[const]` trait bounds
     .trait_impl_assoc_ty = associated types in non-const impls cannot have `[const]` trait bounds
     .inherent_assoc_ty = inherent associated types cannot have `[const]` trait bounds
+    .struct = structs cannot have `[const]` trait bounds
+    .enum = enums cannot have `[const]` trait bounds
+    .union = unions cannot have `[const]` trait bounds
+    .anon_const = anonymous constants cannot have `[const]` trait bounds
     .object = trait objects cannot have `[const]` trait bounds
     .item = this item cannot have `[const]` trait bounds
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1124,7 +1124,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         );
                     }
                 }
-                visit::walk_item(self, item)
+                self.with_tilde_const(Some(TildeConstReason::Enum { span: item.span }), |this| {
+                    visit::walk_item(this, item)
+                });
             }
             ItemKind::Trait(box Trait {
                 constness,
@@ -1175,26 +1177,32 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 }
                 visit::walk_item(self, item)
             }
-            ItemKind::Struct(ident, generics, vdata) => match vdata {
-                VariantData::Struct { fields, .. } => {
-                    self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
-                    self.visit_generics(generics);
-                    walk_list!(self, visit_field_def, fields);
-                }
-                _ => visit::walk_item(self, item),
-            },
+            ItemKind::Struct(ident, generics, vdata) => {
+                self.with_tilde_const(Some(TildeConstReason::Struct { span: item.span }), |this| {
+                    match vdata {
+                        VariantData::Struct { fields, .. } => {
+                            this.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
+                            this.visit_generics(generics);
+                            walk_list!(this, visit_field_def, fields);
+                        }
+                        _ => visit::walk_item(this, item),
+                    }
+                })
+            }
             ItemKind::Union(ident, generics, vdata) => {
                 if vdata.fields().is_empty() {
                     self.dcx().emit_err(errors::FieldlessUnion { span: item.span });
                 }
-                match vdata {
-                    VariantData::Struct { fields, .. } => {
-                        self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
-                        self.visit_generics(generics);
-                        walk_list!(self, visit_field_def, fields);
+                self.with_tilde_const(Some(TildeConstReason::Union { span: item.span }), |this| {
+                    match vdata {
+                        VariantData::Struct { fields, .. } => {
+                            this.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
+                            this.visit_generics(generics);
+                            walk_list!(this, visit_field_def, fields);
+                        }
+                        _ => visit::walk_item(this, item),
                     }
-                    _ => visit::walk_item(self, item),
-                }
+                });
             }
             ItemKind::Const(box ConstItem { defaultness, expr, .. }) => {
                 self.check_defaultness(item.span, *defaultness);
@@ -1623,6 +1631,13 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             _ => self.with_in_trait_impl(None, |this| visit::walk_assoc_item(this, item, ctxt)),
         }
     }
+
+    fn visit_anon_const(&mut self, anon_const: &'a AnonConst) {
+        self.with_tilde_const(
+            Some(TildeConstReason::AnonConst { span: anon_const.value.span }),
+            |this| visit::walk_anon_const(this, anon_const),
+        )
+    }
 }
 
 /// When encountering an equality constraint in a `where` clause, emit an error. If the code seems

compiler/rustc_ast_passes/src/errors.rs

@@ -623,6 +623,26 @@ pub(crate) enum TildeConstReason {
         #[primary_span]
         span: Span,
     },
+    #[note(ast_passes_struct)]
+    Struct {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_enum)]
+    Enum {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_union)]
+    Union {
+        #[primary_span]
+        span: Span,
+    },
+    #[note(ast_passes_anon_const)]
+    AnonConst {
+        #[primary_span]
+        span: Span,
+    },
     #[note(ast_passes_object)]
     TraitObject,
     #[note(ast_passes_item)]

tests/ui/traits/const-traits/conditionally-const-in-anon-const.rs

@@ -0,0 +1,28 @@
+#![feature(const_trait_impl, impl_trait_in_bindings)]
+
+struct S;
+#[const_trait]
+trait Trait<const N: u32> {}
+
+impl const Trait<0> for () {}
+
+const fn f<
+    T: Trait<
+        {
+            const fn g<U: [const] Trait<0>>() {}
+
+            struct I<U: [const] Trait<0>>(U);
+            //~^ ERROR `[const]` is not allowed here
+
+            let x: &impl [const] Trait<0> = &();
+            //~^ ERROR `[const]` is not allowed here
+
+            0
+        },
+    >,
+>(x: &T) {
+    // Should be allowed here
+    let y: &impl [const] Trait<0> = x;
+}
+
+pub fn main() {}

tests/ui/traits/const-traits/conditionally-const-in-anon-const.stderr

@@ -0,0 +1,32 @@
+error: `[const]` is not allowed here
+  --> $DIR/conditionally-const-in-anon-const.rs:14:25
+   |
+LL |             struct I<U: [const] Trait<0>>(U);
+   |                         ^^^^^^^
+   |
+note: structs cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-in-anon-const.rs:14:13
+   |
+LL |             struct I<U: [const] Trait<0>>(U);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `[const]` is not allowed here
+  --> $DIR/conditionally-const-in-anon-const.rs:17:26
+   |
+LL |             let x: &impl [const] Trait<0> = &();
+   |                          ^^^^^^^
+   |
+note: anonymous constants cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-in-anon-const.rs:11:9
+   |
+LL | /         {
+LL | |             const fn g<U: [const] Trait<0>>() {}
+LL | |
+LL | |             struct I<U: [const] Trait<0>>(U);
+...  |
+LL | |             0
+LL | |         },
+   | |_________^
+
+error: aborting due to 2 previous errors
+

tests/ui/traits/const-traits/conditionally-const-invalid-places.stderr

@@ -16,39 +16,59 @@ error: `[const]` is not allowed here
 LL | struct Struct<T: [const] Trait> { field: T }
    |                  ^^^^^^^
    |
-   = note: this item cannot have `[const]` trait bounds
+note: structs cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-invalid-places.rs:9:1
+   |
+LL | struct Struct<T: [const] Trait> { field: T }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `[const]` is not allowed here
   --> $DIR/conditionally-const-invalid-places.rs:10:23
    |
 LL | struct TupleStruct<T: [const] Trait>(T);
    |                       ^^^^^^^
    |
-   = note: this item cannot have `[const]` trait bounds
+note: structs cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-invalid-places.rs:10:1
+   |
+LL | struct TupleStruct<T: [const] Trait>(T);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `[const]` is not allowed here
   --> $DIR/conditionally-const-invalid-places.rs:11:22
    |
 LL | struct UnitStruct<T: [const] Trait>;
    |                      ^^^^^^^
    |
-   = note: this item cannot have `[const]` trait bounds
+note: structs cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-invalid-places.rs:11:1
+   |
+LL | struct UnitStruct<T: [const] Trait>;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `[const]` is not allowed here
   --> $DIR/conditionally-const-invalid-places.rs:14:14
    |
 LL | enum Enum<T: [const] Trait> { Variant(T) }
    |              ^^^^^^^
    |
-   = note: this item cannot have `[const]` trait bounds
+note: enums cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-invalid-places.rs:14:1
+   |
+LL | enum Enum<T: [const] Trait> { Variant(T) }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `[const]` is not allowed here
   --> $DIR/conditionally-const-invalid-places.rs:16:16
    |
 LL | union Union<T: [const] Trait> { field: T }
    |                ^^^^^^^
    |
-   = note: this item cannot have `[const]` trait bounds
+note: unions cannot have `[const]` trait bounds
+  --> $DIR/conditionally-const-invalid-places.rs:16:1
+   |
+LL | union Union<T: [const] Trait> { field: T }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: `[const]` is not allowed here
   --> $DIR/conditionally-const-invalid-places.rs:19:14