Allow ~const bounds on trait assoc functions

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1442,7 +1442,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 if !self.is_tilde_const_allowed {
                     self.err_handler()
                         .struct_span_err(bound.span(), "`~const` is not allowed here")
-                        .note("only allowed on bounds on traits' associated types, const fns, const impls and its associated functions")
+                        .note("only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions")
                         .emit();
                 }
             }
@@ -1616,7 +1616,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 walk_list!(self, visit_ty, ty);
             }
             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, ref body))
-                if self.in_const_trait_impl =>
+                if self.in_const_trait_impl || ctxt == AssocCtxt::Trait =>
             {
                 self.visit_vis(&item.vis);
                 self.visit_ident(item.ident);

src/test/ui/rfc-2632-const-trait-impl/tilde-const-invalid-places.stderr

@@ -4,47 +4,47 @@ error: `~const` is not allowed here
 LL | fn rpit() -> impl ~const T { S }
    |                   ^^^^^^^^
    |
-   = note: only allowed on bounds on traits' associated types, const fns, const impls and its associated functions
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
 
 error: `~const` is not allowed here
   --> $DIR/tilde-const-invalid-places.rs:11:17
    |
 LL | fn apit(_: impl ~const T) {}
    |                 ^^^^^^^^
    |
-   = note: only allowed on bounds on traits' associated types, const fns, const impls and its associated functions
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
 
 error: `~const` is not allowed here
   --> $DIR/tilde-const-invalid-places.rs:14:50
    |
 LL | fn rpit_assoc_bound() -> impl IntoIterator<Item: ~const T> { Some(S) }
    |                                                  ^^^^^^^^
    |
-   = note: only allowed on bounds on traits' associated types, const fns, const impls and its associated functions
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
 
 error: `~const` is not allowed here
   --> $DIR/tilde-const-invalid-places.rs:17:48
    |
 LL | fn apit_assoc_bound(_: impl IntoIterator<Item: ~const T>) {}
    |                                                ^^^^^^^^
    |
-   = note: only allowed on bounds on traits' associated types, const fns, const impls and its associated functions
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
 
 error: `~const` is not allowed here
   --> $DIR/tilde-const-invalid-places.rs:20:15
    |
 LL | fn generic<P: ~const T>() {}
    |               ^^^^^^^^
    |
-   = note: only allowed on bounds on traits' associated types, const fns, const impls and its associated functions
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
 
 error: `~const` is not allowed here
   --> $DIR/tilde-const-invalid-places.rs:23:31
    |
 LL | fn where_clause<P>() where P: ~const T {}
    |                               ^^^^^^^^
    |
-   = note: only allowed on bounds on traits' associated types, const fns, const impls and its associated functions
+   = note: only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions
 
 error: `~const` and `?` are mutually exclusive
   --> $DIR/tilde-const-invalid-places.rs:26:25

src/test/ui/rfc-2632-const-trait-impl/trait-where-clause.rs

@@ -0,0 +1,33 @@
+#![feature(const_fn_trait_bound)]
+#![feature(const_trait_impl)]
+
+trait Bar {}
+
+trait Foo {
+    fn a();
+    fn b() where Self: ~const Bar;
+}
+
+const fn test1<T: ~const Foo + Bar>() {
+    T::a();
+    T::b();
+    //~^ ERROR the trait bound
+}
+
+const fn test2<T: ~const Foo + ~const Bar>() {
+    T::a();
+    T::b();
+}
+
+fn test3<T: Foo>() {
+    T::a();
+    T::b();
+    //~^ ERROR the trait bound
+}
+
+fn test4<T: Foo + Bar>() {
+    T::a();
+    T::b();
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/trait-where-clause.stderr

@@ -0,0 +1,35 @@
+error[E0277]: the trait bound `T: Bar` is not satisfied
+  --> $DIR/trait-where-clause.rs:13:5
+   |
+LL |     T::b();
+   |     ^^^^ the trait `Bar` is not implemented for `T`
+   |
+note: required by `Foo::b`
+  --> $DIR/trait-where-clause.rs:8:5
+   |
+LL |     fn b() where Self: ~const Bar;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: consider further restricting this bound
+   |
+LL | const fn test1<T: ~const Foo + Bar + Bar>() {
+   |                                    +++++
+
+error[E0277]: the trait bound `T: Bar` is not satisfied
+  --> $DIR/trait-where-clause.rs:24:5
+   |
+LL |     T::b();
+   |     ^^^^ the trait `Bar` is not implemented for `T`
+   |
+note: required by `Foo::b`
+  --> $DIR/trait-where-clause.rs:8:5
+   |
+LL |     fn b() where Self: ~const Bar;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: consider further restricting this bound
+   |
+LL | fn test3<T: Foo + Bar>() {
+   |                 +++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.