Allow impl ~const Trait opaque types

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -42,7 +42,6 @@ enum SelfSemantic {
 /// What is the context that prevents using `~const`?
 enum DisallowTildeConstContext<'a> {
     TraitObject,
-    ImplTrait,
     Fn(FnKind<'a>),
 }
 
@@ -187,11 +186,7 @@ impl<'a> AstValidator<'a> {
 
     fn with_impl_trait(&mut self, outer: Option<Span>, f: impl FnOnce(&mut Self)) {
         let old = mem::replace(&mut self.outer_impl_trait, outer);
-        if outer.is_some() {
-            self.with_banned_tilde_const(DisallowTildeConstContext::ImplTrait, f);
-        } else {
-            f(self);
-        }
+        f(self);
         self.outer_impl_trait = old;
     }
 
@@ -1384,7 +1379,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     let mut err = self.err_handler().struct_span_err(bound.span(), "`~const` is not allowed here");
                     match reason {
                         DisallowTildeConstContext::TraitObject => err.note("trait objects cannot have `~const` trait bounds"),
-                        DisallowTildeConstContext::ImplTrait => err.note("`impl Trait`s cannot have `~const` trait bounds"),
                         DisallowTildeConstContext::Fn(FnKind::Closure(..)) => err.note("closures cannot have `~const` trait bounds"),
                         DisallowTildeConstContext::Fn(FnKind::Fn(_, ident, ..)) => err.span_note(ident.span, "this function is not `const`, so it cannot have `~const` trait bounds"),
                     };

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

@@ -0,0 +1,55 @@
+// check-pass
+#![allow(incomplete_features)]
+#![feature(
+    associated_type_bounds,
+    const_trait_impl,
+    const_cmp,
+    return_position_impl_trait_in_trait,
+)]
+
+use std::marker::Destruct;
+
+const fn cmp(a: &impl ~const PartialEq) -> bool {
+    a == a
+}
+
+const fn wrap(x: impl ~const PartialEq + ~const Destruct)
+    -> impl ~const PartialEq + ~const Destruct
+{
+    x
+}
+
+#[const_trait]
+trait Foo {
+    fn huh() -> impl ~const PartialEq + ~const Destruct + Copy;
+}
+
+impl const Foo for () {
+    fn huh() -> impl ~const PartialEq + ~const Destruct + Copy {
+        123
+    }
+}
+
+const _: () = {
+    assert!(cmp(&0xDEADBEEFu32));
+    assert!(cmp(&()));
+    assert!(wrap(123) == wrap(123));
+    assert!(wrap(123) != wrap(456));
+    let x = <() as Foo>::huh();
+    assert!(x == x);
+};
+
+#[const_trait]
+trait T {}
+struct S;
+impl const T for S {}
+
+const fn rpit() -> impl ~const T { S }
+
+const fn apit(_: impl ~const T + ~const Destruct) {}
+
+const fn rpit_assoc_bound() -> impl IntoIterator<Item: ~const T> { Some(S) }
+
+const fn apit_assoc_bound(_: impl IntoIterator<Item: ~const T> + ~const Destruct) {}
+
+fn main() {}

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

@@ -1,23 +1,6 @@
 #![feature(const_trait_impl)]
 #![feature(associated_type_bounds)]
 
-#[const_trait]
-trait T {}
-struct S;
-impl T for S {}
-
-fn rpit() -> impl ~const T { S }
-//~^ ERROR `~const` is not allowed
-
-fn apit(_: impl ~const T) {}
-//~^ ERROR `~const` is not allowed
-
-fn rpit_assoc_bound() -> impl IntoIterator<Item: ~const T> { Some(S) }
-//~^ ERROR `~const` is not allowed
-
-fn apit_assoc_bound(_: impl IntoIterator<Item: ~const T>) {}
-//~^ ERROR `~const` is not allowed
-
 struct TildeQuestion<T: ~const ?Sized>(std::marker::PhantomData<T>);
 //~^ ERROR `~const` and `?` are mutually exclusive
 

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

@@ -1,40 +1,8 @@
-error: `~const` is not allowed here
-  --> $DIR/tilde-const-invalid-places.rs:9:19
-   |
-LL | fn rpit() -> impl ~const T { S }
-   |                   ^^^^^^^^
-   |
-   = note: `impl Trait`s cannot have `~const` trait bounds
-
-error: `~const` is not allowed here
-  --> $DIR/tilde-const-invalid-places.rs:12:17
-   |
-LL | fn apit(_: impl ~const T) {}
-   |                 ^^^^^^^^
-   |
-   = note: `impl Trait`s cannot have `~const` trait bounds
-
-error: `~const` is not allowed here
-  --> $DIR/tilde-const-invalid-places.rs:15:50
-   |
-LL | fn rpit_assoc_bound() -> impl IntoIterator<Item: ~const T> { Some(S) }
-   |                                                  ^^^^^^^^
-   |
-   = note: `impl Trait`s cannot have `~const` trait bounds
-
-error: `~const` is not allowed here
-  --> $DIR/tilde-const-invalid-places.rs:18:48
-   |
-LL | fn apit_assoc_bound(_: impl IntoIterator<Item: ~const T>) {}
-   |                                                ^^^^^^^^
-   |
-   = note: `impl Trait`s cannot have `~const` trait bounds
-
 error: `~const` and `?` are mutually exclusive
-  --> $DIR/tilde-const-invalid-places.rs:21:25
+  --> $DIR/tilde-const-invalid-places.rs:4:25
    |
 LL | struct TildeQuestion<T: ~const ?Sized>(std::marker::PhantomData<T>);
    |                         ^^^^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error: aborting due to previous error