Introduce ~const

 - [x] Removed `?const` and change uses of `?const`
 - [x] Added `~const` to the AST. It is gated behind const_trait_impl.
 - [x] Validate `~const` in ast_validation.
 - [ ] Add enum `BoundConstness` to the HIR. (With variants `NotConst` and
 `ConstIfConst` allowing future extensions)
 - [ ] Adjust trait selection and pre-existing code to use `BoundConstness`.
 - [ ] Optional steps (*for this PR, obviously*)
      - [ ] Fix rust-lang#88155
      - [ ] Do something with constness bounds in chalk

src/types.rs

@@ -537,10 +537,10 @@ impl Rewrite for ast::GenericBound {
                         .map(|s| format!("?{}", s)),
                     ast::TraitBoundModifier::MaybeConst => poly_trait_ref
                         .rewrite(context, shape.offset_left(7)?)
-                        .map(|s| format!("?const {}", s)),
+                        .map(|s| format!("~const {}", s)),
                     ast::TraitBoundModifier::MaybeConstMaybe => poly_trait_ref
                         .rewrite(context, shape.offset_left(8)?)
-                        .map(|s| format!("?const ?{}", s)),
+                        .map(|s| format!("~const ?{}", s)),
                 };
                 rewrite.map(|s| if has_paren { format!("({})", s) } else { s })
             }

tests/target/type.rs

@@ -145,35 +145,33 @@ type MyFn = fn(
     b: SomeOtherLongComplexType,
 ) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>>;
 
-// Const opt-out
+// Const bound
 
-trait T: ?const Super {}
+trait T: ~const Super {}
 
-const fn maybe_const<S: ?const T>() -> i32 {
+const fn not_quite_const<S: ~const T>() -> i32 {
     <S as T>::CONST
 }
 
-struct S<T: ?const ?Sized>(std::marker::PhantomData<T>);
+struct S<T: ~const ?Sized>(std::marker::PhantomData<T>);
 
-impl ?const T {}
+impl ~const T {}
 
-fn trait_object() -> &'static dyn ?const T {
+fn trait_object() -> &'static dyn ~const T {
     &S
 }
 
-fn i(_: impl IntoIterator<Item = Box<dyn ?const T>>) {}
+fn i(_: impl IntoIterator<Item = Box<dyn ~const T>>) {}
 
-fn apit(_: impl ?const T) {}
+fn apit(_: impl ~const T) {}
 
-fn rpit() -> impl ?const T {
+fn rpit() -> impl ~const T {
     S
 }
 
 pub struct Foo<T: Trait>(T);
-impl<T: ?const Trait> Foo<T> {
+impl<T: ~const Trait> Foo<T> {
     fn new(t: T) -> Self {
-        // not calling methods on `t`, so we opt out of requiring
-        // `<T as Trait>` to have const methods via `?const`
         Self(t)
     }
 }