Add Self: ~const Trait to traits with #[const_trait]

Author: fee1-dead

Diff for: compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2556,7 +2556,7 @@ define_print_and_forward_display! {
 
     ty::TraitPredicate<'tcx> {
         p!(print(self.trait_ref.self_ty()), ": ");
-        if let ty::BoundConstness::ConstIfConst = self.constness {
+        if let ty::BoundConstness::ConstIfConst = self.constness && cx.tcx().features().const_trait_impl {
             p!("~const ");
         }
         p!(print(self.trait_ref.print_only_trait_path()))

Diff for: compiler/rustc_typeck/src/collect.rs

@@ -2090,10 +2090,17 @@ fn predicates_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::GenericPredicates<'_> {
         // from the trait itself that *shouldn't* be shown as the source of
         // an obligation and instead be skipped. Otherwise we'd use
         // `tcx.def_span(def_id);`
+
+        let constness = if tcx.has_attr(def_id, sym::const_trait) {
+            ty::BoundConstness::ConstIfConst
+        } else {
+            ty::BoundConstness::NotConst
+        };
+
         let span = rustc_span::DUMMY_SP;
         result.predicates =
             tcx.arena.alloc_from_iter(result.predicates.iter().copied().chain(std::iter::once((
-                ty::TraitRef::identity(tcx, def_id).without_const().to_predicate(tcx),
+                ty::TraitRef::identity(tcx, def_id).with_constness(constness).to_predicate(tcx),
                 span,
             ))));
     }

Diff for: library/core/src/cmp.rs

@@ -1503,9 +1503,10 @@ mod impls {
     // & pointers
 
     #[stable(feature = "rust1", since = "1.0.0")]
-    impl<A: ?Sized, B: ?Sized> PartialEq<&B> for &A
+    #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
+    impl<A: ?Sized, B: ?Sized> const PartialEq<&B> for &A
     where
-        A: PartialEq<B>,
+        A: ~const PartialEq<B>,
     {
         #[inline]
         fn eq(&self, other: &&B) -> bool {