Implement const conditions for GATs

Author: compiler-errors

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -884,10 +884,9 @@ pub(super) fn const_conditions<'tcx>(
                 }
             }
             DefKind::Impl { .. } => {
-                if !tcx
-                    .trait_id_of_impl(def_id.to_def_id())
-                    .is_some_and(|def_id| tcx.is_const_trait(def_id))
-                {
+                // FIXME(effects): Should be using a dedicated function to
+                // test if this is a const trait impl.
+                if tcx.constness(def_id) != hir::Constness::Const {
                     return Default::default();
                 }
             }
@@ -900,10 +899,9 @@ pub(super) fn const_conditions<'tcx>(
                         }
                     }
                     ty::AssocItemContainer::ImplContainer => {
-                        if !tcx
-                            .trait_id_of_impl(parent_def_id)
-                            .is_some_and(|def_id| tcx.is_const_trait(def_id))
-                        {
+                        // FIXME(effects): Should be using a dedicated function to
+                        // test if this is a const trait impl.
+                        if tcx.constness(parent_def_id) != hir::Constness::Const {
                             return Default::default();
                         }
                     }
@@ -922,12 +920,18 @@ pub(super) fn const_conditions<'tcx>(
             }
             _ => return Default::default(),
         },
+        // While associated types are not really const, we do allow them to have `~const`
+        // bounds and where clauses. `const_conditions` is responsible for gathering
+        // these up so we can check them in `compare_type_predicate_entailment`, and
+        // in `HostEffect` goal computation.
         Node::TraitItem(item) => match item.kind {
-            hir::TraitItemKind::Fn(_, _) => (item.generics, None, true),
+            hir::TraitItemKind::Fn(_, _) | hir::TraitItemKind::Type(_, _) => {
+                (item.generics, None, true)
+            }
             _ => return Default::default(),
         },
         Node::ImplItem(item) => match item.kind {
-            hir::ImplItemKind::Fn(_, _) => (item.generics, None, true),
+            hir::ImplItemKind::Fn(_, _) | hir::ImplItemKind::Type(_) => (item.generics, None, true),
             _ => return Default::default(),
         },
         _ => return Default::default(),

tests/ui/rfcs/rfc-2632-const-trait-impl/const_derives/derive-const-with-params.stderr

@@ -30,5 +30,14 @@ LL | #[derive_const(PartialEq)]
    |
    = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 3 previous errors; 1 warning emitted
+error: `~const` can only be applied to `#[const_trait]` traits
+  --> $DIR/derive-const-with-params.rs:7:16
+   |
+LL | #[derive_const(PartialEq)]
+   |                ^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+   = note: this error originates in the derive macro `PartialEq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 4 previous errors; 1 warning emitted
 

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/spec-effectvar-ice.rs

@@ -14,6 +14,7 @@ impl<T> const Foo for T {}
 impl<T> const Foo for T where T: const Specialize {}
 //~^ error: const `impl` for trait `Foo` which is not marked with `#[const_trait]`
 //~| error: `const` can only be applied to `#[const_trait]` traits
+//~| error: `const` can only be applied to `#[const_trait]` traits
 //~| error: specialization impl does not specialize any associated items
 //~| error: cannot specialize on trait `Specialize`
 //~| ERROR cannot specialize on predicate

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/spec-effectvar-ice.stderr

@@ -42,6 +42,14 @@ error: `const` can only be applied to `#[const_trait]` traits
 LL | impl<T> const Foo for T where T: const Specialize {}
    |                                        ^^^^^^^^^^
 
+error: `const` can only be applied to `#[const_trait]` traits
+  --> $DIR/spec-effectvar-ice.rs:14:40
+   |
+LL | impl<T> const Foo for T where T: const Specialize {}
+   |                                        ^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error: specialization impl does not specialize any associated items
   --> $DIR/spec-effectvar-ice.rs:14:1
    |
@@ -66,5 +74,5 @@ error: cannot specialize on trait `Specialize`
 LL | impl<T> const Foo for T where T: const Specialize {}
    |                                  ^^^^^^^^^^^^^^^^
 
-error: aborting due to 7 previous errors; 1 warning emitted
+error: aborting due to 8 previous errors; 1 warning emitted