Add support for ~const item bounds

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -100,6 +100,15 @@ where
         })
     }
 
+    /// Assemble additional assumptions for an alias that are not included
+    /// in the item bounds of the alias. For now, this is limited to the
+    /// `implied_const_bounds` for an associated type.
+    fn consider_additional_alias_assumptions(
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
+        alias_ty: ty::AliasTy<I>,
+    ) -> Vec<Candidate<I>>;
+
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,
@@ -594,6 +603,8 @@ where
             ));
         }
 
+        candidates.extend(G::consider_additional_alias_assumptions(self, goal, alias_ty));
+
         if kind != ty::Projection {
             return;
         }

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -3,7 +3,7 @@
 
 use rustc_type_ir::fast_reject::DeepRejectCtxt;
 use rustc_type_ir::inherent::*;
-use rustc_type_ir::{self as ty, Interner};
+use rustc_type_ir::{self as ty, Interner, elaborate};
 use tracing::instrument;
 
 use super::assembly::Candidate;
@@ -70,6 +70,55 @@ where
         }
     }
 
+    /// Register additional assumptions for aliases corresponding to `~const` item bounds.
+    ///
+    /// Unlike item bounds, they are not simply implied by the well-formedness of the alias.
+    /// Instead, they only hold if the const conditons on the alias also hold. This is why
+    /// we also register the const conditions of the alias after matching the goal against
+    /// the assumption.
+    fn consider_additional_alias_assumptions(
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
+        alias_ty: ty::AliasTy<I>,
+    ) -> Vec<Candidate<I>> {
+        let cx = ecx.cx();
+        let mut candidates = vec![];
+
+        // FIXME(effects): We elaborate here because the implied const bounds
+        // aren't necessarily elaborated. We probably should prefix this query
+        // with `explicit_`...
+        for clause in elaborate::elaborate(
+            cx,
+            cx.implied_const_bounds(alias_ty.def_id)
+                .iter_instantiated(cx, alias_ty.args)
+                .map(|trait_ref| trait_ref.to_host_effect_clause(cx, goal.predicate.host)),
+        ) {
+            candidates.extend(Self::probe_and_match_goal_against_assumption(
+                ecx,
+                CandidateSource::AliasBound,
+                goal,
+                clause,
+                |ecx| {
+                    // Const conditions must hold for the implied const bound to hold.
+                    ecx.add_goals(
+                        GoalSource::Misc,
+                        cx.const_conditions(alias_ty.def_id)
+                            .iter_instantiated(cx, alias_ty.args)
+                            .map(|trait_ref| {
+                                goal.with(
+                                    cx,
+                                    trait_ref.to_host_effect_clause(cx, goal.predicate.host),
+                                )
+                            }),
+                    );
+                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                },
+            ));
+        }
+
+        candidates
+    }
+
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, Self>,

compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

@@ -193,6 +193,14 @@ where
         }
     }
 
+    fn consider_additional_alias_assumptions(
+        _ecx: &mut EvalCtxt<'_, D>,
+        _goal: Goal<I, Self>,
+        _alias_ty: ty::AliasTy<I>,
+    ) -> Vec<Candidate<I>> {
+        vec![]
+    }
+
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, NormalizesTo<I>>,

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -39,6 +39,14 @@ where
         self.def_id()
     }
 
+    fn consider_additional_alias_assumptions(
+        _ecx: &mut EvalCtxt<'_, D>,
+        _goal: Goal<I, Self>,
+        _alias_ty: ty::AliasTy<I>,
+    ) -> Vec<Candidate<I>> {
+        vec![]
+    }
+
     fn consider_impl_candidate(
         ecx: &mut EvalCtxt<'_, D>,
         goal: Goal<I, TraitPredicate<I>>,

tests/ui/traits/const-traits/assoc-type-const-bound-usage-0.rs

@@ -1,5 +1,5 @@
 //@ compile-flags: -Znext-solver
-//@ known-bug: unknown
+//@ check-pass
 
 #![allow(incomplete_features)]
 #![feature(const_trait_impl, effects)]

tests/ui/traits/const-traits/assoc-type-const-bound-usage-1.stderr

@@ -6,18 +6,30 @@ LL | #![feature(const_trait_impl, effects, generic_const_exprs)]
    |
    = help: remove one of these features
 
-error[E0271]: type mismatch resolving `<T as Trait>::Assoc == T`
-  --> $DIR/assoc-type-const-bound-usage-1.rs:15:44
+error[E0284]: type annotations needed: cannot normalize `unqualified<T>::{constant#0}`
+  --> $DIR/assoc-type-const-bound-usage-1.rs:15:37
    |
 LL | fn unqualified<T: const Trait>() -> Type<{ T::Assoc::func() }> {
-   |                                            ^^^^^^^^^^^^^^^^ types differ
+   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot normalize `unqualified<T>::{constant#0}`
 
-error[E0271]: type mismatch resolving `<T as Trait>::Assoc == T`
-  --> $DIR/assoc-type-const-bound-usage-1.rs:19:42
+error[E0284]: type annotations needed: cannot normalize `qualified<T>::{constant#0}`
+  --> $DIR/assoc-type-const-bound-usage-1.rs:19:35
    |
 LL | fn qualified<T: const Trait>() -> Type<{ <T as Trait>::Assoc::func() }> {
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot normalize `qualified<T>::{constant#0}`
 
-error: aborting due to 3 previous errors
+error[E0284]: type annotations needed: cannot normalize `unqualified<T>::{constant#0}`
+  --> $DIR/assoc-type-const-bound-usage-1.rs:16:5
+   |
+LL |     Type
+   |     ^^^^ cannot normalize `unqualified<T>::{constant#0}`
+
+error[E0284]: type annotations needed: cannot normalize `qualified<T>::{constant#0}`
+  --> $DIR/assoc-type-const-bound-usage-1.rs:20:5
+   |
+LL |     Type
+   |     ^^^^ cannot normalize `qualified<T>::{constant#0}`
+
+error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0271`.
+For more information about this error, try `rustc --explain E0284`.

tests/ui/traits/const-traits/assoc-type-const-bound-usage-fail-2.rs

@@ -0,0 +1,35 @@
+//@ compile-flags: -Znext-solver
+
+// Check that `~const` item bounds only hold if the where clauses on the
+// associated type are also const.
+// i.e. check that we validate the const conditions for the associated type
+// when considering one of implied const bounds.
+
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
+
+#[const_trait]
+trait Trait {
+    type Assoc<U>: ~const Trait
+    where
+        U: ~const Other;
+
+    fn func();
+}
+
+#[const_trait]
+trait Other {}
+
+const fn fails<T: ~const Trait, U: Other>() {
+    T::Assoc::<U>::func();
+    //~^ ERROR the trait bound `<T as Trait>::Assoc<U>: ~const Trait` is not satisfied
+    <T as Trait>::Assoc::<U>::func();
+    //~^ ERROR the trait bound `<T as Trait>::Assoc<U>: ~const Trait` is not satisfied
+}
+
+const fn works<T: ~const Trait, U: ~const Other>() {
+    T::Assoc::<U>::func();
+    <T as Trait>::Assoc::<U>::func();
+}
+
+fn main() {}

tests/ui/traits/const-traits/assoc-type-const-bound-usage-fail-2.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `<T as Trait>::Assoc<U>: ~const Trait` is not satisfied
+  --> $DIR/assoc-type-const-bound-usage-fail-2.rs:24:5
+   |
+LL |     T::Assoc::<U>::func();
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the trait bound `<T as Trait>::Assoc<U>: ~const Trait` is not satisfied
+  --> $DIR/assoc-type-const-bound-usage-fail-2.rs:26:5
+   |
+LL |     <T as Trait>::Assoc::<U>::func();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/assoc-type-const-bound-usage-fail.rs

@@ -0,0 +1,28 @@
+//@ compile-flags: -Znext-solver
+
+// Check that `~const` item bounds only hold if the parent trait is `~const`.
+// i.e. check that we validate the const conditions for the associated type
+// when considering one of implied const bounds.
+
+#![allow(incomplete_features)]
+#![feature(const_trait_impl, effects)]
+
+#[const_trait]
+trait Trait {
+    type Assoc: ~const Trait;
+    fn func();
+}
+
+const fn unqualified<T: Trait>() {
+    T::Assoc::func();
+    //~^ ERROR the trait bound `T: ~const Trait` is not satisfied
+    <T as Trait>::Assoc::func();
+    //~^ ERROR the trait bound `T: ~const Trait` is not satisfied
+}
+
+const fn works<T: ~const Trait>() {
+    T::Assoc::func();
+    <T as Trait>::Assoc::func();
+}
+
+fn main() {}

tests/ui/traits/const-traits/assoc-type-const-bound-usage-fail.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `T: ~const Trait` is not satisfied
+  --> $DIR/assoc-type-const-bound-usage-fail.rs:17:5
+   |
+LL |     T::Assoc::func();
+   |     ^^^^^^^^^^^^^^^^
+
+error[E0277]: the trait bound `T: ~const Trait` is not satisfied
+  --> $DIR/assoc-type-const-bound-usage-fail.rs:19:5
+   |
+LL |     <T as Trait>::Assoc::func();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.