Make const trait aliases work in the old solver

Author: oli-obk

compiler/rustc_trait_selection/src/traits/effects.rs

@@ -69,6 +69,12 @@ pub fn evaluate_host_effect_obligation<'tcx>(
         Err(EvaluationFailure::NoSolution) => {}
     }
 
+    match evaluate_host_effect_from_trait_alias(selcx, obligation) {
+        Ok(result) => return Ok(result),
+        Err(EvaluationFailure::Ambiguous) => return Err(EvaluationFailure::Ambiguous),
+        Err(EvaluationFailure::NoSolution) => {}
+    }
+
     Err(EvaluationFailure::NoSolution)
 }
 
@@ -496,3 +502,37 @@ fn evaluate_host_effect_from_selection_candidate<'tcx>(
         }
     })
 }
+
+fn evaluate_host_effect_from_trait_alias<'tcx>(
+    selcx: &mut SelectionContext<'_, 'tcx>,
+    obligation: &HostEffectObligation<'tcx>,
+) -> Result<ThinVec<PredicateObligation<'tcx>>, EvaluationFailure> {
+    let tcx = selcx.tcx();
+    let def_id = obligation.predicate.def_id();
+    if !tcx.trait_is_alias(def_id) {
+        return Err(EvaluationFailure::NoSolution);
+    }
+
+    Ok(tcx
+        .const_conditions(def_id)
+        .instantiate(tcx, obligation.predicate.trait_ref.args)
+        .into_iter()
+        .map(|(trait_ref, span)| {
+            Obligation::new(
+                tcx,
+                obligation.cause.clone().derived_host_cause(
+                    ty::Binder::dummy(obligation.predicate),
+                    |derived| {
+                        ObligationCauseCode::ImplDerivedHost(Box::new(ImplDerivedHostCause {
+                            derived,
+                            impl_def_id: def_id,
+                            span,
+                        }))
+                    },
+                ),
+                obligation.param_env,
+                trait_ref.to_host_effect_clause(tcx, obligation.predicate.constness),
+            )
+        })
+        .collect())
+}

tests/ui/consts/trait_alias.fail.stderr

@@ -1,23 +1,9 @@
 error[E0277]: the trait bound `T: [const] Baz` is not satisfied
-  --> $DIR/trait_alias.rs:23:11
+  --> $DIR/trait_alias.rs:24:11
    |
 LL |         x.baz();
    |           ^^^
 
-error[E0277]: the trait bound `(): const Foo` is not satisfied
-  --> $DIR/trait_alias.rs:28:19
-   |
-LL | const _: () = foo(&());
-   |               --- ^^^
-   |               |
-   |               required by a bound introduced by this call
-   |
-note: required by a bound in `foo`
-  --> $DIR/trait_alias.rs:19:17
-   |
-LL | const fn foo<T: [const] Foo>(x: &T) {
-   |                 ^^^^^^^^^^^ required by this bound in `foo`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/consts/trait_alias.next_fail.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `T: [const] Baz` is not satisfied
-  --> $DIR/trait_alias.rs:23:11
+  --> $DIR/trait_alias.rs:24:11
    |
 LL |         x.baz();
    |           ^^^

tests/ui/consts/trait_alias.pass.stderr

@@ -3,6 +3,7 @@
 //@[next_pass] compile-flags: -Znext-solver
 //@[next_fail] compile-flags: -Znext-solver
 //@[next_pass] check-pass
+//@[pass] check-pass
 
 const trait Bar {
     fn bar(&self) {}
@@ -26,6 +27,5 @@ const fn foo<T: [const] Foo>(x: &T) {
 }
 
 const _: () = foo(&());
-//[fail,pass]~^ ERROR: `(): const Foo` is not satisfied
 
 fn main() {}