elaborate obligations in coherence

Author: lcnr

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -12,11 +12,8 @@ use crate::solve::{deeply_normalize_for_diagnostics, inspect, FulfillmentCtxt};
 use crate::traits::engine::TraitEngineExt as _;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::structural_normalize::StructurallyNormalizeExt;
-use crate::traits::NormalizeExt;
-use crate::traits::SkipLeakCheck;
-use crate::traits::{
-    Obligation, ObligationCause, PredicateObligation, PredicateObligations, SelectionContext,
-};
+use crate::traits::{NormalizeExt, SelectionContext, SkipLeakCheck};
+use crate::traits::{Obligation, ObligationCause, PredicateObligation, PredicateObligations};
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir::def::DefKind;
@@ -346,9 +343,15 @@ fn impl_intersection_has_impossible_obligation<'a, 'cx, 'tcx>(
 ) -> IntersectionHasImpossibleObligations<'tcx> {
     let infcx = selcx.infcx;
 
+    // Elaborate obligations in case the current obligation is unknowable,
+    // but its super trait bound is not. See #124532 for more details.
+    //
+    // FIXME(-Znext-solver=coinductive): Once we always prove super trait
+    // bounds when using impls this `elaborate` call can be ignored.
+    let obligations = util::elaborate(infcx.tcx, obligations.iter().cloned());
     if infcx.next_trait_solver() {
         let mut fulfill_cx = FulfillmentCtxt::new(infcx);
-        fulfill_cx.register_predicate_obligations(infcx, obligations.iter().cloned());
+        fulfill_cx.register_predicate_obligations(infcx, obligations);
 
         // We only care about the obligations that are *definitely* true errors.
         // Ambiguities do not prove the disjointness of two impls.
@@ -371,7 +374,7 @@ fn impl_intersection_has_impossible_obligation<'a, 'cx, 'tcx>(
         for obligation in obligations {
             // We use `evaluate_root_obligation` to correctly track intercrate
             // ambiguity clauses.
-            let evaluation_result = selcx.evaluate_root_obligation(obligation);
+            let evaluation_result = selcx.evaluate_root_obligation(&obligation);
 
             match evaluation_result {
                 Ok(result) => {

tests/ui/coherence/normalize-for-errors.current.stderr

@@ -8,6 +8,7 @@ LL | impl<S: Iterator> MyTrait<S> for (Box<<(MyType,) as Mirror>::Assoc>, S::Ite
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(Box<(MyType,)>, _)`
    |
    = note: upstream crates may add a new impl of trait `std::marker::Copy` for type `std::boxed::Box<(MyType,)>` in future versions
+   = note: upstream crates may add a new impl of trait `std::clone::Clone` for type `(MyType,)` in future versions
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/normalize-for-errors.rs

@@ -18,5 +18,6 @@ impl<S: Iterator> MyTrait<S> for (Box<<(MyType,) as Mirror>::Assoc>, S::Item) {}
 //~^ ERROR conflicting implementations of trait `MyTrait<_>` for type `(Box<(MyType,)>,
 //~| NOTE conflicting implementation for `(Box<(MyType,)>,
 //~| NOTE upstream crates may add a new impl of trait `std::marker::Copy` for type `std::boxed::Box<(MyType,)>` in future versions
+//[current]~| NOTE upstream crates may add a new impl of trait `std::clone::Clone` for type `(MyType,)` in future versions
 
 fn main() {}

tests/ui/coherence/super-traits/super-trait-knowable-1.rs

@@ -0,0 +1,18 @@
+// Added in #124532. While `(): Super` is knowable, `(): Sub<?t>` is not.
+//
+// We therefore elaborate super trait bounds in the implicit negative
+// overlap check.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+trait Super {}
+trait Sub<T>: Super {}
+
+trait Overlap<T> {}
+impl<T, U: Sub<T>> Overlap<T> for U {}
+impl<T> Overlap<T> for () {}
+
+fn main() {}

tests/ui/coherence/super-traits/super-trait-knowable-2.rs

@@ -0,0 +1,33 @@
+// A regression test for pyella-0.1.5 which broke when
+// enabling the new solver in coherence.
+//
+// `Tensor: TensorValue` is knowable while `Tensor: TensorOp<?t2>`
+// may be implemented downstream. We previously didn't check the
+// super trait bound in coherence, causing these impls to overlap.
+//
+// However, we did fail to normalize `<Tensor as TensorValue::Unmasked`
+// which caused the old solver to emit a `Tensor: TensorValue` goal
+// which then failed, causing this test to pass.
+
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+pub trait TensorValue {
+    type Unmasked;
+}
+
+trait TensorCompare<T> {}
+pub trait TensorOp<T>: TensorValue {}
+
+pub struct Tensor;
+impl<T2> TensorCompare<T2> for Tensor {}
+impl<T1, T2> TensorCompare<T2> for T1
+where
+    T1: TensorOp<T2>,
+    T1::Unmasked: Sized,
+{}
+
+
+fn main() {}

tests/ui/issues/issue-48728.rs

@@ -1,7 +1,12 @@
 // Regression test for #48728, an ICE that occurred computing
 // coherence "help" information.
 
-#[derive(Clone)] //~ ERROR conflicting implementations of trait `Clone`
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+#[derive(Clone)]
 struct Node<T: ?Sized>(Box<T>);
 
 impl<T: Clone + ?Sized> Clone for Node<[T]> {