Rollup merge of #109447 - lcnr:coherence, r=compiler-errors

new solver cleanup + implement coherence

the cleanup:
- change `Certainty::unify_and` to consider ambig + overflow to be ambig
- rename `trait_candidate_should_be_dropped_in_favor_of` to `candidate_should_be_dropped_in_favor_of`
- remove outdated fixme

For coherence I mostly just add an ambiguous candidate if the current trait ref is unknowable. I am doing the same for reservation impl where I also just add an ambiguous candidate.

compiler/rustc_infer/src/infer/mod.rs

@@ -585,8 +585,8 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
         self
     }
 
-    pub fn intercrate(mut self) -> Self {
+    pub fn intercrate(mut self, intercrate: bool) -> Self {
-        self.intercrate = true;
+        self.intercrate = intercrate;
         self
     }
 

compiler/rustc_middle/src/traits/solve.rs

@@ -63,15 +63,14 @@ impl Certainty {
             (Certainty::Yes, Certainty::Yes) => Certainty::Yes,
             (Certainty::Yes, Certainty::Maybe(_)) => other,
             (Certainty::Maybe(_), Certainty::Yes) => self,
-            (Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Overflow)) => {
+            (Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(MaybeCause::Ambiguity)) => {
-                Certainty::Maybe(MaybeCause::Overflow)
-            }
-            // If at least one of the goals is ambiguous, hide the overflow as the ambiguous goal
-            // may still result in failure.
-            (Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(_))
-            | (Certainty::Maybe(_), Certainty::Maybe(MaybeCause::Ambiguity)) => {
                 Certainty::Maybe(MaybeCause::Ambiguity)
             }
+            (Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(MaybeCause::Overflow))
+            | (Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Ambiguity))
+            | (Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Overflow)) => {
+                Certainty::Maybe(MaybeCause::Overflow)
+            }
         }
     }
 }

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -2,7 +2,8 @@
 
 #[cfg(doc)]
 use super::trait_goals::structural_traits::*;
-use super::EvalCtxt;
+use super::{EvalCtxt, SolverMode};
+use crate::traits::coherence;
 use itertools::Itertools;
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::query::NoSolution;
@@ -87,6 +88,8 @@ pub(super) enum CandidateSource {
 pub(super) trait GoalKind<'tcx>: TypeFoldable<TyCtxt<'tcx>> + Copy + Eq {
     fn self_ty(self) -> Ty<'tcx>;
 
+    fn trait_ref(self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx>;
+
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
 
     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;
@@ -244,15 +247,16 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         self.assemble_object_bound_candidates(goal, &mut candidates);
 
+        self.assemble_coherence_unknowable_candidates(goal, &mut candidates);
+
         candidates
     }
 
     /// If the self type of a goal is a projection, computing the relevant candidates is difficult.
     ///
     /// To deal with this, we first try to normalize the self type and add the candidates for the normalized
-    /// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in
+    /// self type to the list of candidates in case that succeeds. We also have to consider candidates with the
-    /// this case as projections as self types add
+    /// projection as a self type as well
-    // FIXME complete the unfinished sentence above
     fn assemble_candidates_after_normalizing_self_ty<G: GoalKind<'tcx>>(
         &mut self,
         goal: Goal<'tcx, G>,
@@ -468,14 +472,40 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         }
     }
 
+    fn assemble_coherence_unknowable_candidates<G: GoalKind<'tcx>>(
+        &mut self,
+        goal: Goal<'tcx, G>,
+        candidates: &mut Vec<Candidate<'tcx>>,
+    ) {
+        match self.solver_mode() {
+            SolverMode::Normal => return,
+            SolverMode::Coherence => {
+                let trait_ref = goal.predicate.trait_ref(self.tcx());
+                match coherence::trait_ref_is_knowable(self.tcx(), trait_ref) {
+                    Ok(()) => {}
+                    Err(_) => match self
+                        .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+                    {
+                        Ok(result) => candidates
+                            .push(Candidate { source: CandidateSource::BuiltinImpl, result }),
+                        // FIXME: This will be reachable at some point if we're in
+                        // `assemble_candidates_after_normalizing_self_ty` and we get a
+                        // universe error. We'll deal with it at this point.
+                        Err(NoSolution) => bug!("coherence candidate resulted in NoSolution"),
+                    },
+                }
+            }
+        }
+    }
+
     #[instrument(level = "debug", skip(self), ret)]
-    pub(super) fn merge_candidates_and_discard_reservation_impls(
+    pub(super) fn merge_candidates(
         &mut self,
         mut candidates: Vec<Candidate<'tcx>>,
     ) -> QueryResult<'tcx> {
         match candidates.len() {
             0 => return Err(NoSolution),
-            1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
+            1 => return Ok(candidates.pop().unwrap().result),
             _ => {}
         }
 
@@ -483,10 +513,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             let mut i = 0;
             'outer: while i < candidates.len() {
                 for j in (0..candidates.len()).filter(|&j| i != j) {
-                    if self.trait_candidate_should_be_dropped_in_favor_of(
+                    if self.candidate_should_be_dropped_in_favor_of(&candidates[i], &candidates[j])
-                        &candidates[i],
+                    {
-                        &candidates[j],
-                    ) {
                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
                         candidates.swap_remove(i);
                         continue 'outer;
@@ -511,11 +539,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             }
         }
 
-        // FIXME: What if there are >1 candidates left with the same response, and one is a reservation impl?
+        Ok(candidates.pop().unwrap().result)
-        Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
     }
 
-    fn trait_candidate_should_be_dropped_in_favor_of(
+    fn candidate_should_be_dropped_in_favor_of(
         &self,
         candidate: &Candidate<'tcx>,
         other: &Candidate<'tcx>,
@@ -528,20 +555,4 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             | (CandidateSource::BuiltinImpl, _) => false,
         }
     }
-
-    fn discard_reservation_impl(&mut self, mut candidate: Candidate<'tcx>) -> Candidate<'tcx> {
-        if let CandidateSource::Impl(def_id) = candidate.source {
-            if let ty::ImplPolarity::Reservation = self.tcx().impl_polarity(def_id) {
-                debug!("Selected reservation impl");
-                // We assemble all candidates inside of a probe so by
-                // making a new canonical response here our result will
-                // have no constraints.
-                candidate.result = self
-                    .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                    .unwrap();
-            }
-        }
-
-        candidate
-    }
 }

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -17,6 +17,7 @@ use rustc_span::DUMMY_SP;
 use std::ops::ControlFlow;
 
 use super::search_graph::{self, OverflowHandler};
+use super::SolverMode;
 use super::{search_graph::SearchGraph, Goal};
 
 pub struct EvalCtxt<'a, 'tcx> {
@@ -78,7 +79,9 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
         &self,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
     ) -> Result<(bool, Certainty), NoSolution> {
-        let mut search_graph = search_graph::SearchGraph::new(self.tcx);
+        let mode = if self.intercrate { SolverMode::Coherence } else { SolverMode::Normal };
+
+        let mut search_graph = search_graph::SearchGraph::new(self.tcx, mode);
 
         let mut ecx = EvalCtxt {
             search_graph: &mut search_graph,
@@ -101,6 +104,10 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
 }
 
 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
+    pub(super) fn solver_mode(&self) -> SolverMode {
+        self.search_graph.solver_mode()
+    }
+
     /// The entry point of the solver.
     ///
     /// This function deals with (coinductive) cycles, overflow, and caching
@@ -120,8 +127,14 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         //
         // The actual solver logic happens in `ecx.compute_goal`.
         search_graph.with_new_goal(tcx, canonical_goal, |search_graph| {
-            let (ref infcx, goal, var_values) =
+            let intercrate = match search_graph.solver_mode() {
-                tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
+                SolverMode::Normal => false,
+                SolverMode::Coherence => true,
+            };
+            let (ref infcx, goal, var_values) = tcx
+                .infer_ctxt()
+                .intercrate(intercrate)
+                .build_with_canonical(DUMMY_SP, &canonical_goal);
             let mut ecx = EvalCtxt {
                 infcx,
                 var_values,

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -9,10 +9,6 @@
 //! FIXME(@lcnr): Write that section. If you read this before then ask me
 //! about it on zulip.
 
-// FIXME: Instead of using `infcx.canonicalize_query` we have to add a new routine which
-// preserves universes and creates a unique var (in the highest universe) for each
-// appearance of a region.
-
 // FIXME: uses of `infcx.at` need to enable deferred projection equality once that's implemented.
 
 use rustc_hir::def_id::DefId;
@@ -41,6 +37,19 @@ mod trait_goals;
 pub use eval_ctxt::{EvalCtxt, InferCtxtEvalExt};
 pub use fulfill::FulfillmentCtxt;
 
+#[derive(Debug, Clone, Copy)]
+enum SolverMode {
+    /// Ordinary trait solving, using everywhere except for coherence.
+    Normal,
+    /// Trait solving during coherence. There are a few notable differences
+    /// between coherence and ordinary trait solving.
+    ///
+    /// Most importantly, trait solving during coherence must not be incomplete,
+    /// i.e. return `Err(NoSolution)` for goals for which a solution exists.
+    /// This means that we must not make any guesses or arbitrary choices.
+    Coherence,
+}
+
 trait CanonicalResponseExt {
     fn has_no_inference_or_external_constraints(&self) -> bool;
 }
@@ -255,7 +264,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             return Err(NoSolution);
         }
 
-        // FIXME(-Ztreat-solver=next): We should instead try to find a `Certainty::Yes` response with
+        // FIXME(-Ztrait-solver=next): We should instead try to find a `Certainty::Yes` response with
         // a subset of the constraints that all the other responses have.
         let one = candidates[0];
         if candidates[1..].iter().all(|resp| resp == &one) {

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -34,7 +34,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         // projection cache in the solver.
         if self.term_is_fully_unconstrained(goal) {
             let candidates = self.assemble_and_evaluate_candidates(goal);
-            self.merge_candidates_and_discard_reservation_impls(candidates)
+            self.merge_candidates(candidates)
         } else {
             let predicate = goal.predicate;
             let unconstrained_rhs = self.next_term_infer_of_kind(predicate.term);
@@ -56,6 +56,10 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         self.self_ty()
     }
 
+    fn trait_ref(self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
+        self.projection_ty.trait_ref(tcx)
+    }
+
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
         self.with_self_ty(tcx, self_ty)
     }

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -1,8 +1,9 @@
 mod cache;
 mod overflow;
 
+pub(super) use overflow::OverflowHandler;
+
 use self::cache::ProvisionalEntry;
-pub(super) use crate::solve::search_graph::overflow::OverflowHandler;
 use cache::ProvisionalCache;
 use overflow::OverflowData;
 use rustc_index::vec::IndexVec;
@@ -11,6 +12,8 @@ use rustc_middle::traits::solve::{CanonicalGoal, Certainty, MaybeCause, QueryRes
 use rustc_middle::ty::TyCtxt;
 use std::{collections::hash_map::Entry, mem};
 
+use super::SolverMode;
+
 rustc_index::newtype_index! {
     pub struct StackDepth {}
 }
@@ -21,6 +24,7 @@ struct StackElem<'tcx> {
 }
 
 pub(super) struct SearchGraph<'tcx> {
+    mode: SolverMode,
     /// The stack of goals currently being computed.
     ///
     /// An element is *deeper* in the stack if its index is *lower*.
@@ -30,14 +34,19 @@ pub(super) struct SearchGraph<'tcx> {
 }
 
 impl<'tcx> SearchGraph<'tcx> {
-    pub(super) fn new(tcx: TyCtxt<'tcx>) -> SearchGraph<'tcx> {
+    pub(super) fn new(tcx: TyCtxt<'tcx>, mode: SolverMode) -> SearchGraph<'tcx> {
         Self {
+            mode,
             stack: Default::default(),
             overflow_data: OverflowData::new(tcx),
             provisional_cache: ProvisionalCache::empty(),
         }
     }
 
+    pub(super) fn solver_mode(&self) -> SolverMode {
+        self.mode
+    }
+
     pub(super) fn is_empty(&self) -> bool {
         self.stack.is_empty() && self.provisional_cache.is_empty()
     }
@@ -245,7 +254,8 @@ impl<'tcx> SearchGraph<'tcx> {
             // dependencies, our non-root goal may no longer appear as child of the root goal.
             //
             // See https://github.com/rust-lang/rust/pull/108071 for some additional context.
-            let should_cache_globally = !self.overflow_data.did_overflow() || self.stack.is_empty();
+            let should_cache_globally = matches!(self.solver_mode(), SolverMode::Normal)
+                && (!self.overflow_data.did_overflow() || self.stack.is_empty());
             if should_cache_globally {
                 tcx.new_solver_evaluation_cache.insert(
                     current_goal.goal,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -2,7 +2,7 @@
 
 use std::iter;
 
-use super::{assembly, EvalCtxt};
+use super::{assembly, EvalCtxt, SolverMode};
 use rustc_hir::def_id::DefId;
 use rustc_hir::LangItem;
 use rustc_infer::traits::query::NoSolution;
@@ -20,6 +20,10 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         self.self_ty()
     }
 
+    fn trait_ref(self, _: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
+        self.trait_ref
+    }
+
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
         self.with_self_ty(tcx, self_ty)
     }
@@ -43,6 +47,22 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             return Err(NoSolution);
         }
 
+        let impl_polarity = tcx.impl_polarity(impl_def_id);
+        // An upper bound of the certainty of this goal, used to lower the certainty
+        // of reservation impl to ambiguous during coherence.
+        let maximal_certainty = match impl_polarity {
+            ty::ImplPolarity::Positive | ty::ImplPolarity::Negative => {
+                match impl_polarity == goal.predicate.polarity {
+                    true => Certainty::Yes,
+                    false => return Err(NoSolution),
+                }
+            }
+            ty::ImplPolarity::Reservation => match ecx.solver_mode() {
+                SolverMode::Normal => return Err(NoSolution),
+                SolverMode::Coherence => Certainty::AMBIGUOUS,
+            },
+        };
+
         ecx.probe(|ecx| {
             let impl_substs = ecx.fresh_substs_for_item(impl_def_id);
             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
@@ -55,7 +75,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                 .into_iter()
                 .map(|pred| goal.with(tcx, pred));
             ecx.add_goals(where_clause_bounds);
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+
+            ecx.evaluate_added_goals_and_make_canonical_response(maximal_certainty)
         })
     }
 
@@ -547,6 +568,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         goal: Goal<'tcx, TraitPredicate<'tcx>>,
     ) -> QueryResult<'tcx> {
         let candidates = self.assemble_and_evaluate_candidates(goal);
-        self.merge_candidates_and_discard_reservation_impls(candidates)
+        self.merge_candidates(candidates)
     }
 }

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -95,8 +95,11 @@ pub fn overlapping_impls(
         return None;
     }
 
-    let infcx =
+    let infcx = tcx
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
+        .infer_ctxt()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .intercrate(true)
+        .build();
     let selcx = &mut SelectionContext::new(&infcx);
     let overlaps =
         overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
@@ -107,8 +110,11 @@ pub fn overlapping_impls(
     // In the case where we detect an error, run the check again, but
     // this time tracking intercrate ambiguity causes for better
     // diagnostics. (These take time and can lead to false errors.)
-    let infcx =
+    let infcx = tcx
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
+        .infer_ctxt()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .intercrate(true)
+        .build();
     let selcx = &mut SelectionContext::new(&infcx);
     selcx.enable_tracking_intercrate_ambiguity_causes();
     Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -4,7 +4,7 @@
 
 pub mod auto_trait;
 mod chalk_fulfill;
-mod coherence;
+pub(crate) mod coherence;
 pub mod const_evaluatable;
 mod engine;
 pub mod error_reporting;

tests/ui/traits/new-solver/coherence/issue-102048.rs

@@ -0,0 +1,44 @@
+// This must fail coherence.
+//
+// Getting this to pass was fairly difficult, so here's an explanation
+// of what's happening:
+//
+// Normalizing projections currently tries to replace them with inference variables
+// while emitting a nested `Projection` obligation. This cannot be done if the projection
+// has bound variables which is the case here.
+//
+// So the projections stay until after normalization. When unifying two projections we
+// currently treat them as if they are injective, so we **incorrectly** unify their
+// substs. This means that coherence for the two impls ends up unifying `?T` and `?U`
+// as it tries to unify `<?T as WithAssoc1<'a>>::Assoc` with `<?U as WithAssoc1<'a>>::Assoc`.
+//
+// `impl1` therefore has the projection `<?T as WithAssoc2<'a>>::Assoc` and we have the
+// assumption `?T: for<'a> WithAssoc2<'a, Assoc = i32>` in the `param_env`, so we normalize
+// that to `i32`. We then try to unify `i32` from `impl1` with `u32` from `impl2` which fails,
+// causing coherence to consider these two impls distinct.
+
+// compile-flags: -Ztrait-solver=next
+pub trait Trait<T> {}
+
+pub trait WithAssoc1<'a> {
+    type Assoc;
+}
+pub trait WithAssoc2<'a> {
+    type Assoc;
+}
+
+// impl 1
+impl<T, U> Trait<for<'a> fn(<T as WithAssoc1<'a>>::Assoc, <U as WithAssoc2<'a>>::Assoc)> for (T, U)
+where
+    T: for<'a> WithAssoc1<'a> + for<'a> WithAssoc2<'a, Assoc = i32>,
+    U: for<'a> WithAssoc2<'a>,
+{
+}
+
+// impl 2
+impl<T, U> Trait<for<'a> fn(<U as WithAssoc1<'a>>::Assoc, u32)> for (T, U) where
+    U: for<'a> WithAssoc1<'a> //~^ ERROR conflicting implementations of trait
+{
+}
+
+fn main() {}

tests/ui/traits/new-solver/coherence/issue-102048.stderr

@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `Trait<for<'a> fn(<_ as WithAssoc1<'a>>::Assoc, <_ as WithAssoc2<'a>>::Assoc)>` for type `(_, _)`
+  --> $DIR/issue-102048.rs:39:1
+   |
+LL | impl<T, U> Trait<for<'a> fn(<T as WithAssoc1<'a>>::Assoc, <U as WithAssoc2<'a>>::Assoc)> for (T, U)
+   | --------------------------------------------------------------------------------------------------- first implementation here
+...
+LL | impl<T, U> Trait<for<'a> fn(<U as WithAssoc1<'a>>::Assoc, u32)> for (T, U) where
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(_, _)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/reservation-impl/coherence-conflict.next.stderr

@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `OtherTrait` for type `()`
+  --> $DIR/coherence-conflict.rs:12:1
+   |
+LL | impl OtherTrait for () {}
+   | ---------------------- first implementation here
+LL | impl<T: MyTrait> OtherTrait for T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/reservation-impl/coherence-conflict.old.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `OtherTrait` for type `()`
-  --> $DIR/coherence-conflict.rs:11:1
+  --> $DIR/coherence-conflict.rs:12:1
    |
 LL | impl OtherTrait for () {}
    | ---------------------- first implementation here

tests/ui/traits/reservation-impl/coherence-conflict.rs

@@ -1,5 +1,6 @@
 // check that reservation impls are accounted for in negative reasoning.
-
+// revisions: old next
+//[next] compile-flags: -Ztrait-solver=next
 #![feature(rustc_attrs)]
 
 trait MyTrait {}

tests/ui/traits/reservation-impl/no-use.next.stderr

@@ -1,5 +1,5 @@
 error[E0277]: the trait bound `(): MyTrait` is not satisfied
-  --> $DIR/no-use.rs:10:26
+  --> $DIR/no-use.rs:11:26
    |
 LL |     <() as MyTrait>::foo(&());
    |     -------------------- ^^^ the trait `MyTrait` is not implemented for `()`

tests/ui/traits/reservation-impl/no-use.old.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `(): MyTrait` is not satisfied
+  --> $DIR/no-use.rs:11:26
+   |
+LL |     <() as MyTrait>::foo(&());
+   |     -------------------- ^^^ the trait `MyTrait` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `MyTrait` is implemented for `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/reservation-impl/no-use.rs

@@ -1,5 +1,6 @@
 // check that reservation impls can't be used as normal impls in positive reasoning.
-
+// revisions: old next
+//[next] compile-flags: -Ztrait-solver=next
 #![feature(rustc_attrs)]
 
 trait MyTrait { fn foo(&self); }

tests/ui/traits/reservation-impl/non-lattice-ok.rs

@@ -30,6 +30,12 @@
 //
 // [ii]: https://smallcultfollowing.com/babysteps/blog/2016/09/24/intersection-impls/
 
+
+// check that reservation impls can't be used as normal impls in positive reasoning.
+
+// revisions: old next
+//[next] compile-flags: -Ztrait-solver=next
+
 #![feature(rustc_attrs, never_type)]
 
 trait MyTrait {}

tests/ui/traits/reservation-impl/ok.rs

@@ -3,6 +3,9 @@
 // rpass test for reservation impls. Not 100% required because `From` uses them,
 // but still.
 
+// revisions: old next
+//[next] compile-flags: -Ztrait-solver=next
+
 #![feature(rustc_attrs)]
 
 use std::mem;