Make negative trait bounds work with the old trait solver

Author: compiler-errors

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -701,6 +701,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         let mut dup_bindings = FxHashMap::default();
         for binding in &assoc_bindings {
+            // Don't register additional associated type bounds for negative bounds,
+            // since we should have emitten an error for them earlier, and they will
+            // not be well-formed!
+            if polarity == ty::ImplPolarity::Negative {
+                self.tcx()
+                    .sess
+                    .delay_span_bug(binding.span, "negative trait bounds should not have bindings");
+                continue;
+            }
+
             // Specify type to assert that error was already reported in `Err` case.
             let _: Result<_, ErrorGuaranteed> = self.add_predicates_for_ast_type_binding(
                 hir_id,

compiler/rustc_infer/src/traits/util.rs

@@ -200,6 +200,10 @@ impl<'tcx, O: Elaboratable<'tcx>> Elaborator<'tcx, O> {
         let bound_predicate = elaboratable.predicate().kind();
         match bound_predicate.skip_binder() {
             ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
+                // Negative trait bounds do not imply any supertrait bounds
+                if data.polarity == ty::ImplPolarity::Negative {
+                    return;
+                }
                 // Get predicates implied by the trait, or only super predicates if we only care about self predicates.
                 let predicates = if self.only_self {
                     tcx.super_predicates_of(data.def_id())

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -57,6 +57,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if obligation.polarity() == ty::ImplPolarity::Negative {
             self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
             self.assemble_candidates_from_impls(obligation, &mut candidates);
+            self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
         } else {
             self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
 
@@ -187,6 +188,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // Keep only those bounds which may apply, and propagate overflow if it occurs.
         for bound in matching_bounds {
+            if bound.skip_binder().polarity != stack.obligation.predicate.skip_binder().polarity {
+                continue;
+            }
+
             // FIXME(oli-obk): it is suspicious that we are dropping the constness and
             // polarity here.
             let wc = self.where_clause_may_apply(stack, bound.map_bound(|t| t.trait_ref))?;

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -328,6 +328,13 @@ impl<'tcx> WfPredicates<'tcx> {
         let tcx = self.tcx;
         let trait_ref = &trait_pred.trait_ref;
 
+        // Negative trait predicates don't require supertraits to hold, just
+        // that their substs are WF.
+        if trait_pred.polarity == ty::ImplPolarity::Negative {
+            self.compute_negative_trait_pred(trait_ref);
+            return;
+        }
+
         // if the trait predicate is not const, the wf obligations should not be const as well.
         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
@@ -393,6 +400,14 @@ impl<'tcx> WfPredicates<'tcx> {
         );
     }
 
+    // Compute the obligations that are required for `trait_ref` to be WF,
+    // given that it is a *negative* trait predicate.
+    fn compute_negative_trait_pred(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
+        for arg in trait_ref.substs {
+            self.compute(arg);
+        }
+    }
+
     /// Pushes the obligations required for `trait_ref::Item` to be WF
     /// into `self.out`.
     fn compute_projection(&mut self, data: ty::AliasTy<'tcx>) {

tests/ui/traits/negative-bounds/simple.stderr

@@ -7,19 +7,7 @@ LL | #![feature(negative_bounds, negative_impls)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0277]: the trait bound `T: !Copy` is not satisfied
-  --> $DIR/simple.rs:6:16
-   |
-LL |     not_copy::<T>();
-   |                ^ the trait `!Copy` is not implemented for `T`
-   |
-note: required by a bound in `not_copy`
-  --> $DIR/simple.rs:3:16
-   |
-LL | fn not_copy<T: !Copy>() {}
-   |                ^^^^^ required by this bound in `not_copy`
-
-error[E0277]: the trait bound `T: !Copy` is not satisfied
-  --> $DIR/simple.rs:10:16
+  --> $DIR/simple.rs:11:16
    |
 LL |     not_copy::<T>();
    |                ^ the trait `!Copy` is not implemented for `T`
@@ -77,6 +65,6 @@ LL + #[derive(Copy)]
 LL | struct NotNecessarilyCopyable;
    |
 
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.