Rollup merge of #110791 - compiler-errors:negative-bounds, r=oli-obk

Implement negative bounds for internal testing purposes

Implements partial support the `!` negative polarity on trait bounds. This is incomplete, but should allow us to at least be able to play with the feature.

Not even gonna consider them as a public-facing feature, but I'm implementing them because would've been nice to have in UI tests, for example in #110671.

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -530,6 +530,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         associated_ty: Option<(&'static str, Ty<'tcx>)>,
         mut body_id: LocalDefId,
     ) {
+        if trait_pred.skip_binder().polarity == ty::ImplPolarity::Negative {
+            return;
+        }
+
         let trait_pred = self.resolve_numeric_literals_with_default(trait_pred);
 
         let self_ty = trait_pred.skip_binder().self_ty();

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>) {