Rollup merge of #103488 - oli-obk:impl_trait_for_tait, r=lcnr

Allow opaque types in trait impl headers and rely on coherence to reject unsound cases

r? ````@lcnr````

fixes #99840

compiler/rustc_infer/src/infer/at.rs

@@ -81,6 +81,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 .normalize_fn_sig_for_diagnostic
                 .as_ref()
                 .map(|f| f.clone()),
+            intercrate: self.intercrate,
         }
     }
 }

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -23,11 +23,10 @@ use rustc_index::vec::Idx;
 use rustc_index::vec::IndexVec;
 use rustc_middle::arena::ArenaAllocatable;
 use rustc_middle::mir::ConstraintCategory;
-use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
-use rustc_middle::ty::{self, BoundVar, Const, ToPredicate, Ty, TyCtxt};
+use rustc_middle::ty::{self, BoundVar, ToPredicate, Ty, TyCtxt};
 use rustc_span::Span;
 use std::fmt::Debug;
 use std::iter;
@@ -729,10 +728,6 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
         });
     }
 
-    fn const_equate(&mut self, _a: Const<'tcx>, _b: Const<'tcx>) {
-        span_bug!(self.cause.span(), "generic_const_exprs: unreachable `const_equate`");
-    }
-
     fn normalization() -> NormalizationStrategy {
         NormalizationStrategy::Eager
     }
@@ -741,11 +736,7 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
         true
     }
 
-    fn register_opaque_type_obligations(
-        &mut self,
-        obligations: PredicateObligations<'tcx>,
-    ) -> Result<(), TypeError<'tcx>> {
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.obligations.extend(obligations);
-        Ok(())
     }
 }

compiler/rustc_infer/src/infer/combine.rs

@@ -450,6 +450,15 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             ty::Binder::dummy(predicate),
         ));
     }
+
+    pub fn mark_ambiguous(&mut self) {
+        self.obligations.push(Obligation::new(
+            self.tcx(),
+            self.trace.cause.clone(),
+            self.param_env,
+            ty::Binder::dummy(ty::PredicateKind::Ambiguous),
+        ));
+    }
 }
 
 struct Generalizer<'cx, 'tcx> {
@@ -521,6 +530,11 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.infcx.tcx
     }
+
+    fn intercrate(&self) -> bool {
+        self.infcx.intercrate
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
@@ -533,6 +547,10 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        span_bug!(self.cause.span, "opaque types are handled in `tys`");
+    }
+
     fn binders<T>(
         &mut self,
         a: ty::Binder<'tcx, T>,
@@ -657,6 +675,10 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
                 // relatable.
                 Ok(t)
             }
+            ty::Opaque(def_id, substs) => {
+                let s = self.relate(substs, substs)?;
+                Ok(if s == substs { t } else { self.infcx.tcx.mk_opaque(def_id, s) })
+            }
             _ => relate::super_relate_tys(self, t, t),
         }?;
 
@@ -799,6 +821,11 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         self.infcx.tcx
     }
 
+    fn intercrate(&self) -> bool {
+        assert!(!self.infcx.intercrate);
+        false
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
@@ -811,6 +838,10 @@ impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _variance: ty::Variance,

compiler/rustc_infer/src/infer/equate.rs

@@ -32,6 +32,10 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.fields.tcx()
     }
 
+    fn intercrate(&self) -> bool {
+        self.fields.infcx.intercrate
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.fields.param_env
     }
@@ -40,6 +44,10 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        self.fields.mark_ambiguous();
+    }
+
     fn relate_item_substs(
         &mut self,
         _item_def_id: DefId,

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2937,6 +2937,11 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
         self.0.tcx
     }
 
+    fn intercrate(&self) -> bool {
+        assert!(!self.0.intercrate);
+        false
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         // Unused, only for consts which we treat as always equal
         ty::ParamEnv::empty()
@@ -2950,6 +2955,10 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: relate::Relate<'tcx>>(
         &mut self,
         _variance: ty::Variance,

compiler/rustc_infer/src/infer/glb.rs

@@ -30,6 +30,11 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         "Glb"
     }
 
+    fn intercrate(&self) -> bool {
+        assert!(!self.fields.infcx.intercrate);
+        false
+    }
+
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.fields.tcx()
     }
@@ -42,6 +47,10 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!("mark_ambiguous used outside of coherence");
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

compiler/rustc_infer/src/infer/lub.rs

@@ -30,6 +30,11 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         "Lub"
     }
 
+    fn intercrate(&self) -> bool {
+        assert!(!self.fields.infcx.intercrate);
+        false
+    }
+
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.fields.tcx()
     }
@@ -42,6 +47,10 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!("mark_ambiguous used outside of coherence");
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

compiler/rustc_infer/src/infer/mod.rs

@@ -337,6 +337,26 @@ pub struct InferCtxt<'tcx> {
 
     normalize_fn_sig_for_diagnostic:
         Option<Lrc<dyn Fn(&InferCtxt<'tcx>, ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx>>>,
+
+    /// During coherence we have to assume that other crates may add
+    /// additional impls which we currently don't know about.
+    ///
+    /// To deal with this evaluation should be conservative
+    /// and consider the possibility of impls from outside this crate.
+    /// This comes up primarily when resolving ambiguity. Imagine
+    /// there is some trait reference `$0: Bar` where `$0` is an
+    /// inference variable. If `intercrate` is true, then we can never
+    /// say for sure that this reference is not implemented, even if
+    /// there are *no impls at all for `Bar`*, because `$0` could be
+    /// bound to some type that in a downstream crate that implements
+    /// `Bar`.
+    ///
+    /// Outside of coherence we set this to false because we are only
+    /// interested in types that the user could actually have written.
+    /// In other words, we consider `$0: Bar` to be unimplemented if
+    /// there is no type that the user could *actually name* that
+    /// would satisfy it. This avoids crippling inference, basically.
+    pub intercrate: bool,
 }
 
 /// See the `error_reporting` module for more details.
@@ -552,6 +572,8 @@ pub struct InferCtxtBuilder<'tcx> {
     tcx: TyCtxt<'tcx>,
     defining_use_anchor: DefiningAnchor,
     considering_regions: bool,
+    /// Whether we are in coherence mode.
+    intercrate: bool,
     normalize_fn_sig_for_diagnostic:
         Option<Lrc<dyn Fn(&InferCtxt<'tcx>, ty::PolyFnSig<'tcx>) -> ty::PolyFnSig<'tcx>>>,
 }
@@ -567,6 +589,7 @@ impl<'tcx> TyCtxtInferExt<'tcx> for TyCtxt<'tcx> {
             defining_use_anchor: DefiningAnchor::Error,
             considering_regions: true,
             normalize_fn_sig_for_diagnostic: None,
+            intercrate: false,
         }
     }
 }
@@ -583,6 +606,11 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
         self
     }
 
+    pub fn intercrate(mut self) -> Self {
+        self.intercrate = true;
+        self
+    }
+
     pub fn ignoring_regions(mut self) -> Self {
         self.considering_regions = false;
         self
@@ -622,6 +650,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             defining_use_anchor,
             considering_regions,
             ref normalize_fn_sig_for_diagnostic,
+            intercrate,
         } = *self;
         InferCtxt {
             tcx,
@@ -641,6 +670,7 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             normalize_fn_sig_for_diagnostic: normalize_fn_sig_for_diagnostic
                 .as_ref()
                 .map(|f| f.clone()),
+            intercrate,
         }
     }
 }

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -25,7 +25,7 @@ use crate::infer::combine::ConstEquateRelation;
 use crate::infer::InferCtxt;
 use crate::infer::{ConstVarValue, ConstVariableValue};
 use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
-use crate::traits::PredicateObligation;
+use crate::traits::{Obligation, PredicateObligation};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::TypeError;
@@ -92,11 +92,7 @@ pub trait TypeRelatingDelegate<'tcx> {
         info: ty::VarianceDiagInfo<'tcx>,
     );
 
-    fn const_equate(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
-    fn register_opaque_type_obligations(
-        &mut self,
-        obligations: Vec<PredicateObligation<'tcx>>,
-    ) -> Result<(), TypeError<'tcx>>;
+    fn register_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>);
 
     /// Creates a new universe index. Used when instantiating placeholders.
     fn create_next_universe(&mut self) -> ty::UniverseIndex;
@@ -419,7 +415,7 @@ where
             .infcx
             .handle_opaque_type(a, b, true, &cause, self.delegate.param_env())?
             .obligations;
-        self.delegate.register_opaque_type_obligations(obligations)?;
+        self.delegate.register_obligations(obligations);
         trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
         Ok(a)
     }
@@ -531,6 +527,10 @@ where
         self.infcx.tcx
     }
 
+    fn intercrate(&self) -> bool {
+        self.infcx.intercrate
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.delegate.param_env()
     }
@@ -543,6 +543,17 @@ where
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        let cause = ObligationCause::dummy_with_span(self.delegate.span());
+        let param_env = self.delegate.param_env();
+        self.delegate.register_obligations(vec![Obligation::new(
+            self.tcx(),
+            cause,
+            param_env,
+            ty::Binder::dummy(ty::PredicateKind::Ambiguous),
+        )]);
+    }
+
     #[instrument(skip(self, info), level = "trace", ret)]
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
@@ -800,8 +811,12 @@ impl<'tcx, D> ConstEquateRelation<'tcx> for TypeRelating<'_, 'tcx, D>
 where
     D: TypeRelatingDelegate<'tcx>,
 {
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        self.delegate.const_equate(a, b);
+    fn const_equate_obligation(&mut self, _a: ty::Const<'tcx>, _b: ty::Const<'tcx>) {
+        // We don't have to worry about the equality of consts during borrow checking
+        // as consts always have a static lifetime.
+        // FIXME(oli-obk): is this really true? We can at least have HKL and with
+        // inline consts we may have further lifetimes that may be unsound to treat as
+        // 'static.
     }
 }
 
@@ -898,6 +913,11 @@ where
         self.infcx.tcx
     }
 
+    fn intercrate(&self) -> bool {
+        assert!(!self.infcx.intercrate);
+        false
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.delegate.param_env()
     }
@@ -910,6 +930,10 @@ where
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         variance: ty::Variance,

compiler/rustc_infer/src/infer/outlives/mod.rs

@@ -29,6 +29,7 @@ pub fn explicit_outlives_bounds<'tcx>(
             | ty::PredicateKind::TypeOutlives(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::Ambiguous
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(r_a, r_b)) => {
                 Some(OutlivesBound::RegionSubRegion(r_b, r_a))

compiler/rustc_infer/src/infer/outlives/test_type_match.rs

@@ -136,6 +136,11 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
     fn tag(&self) -> &'static str {
         "Match"
     }
+
+    fn intercrate(&self) -> bool {
+        false
+    }
+
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.tcx
     }
@@ -146,6 +151,10 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
         true
     } // irrelevant
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _: ty::Variance,

compiler/rustc_infer/src/infer/sub.rs

@@ -35,6 +35,11 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
     fn tag(&self) -> &'static str {
         "Sub"
     }
+
+    fn intercrate(&self) -> bool {
+        self.fields.infcx.intercrate
+    }
+
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.fields.infcx.tcx
     }
@@ -47,6 +52,10 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         self.a_is_expected
     }
 
+    fn mark_ambiguous(&mut self) {
+        self.fields.mark_ambiguous()
+    }
+
     fn with_cause<F, R>(&mut self, cause: Cause, f: F) -> R
     where
         F: FnOnce(&mut Self) -> R,

compiler/rustc_infer/src/traits/util.rs

@@ -285,6 +285,7 @@ impl<'tcx> Elaborator<'tcx> {
             ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                 // Nothing to elaborate
             }
+            ty::PredicateKind::Ambiguous => {}
         }
     }
 }