Rollup merge of rust-lang#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 rust-lang#99840

Author: Manishearth

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -2,9 +2,8 @@ use rustc_infer::infer::nll_relate::{NormalizationStrategy, TypeRelating, TypeRe
 use rustc_infer::infer::NllRegionVariableOrigin;
 use rustc_infer::traits::PredicateObligations;
 use rustc_middle::mir::ConstraintCategory;
-use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::relate::TypeRelation;
-use rustc_middle::ty::{self, Const, Ty};
+use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
 use rustc_trait_selection::traits::query::Fallible;
 
@@ -141,13 +140,6 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, '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.
-    fn const_equate(&mut self, _a: Const<'tcx>, _b: Const<'tcx>) {}
-
     fn normalization() -> NormalizationStrategy {
         NormalizationStrategy::Eager
     }
@@ -156,10 +148,7 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx>
         true
     }
 
-    fn register_opaque_type_obligations(
-        &mut self,
-        obligations: PredicateObligations<'tcx>,
-    ) -> Result<(), TypeError<'tcx>> {
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.type_checker
             .fully_perform_op(
                 self.locations,
@@ -172,6 +161,5 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx>
                 },
             )
             .unwrap();
-        Ok(())
     }
 }

compiler/rustc_hir_analysis/src/check/dropck.rs

@@ -244,6 +244,10 @@ impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
         self.tcx
     }
 
+    fn intercrate(&self) -> bool {
+        false
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
@@ -256,6 +260,10 @@ impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
         true
     }
 
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _: ty::Variance,

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -5,7 +5,6 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{struct_span_err, DelayDm};
 use rustc_errors::{Diagnostic, ErrorGuaranteed};
 use rustc_hir as hir;
-use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IgnoreRegions;
 use rustc_middle::ty::{
@@ -47,58 +46,6 @@ fn do_orphan_check_impl<'tcx>(
     let sp = tcx.def_span(def_id);
     let tr = impl_.of_trait.as_ref().unwrap();
 
-    // Ensure no opaque types are present in this impl header. See issues #76202 and #86411 for examples,
-    // and #84660 where it would otherwise allow unsoundness.
-    if trait_ref.has_opaque_types() {
-        trace!("{:#?}", item);
-        // First we find the opaque type in question.
-        for ty in trait_ref.substs {
-            for ty in ty.walk() {
-                let ty::subst::GenericArgKind::Type(ty) = ty.unpack() else { continue };
-                let ty::Opaque(def_id, _) = *ty.kind() else { continue };
-                trace!(?def_id);
-
-                // Then we search for mentions of the opaque type's type alias in the HIR
-                struct SpanFinder<'tcx> {
-                    sp: Span,
-                    def_id: DefId,
-                    tcx: TyCtxt<'tcx>,
-                }
-                impl<'v, 'tcx> hir::intravisit::Visitor<'v> for SpanFinder<'tcx> {
-                    #[instrument(level = "trace", skip(self, _id))]
-                    fn visit_path(&mut self, path: &'v hir::Path<'v>, _id: hir::HirId) {
-                        // You can't mention an opaque type directly, so we look for type aliases
-                        if let hir::def::Res::Def(hir::def::DefKind::TyAlias, def_id) = path.res {
-                            // And check if that type alias's type contains the opaque type we're looking for
-                            for arg in self.tcx.type_of(def_id).walk() {
-                                if let GenericArgKind::Type(ty) = arg.unpack() {
-                                    if let ty::Opaque(def_id, _) = *ty.kind() {
-                                        if def_id == self.def_id {
-                                            // Finally we update the span to the mention of the type alias
-                                            self.sp = path.span;
-                                            return;
-                                        }
-                                    }
-                                }
-                            }
-                        }
-                        hir::intravisit::walk_path(self, path)
-                    }
-                }
-
-                let mut visitor = SpanFinder { sp, def_id, tcx };
-                hir::intravisit::walk_item(&mut visitor, item);
-                let reported = tcx
-                    .sess
-                    .struct_span_err(visitor.sp, "cannot implement trait on type alias impl trait")
-                    .span_note(tcx.def_span(def_id), "type alias impl trait defined here")
-                    .emit();
-                return Err(reported);
-            }
-        }
-        span_bug!(sp, "opaque type not found, but `has_opaque_types` is set")
-    }
-
     match traits::orphan_check(tcx, item.owner_id.to_def_id()) {
         Ok(()) => {}
         Err(err) => emit_orphan_check_error(

compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs

@@ -517,6 +517,7 @@ fn trait_predicate_kind<'tcx>(
         | ty::PredicateKind::ClosureKind(..)
         | ty::PredicateKind::ConstEvaluatable(..)
         | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::Ambiguous
         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
     }
 }

compiler/rustc_hir_analysis/src/outlives/explicit.rs

@@ -59,6 +59,7 @@ impl<'tcx> ExplicitPredicatesMap<'tcx> {
                     | ty::PredicateKind::Coerce(..)
                     | ty::PredicateKind::ConstEvaluatable(..)
                     | ty::PredicateKind::ConstEquate(..)
+                    | ty::PredicateKind::Ambiguous
                     | ty::PredicateKind::TypeWellFormedFromEnv(..) => (),
                 }
             }

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -702,6 +702,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // code is looking for a self type of an unresolved
                 // inference variable.
                 | ty::PredicateKind::ClosureKind(..)
+                | ty::PredicateKind::Ambiguous
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             },
         )

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -803,6 +803,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 | ty::PredicateKind::TypeOutlives(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
+                | ty::PredicateKind::Ambiguous
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             }
         });

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,