Don't build ParamEnv and do trait solving in ItemCtxts when lowering IATs

compiler/rustc_hir_analysis/src/collect.rs

@@ -35,17 +35,22 @@ use rustc_infer::traits::ObligationCause;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
-use rustc_middle::ty::{self, AdtKind, Const, IsSuggestable, Ty, TyCtxt, TypingMode, fold_regions};
+use rustc_middle::ty::{
+    self, AdtKind, Const, IsSuggestable, Ty, TyCtxt, TypeFolder, TypeSuperFoldable,
+    TypeVisitableExt, TypingMode, fold_regions,
+};
 use rustc_middle::{bug, span_bug};
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, sym};
 use rustc_trait_selection::error_reporting::traits::suggestions::NextTypeParamName;
 use rustc_trait_selection::infer::InferCtxtExt;
-use rustc_trait_selection::traits::ObligationCtxt;
+use rustc_trait_selection::traits::{FulfillmentError, ObligationCtxt};
 use tracing::{debug, instrument};
 
 use crate::errors;
 use crate::hir_ty_lowering::errors::assoc_tag_str;
-use crate::hir_ty_lowering::{FeedConstTy, HirTyLowerer, RegionInferReason};
+use crate::hir_ty_lowering::{
+    FeedConstTy, HirTyLowerer, InherentAssocCandidate, RegionInferReason,
+};
 
 pub(crate) mod dump;
 mod generics_of;
@@ -444,6 +449,96 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         self.tcx.at(span).type_param_predicates((self.item_def_id, def_id, assoc_ident))
     }
 
+    fn select_inherent_assoc_candidates(
+        &self,
+        span: Span,
+        self_ty: Ty<'tcx>,
+        candidates: &Vec<InherentAssocCandidate>,
+    ) -> (Vec<InherentAssocCandidate>, Vec<FulfillmentError<'tcx>>) {
+        // This is all rather hacky. We attempt to unify impl headers against this self ty
+        // (even though we aren't really in a position to do so) as users of IATs in signatures
+        // are unable to explicitly disambiguate which impl to use other than by specifying the
+        // self type.
+
+        struct ReplaceAliasesWithInfer<'tcx, 'a>(&'a InferCtxt<'tcx>);
+
+        impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReplaceAliasesWithInfer<'tcx, '_> {
+            fn cx(&self) -> TyCtxt<'tcx> {
+                self.0.tcx
+            }
+
+            fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+                if let ty::Alias(_, _) = ty.kind() {
+                    return self.0.next_ty_var(DUMMY_SP);
+                }
+
+                ty.super_fold_with(self)
+            }
+
+            fn fold_const(&mut self, ct: Const<'tcx>) -> Const<'tcx> {
+                if let ty::ConstKind::Unevaluated(_) = ct.kind() {
+                    return self.0.next_const_var(DUMMY_SP);
+                }
+
+                ct.super_fold_with(self)
+            }
+        }
+
+        assert!(!self_ty.has_infer());
+        let infcx =
+            self.tcx().infer_ctxt().ignoring_regions().build(TypingMode::non_body_analysis());
+
+        // We replace all the aliases in the self type with infer vars to avoid filtering out
+        // candidates that only differ in how normalized alias is. This shouldn't be necessary
+        // under the new solver which emits alias relate goals instead of expecting inputs to
+        // already be fully normalized. FIXME(-Znext-solver)
+        //
+        // We don't just call the normal normalization routine here as we can't provide the
+        // correct `ParamEnv` and it seems dubious to invoke arbitrary trait solving under
+        // the wrong `ParamEnv`.
+        let self_ty = ReplaceAliasesWithInfer(&infcx).fold_ty(self_ty);
+
+        let mut universes = if self_ty.has_escaping_bound_vars() {
+            vec![None; self_ty.outer_exclusive_binder().as_usize()]
+        } else {
+            vec![]
+        };
+
+        let candidates = rustc_trait_selection::traits::with_replaced_escaping_bound_vars(
+            &infcx,
+            &mut universes,
+            self_ty,
+            |self_ty| {
+                infcx.probe(|_| {
+                    candidates
+                        .into_iter()
+                        .filter(|&&InherentAssocCandidate { impl_, .. }| {
+                            let impl_ty = self
+                                .tcx()
+                                .type_of(impl_)
+                                .instantiate(self.tcx(), infcx.fresh_args_for_item(span, impl_));
+                            // See comment on doing this operation for `self_ty`
+                            let impl_ty = ReplaceAliasesWithInfer(&infcx).fold_ty(impl_ty);
+
+                            use rustc_trait_selection::infer::DefineOpaqueTypes;
+                            infcx
+                                // Using an empty `ParamEnv` is pretty weird but we don't actually
+                                // do anything with any of the returned obligations so it doesn't
+                                // matter too much. Building the correct `ParamEnv` would also result
+                                // in undesirable query cycles.
+                                .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
+                                .eq(DefineOpaqueTypes::No, self_ty, impl_ty)
+                                .is_ok()
+                        })
+                        .cloned()
+                        .collect()
+                })
+            },
+        );
+
+        (candidates, vec![])
+    }
+
     fn lower_assoc_shared(
         &self,
         span: Span,

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -24,6 +24,7 @@ use rustc_trait_selection::traits::{
 };
 use smallvec::SmallVec;
 
+use super::InherentAssocCandidate;
 use crate::errors::{
     self, AssocItemConstraintsNotAllowedHere, ManualImplementation, MissingTypeParams,
     ParenthesizedFnTraitExpansion, TraitObjectDeclaredWithNoTraits,
@@ -570,7 +571,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         &self,
         name: Ident,
         self_ty: Ty<'tcx>,
-        candidates: Vec<(DefId, (DefId, DefId))>,
+        candidates: Vec<InherentAssocCandidate>,
         fulfillment_errors: Vec<FulfillmentError<'tcx>>,
         span: Span,
         assoc_tag: ty::AssocTag,
@@ -604,8 +605,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             let type_candidates = candidates
                 .iter()
                 .take(limit)
-                .map(|&(impl_, _)| {
-                    format!("- `{}`", tcx.at(span).type_of(impl_).instantiate_identity())
+                .map(|cand| {
+                    format!("- `{}`", tcx.at(span).type_of(cand.impl_).instantiate_identity())
                 })
                 .collect::<Vec<_>>()
                 .join("\n");

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -33,13 +33,13 @@ use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{self as hir, AnonConst, GenericArg, GenericArgs, HirId};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
-use rustc_infer::traits::ObligationCause;
+use rustc_macros::{TypeFoldable, TypeVisitable};
 use rustc_middle::middle::stability::AllowUnstable;
 use rustc_middle::mir::interpret::LitToConstInput;
 use rustc_middle::ty::print::PrintPolyTraitRefExt as _;
 use rustc_middle::ty::{
-    self, AssocTag, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, ParamEnv, Ty,
-    TyCtxt, TypeVisitableExt, TypingMode, Upcast, fold_regions,
+    self, AssocTag, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, Ty, TyCtxt,
+    TypeVisitableExt, TypingMode, Upcast, fold_regions,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::AMBIGUOUS_ASSOCIATED_ITEMS;
@@ -48,7 +48,7 @@ use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, sym};
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::wf::object_region_bounds;
-use rustc_trait_selection::traits::{self, ObligationCtxt};
+use rustc_trait_selection::traits::{self, FulfillmentError};
 use tracing::{debug, instrument};
 
 use self::errors::assoc_tag_str;
@@ -101,6 +101,13 @@ pub enum RegionInferReason<'a> {
     OutlivesBound,
 }
 
+#[derive(Copy, Clone, TypeFoldable, TypeVisitable, Debug)]
+pub struct InherentAssocCandidate {
+    pub impl_: DefId,
+    pub assoc_item: DefId,
+    pub scope: DefId,
+}
+
 /// A context which can lower type-system entities from the [HIR][hir] to
 /// the [`rustc_middle::ty`] representation.
 ///
@@ -150,6 +157,13 @@ pub trait HirTyLowerer<'tcx> {
         assoc_ident: Ident,
     ) -> ty::EarlyBinder<'tcx, &'tcx [(ty::Clause<'tcx>, Span)]>;
 
+    fn select_inherent_assoc_candidates(
+        &self,
+        span: Span,
+        self_ty: Ty<'tcx>,
+        candidates: &Vec<InherentAssocCandidate>,
+    ) -> (Vec<InherentAssocCandidate>, Vec<FulfillmentError<'tcx>>);
+
     /// Lower an associated type/const (from a trait) to a projection.
     ///
     /// This method has to be defined by the concrete lowering context because
@@ -1513,48 +1527,32 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             .filter_map(|&impl_| {
                 let (item, scope) =
                     self.probe_assoc_item_unchecked(name, assoc_tag, block, impl_)?;
-                Some((impl_, (item.def_id, scope)))
+                Some(InherentAssocCandidate { impl_, assoc_item: item.def_id, scope })
             })
             .collect();
 
-        if candidates.is_empty() {
-            return Ok(None);
-        }
-
-        //
-        // Select applicable inherent associated type candidates modulo regions.
-        //
-
-        // In contexts that have no inference context, just make a new one.
-        // We do need a local variable to store it, though.
-        let infcx = match self.infcx() {
-            Some(infcx) => infcx,
-            None => {
-                assert!(!self_ty.has_infer());
-                &tcx.infer_ctxt().ignoring_regions().build(TypingMode::non_body_analysis())
-            }
-        };
+        let (applicable_candidates, fulfillment_errors) =
+            self.select_inherent_assoc_candidates(span, self_ty, &candidates);
 
-        // FIXME(inherent_associated_types): Acquiring the ParamEnv this early leads to cycle errors
-        // when inside of an ADT (#108491) or where clause.
-        let param_env = tcx.param_env(block.owner);
+        let InherentAssocCandidate { impl_, assoc_item, scope: def_scope } =
+            match &applicable_candidates[..] {
+                &[] => Err(self.complain_about_inherent_assoc_not_found(
+                    name,
+                    self_ty,
+                    candidates,
+                    fulfillment_errors,
+                    span,
+                    assoc_tag,
+                )),
 
-        let mut universes = if self_ty.has_escaping_bound_vars() {
-            vec![None; self_ty.outer_exclusive_binder().as_usize()]
-        } else {
-            vec![]
-        };
+                &[applicable_candidate] => Ok(applicable_candidate),
 
-        let (impl_, (assoc_item, def_scope)) = crate::traits::with_replaced_escaping_bound_vars(
-            infcx,
-            &mut universes,
-            self_ty,
-            |self_ty| {
-                self.select_inherent_assoc_candidates(
-                    infcx, name, span, self_ty, param_env, candidates, assoc_tag,
-                )
-            },
-        )?;
+                &[_, ..] => Err(self.complain_about_ambiguous_inherent_assoc(
+                    name,
+                    candidates.into_iter().map(|cand| cand.assoc_item).collect(),
+                    span,
+                )),
+            }?;
 
         self.check_assoc_item(assoc_item, name, def_scope, block, span);
 
@@ -1571,78 +1569,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         Ok(Some((assoc_item, args)))
     }
 
-    fn select_inherent_assoc_candidates(
-        &self,
-        infcx: &InferCtxt<'tcx>,
-        name: Ident,
-        span: Span,
-        self_ty: Ty<'tcx>,
-        param_env: ParamEnv<'tcx>,
-        candidates: Vec<(DefId, (DefId, DefId))>,
-        assoc_tag: ty::AssocTag,
-    ) -> Result<(DefId, (DefId, DefId)), ErrorGuaranteed> {
-        let tcx = self.tcx();
-        let mut fulfillment_errors = Vec::new();
-
-        let applicable_candidates: Vec<_> = candidates
-            .iter()
-            .copied()
-            .filter(|&(impl_, _)| {
-                infcx.probe(|_| {
-                    let ocx = ObligationCtxt::new_with_diagnostics(infcx);
-                    let self_ty = ocx.normalize(&ObligationCause::dummy(), param_env, self_ty);
-
-                    let impl_args = infcx.fresh_args_for_item(span, impl_);
-                    let impl_ty = tcx.type_of(impl_).instantiate(tcx, impl_args);
-                    let impl_ty = ocx.normalize(&ObligationCause::dummy(), param_env, impl_ty);
-
-                    // Check that the self types can be related.
-                    if ocx.eq(&ObligationCause::dummy(), param_env, impl_ty, self_ty).is_err() {
-                        return false;
-                    }
-
-                    // Check whether the impl imposes obligations we have to worry about.
-                    let impl_bounds = tcx.predicates_of(impl_).instantiate(tcx, impl_args);
-                    let impl_bounds =
-                        ocx.normalize(&ObligationCause::dummy(), param_env, impl_bounds);
-                    let impl_obligations = traits::predicates_for_generics(
-                        |_, _| ObligationCause::dummy(),
-                        param_env,
-                        impl_bounds,
-                    );
-                    ocx.register_obligations(impl_obligations);
-
-                    let mut errors = ocx.select_where_possible();
-                    if !errors.is_empty() {
-                        fulfillment_errors.append(&mut errors);
-                        return false;
-                    }
-
-                    true
-                })
-            })
-            .collect();
-
-        match &applicable_candidates[..] {
-            &[] => Err(self.complain_about_inherent_assoc_not_found(
-                name,
-                self_ty,
-                candidates,
-                fulfillment_errors,
-                span,
-                assoc_tag,
-            )),
-
-            &[applicable_candidate] => Ok(applicable_candidate),
-
-            &[_, ..] => Err(self.complain_about_ambiguous_inherent_assoc(
-                name,
-                applicable_candidates.into_iter().map(|(_, (candidate, _))| candidate).collect(),
-                span,
-            )),
-        }
-    }
-
     /// Given name and kind search for the assoc item in the provided scope and check if it's accessible[^1].
     ///
     /// [^1]: I.e., accessible in the provided scope wrt. visibility and stability.