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

compiler/rustc_hir_analysis/src/collect.rs

@@ -34,16 +34,22 @@ use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{DynCompatibilityViolation, ObligationCause};
 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, 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, hir_ty_lowering_dyn_compatibility_violations};
+use rustc_trait_selection::traits::{
+    FulfillmentError, ObligationCtxt, hir_ty_lowering_dyn_compatibility_violations,
+};
 use tracing::{debug, instrument};
 
 use crate::errors;
-use crate::hir_ty_lowering::{FeedConstTy, HirTyLowerer, RegionInferReason};
+use crate::hir_ty_lowering::{
+    FeedConstTy, HirTyLowerer, InherentAssocCandidate, RegionInferReason,
+};
 
 pub(crate) mod dump;
 mod generics_of;
@@ -364,6 +370,58 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         self.tcx.at(span).type_param_predicates((self.item_def_id, def_id, assoc_ident))
     }
 
+    #[instrument(level = "debug", skip(self, _span), ret)]
+    fn select_inherent_assoc_candidates(
+        &self,
+        _span: Span,
+        self_ty: Ty<'tcx>,
+        candidates: Vec<InherentAssocCandidate>,
+    ) -> (Vec<InherentAssocCandidate>, Vec<FulfillmentError<'tcx>>) {
+        assert!(!self_ty.has_infer());
+
+        // We don't just call the normal normalization routine here as we can't provide the
+        // correct `ParamEnv` and it would be wrong to invoke arbitrary trait solving under
+        // the wrong `ParamEnv`. Expanding free aliases doesn't need a `ParamEnv` so we do
+        // this just to make resolution a little bit smarter.
+        let self_ty = self.tcx.expand_free_alias_tys(self_ty);
+        debug!("select_inherent_assoc_candidates: self_ty={:?}", self_ty);
+
+        let candidates = candidates
+            .into_iter()
+            .filter(|&InherentAssocCandidate { impl_, .. }| {
+                let impl_ty = self.tcx().type_of(impl_).instantiate_identity();
+
+                // See comment on doing this operation for `self_ty`
+                let impl_ty = self.tcx.expand_free_alias_tys(impl_ty);
+                debug!("select_inherent_assoc_candidates: impl_ty={:?}", impl_ty);
+
+                // We treat parameters in the self ty as rigid and parameters in the impl ty as infers
+                // because it allows `impl<T> Foo<T>` to unify with `Foo<u8>::IAT`, while also disallowing
+                // `Foo<T>::IAT` from unifying with `impl Foo<u8>`.
+                //
+                // We don't really care about a depth limit here because we're only working with user-written
+                // types and if they wrote a type that would take hours to walk then that's kind of on them. On
+                // the other hand the default depth limit is relatively low and could realistically be hit by
+                // users in normal cases.
+                //
+                // `DeepRejectCtxt` leads to slightly worse IAT resolution than real type equality in cases
+                // where the `impl_ty` has repeated uses of generic parameters. E.g. `impl<T> Foo<T, T>` would
+                // be considered a valid candidate when resolving `Foo<u8, u16>::IAT`.
+                //
+                // Not replacing escaping bound vars in `self_ty` with placeholders also leads to slightly worse
+                // resolution, but it probably won't come up in practice and it would be backwards compatible
+                // to switch over to doing that.
+                ty::DeepRejectCtxt::relate_rigid_infer(self.tcx).types_may_unify_with_depth(
+                    self_ty,
+                    impl_ty,
+                    usize::MAX,
+                )
+            })
+            .collect();
+
+        (candidates, vec![])
+    }
+
     fn lower_assoc_item_path(
         &self,
         span: Span,

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -26,6 +26,7 @@ use rustc_trait_selection::traits::{
 use smallvec::SmallVec;
 use tracing::debug;
 
+use super::InherentAssocCandidate;
 use crate::errors::{
     self, AssocItemConstraintsNotAllowedHere, ManualImplementation, MissingTypeParams,
     ParenthesizedFnTraitExpansion, TraitObjectDeclaredWithNoTraits,
@@ -742,7 +743,7 @@ 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,
@@ -776,8 +777,8 @@ 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

@@ -33,21 +33,22 @@ 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::{DynCompatibilityViolation, ObligationCause};
+use rustc_infer::traits::DynCompatibilityViolation;
+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, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, ParamEnv, Ty, TyCtxt,
-    TypeVisitableExt, TypingMode, Upcast, fold_regions,
+    self, 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;
 use rustc_session::parse::feature_err;
 use rustc_span::{DUMMY_SP, Ident, Span, 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 crate::check::check_abi_fn_ptr;
@@ -99,6 +100,13 @@ 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.
 ///
@@ -148,6 +156,13 @@ 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 a path to an associated item (of a trait) to a projection.
     ///
     /// This method has to be defined by the concrete lowering context because
@@ -1449,48 +1464,32 @@ 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.clone());
 
-        // 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.report_unresolved_inherent_assoc_item(
+                    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.report_ambiguous_inherent_assoc_item(
+                    name,
+                    candidates.into_iter().map(|cand| cand.assoc_item).collect(),
+                    span,
+                )),
+            }?;
 
         self.check_assoc_item(assoc_item, name, def_scope, block, span);
 
@@ -1507,78 +1506,6 @@ 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.report_unresolved_inherent_assoc_item(
-                name,
-                self_ty,
-                candidates,
-                fulfillment_errors,
-                span,
-                assoc_tag,
-            )),
-
-            &[applicable_candidate] => Ok(applicable_candidate),
-
-            &[_, ..] => Err(self.report_ambiguous_inherent_assoc_item(
-                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.