Sync `Instance::resolve` with the projection code

4 files changed, 40 insertions, 53 deletions

diff --git a/src/librustc_trait_selection/traits/mod.rs b/src/librustc_trait_selection/traits/mod.rs
index d6204975a7b..5f988506330 100644
--- a/src/librustc_trait_selection/traits/mod.rs
+++ b/src/librustc_trait_selection/traits/mod.rs
@@ -54,7 +54,6 @@ pub use self::project::{
 };
 pub use self::select::{EvaluationCache, SelectionCache, SelectionContext};
 pub use self::select::{EvaluationResult, IntercrateAmbiguityCause, OverflowError};
-pub use self::specialize::find_associated_item;
 pub use self::specialize::specialization_graph::FutureCompatOverlapError;
 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
 pub use self::specialize::{specialization_graph, translate_substs, OverlapError};
diff --git a/src/librustc_trait_selection/traits/project.rs b/src/librustc_trait_selection/traits/project.rs
index cd89111a177..aae0d467563 100644
--- a/src/librustc_trait_selection/traits/project.rs
+++ b/src/librustc_trait_selection/traits/project.rs
@@ -1016,7 +1016,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                         .map_err(|ErrorReported| ())?;
 
                 if node_item.is_final() {
-                    // Non-specializable items are always projectable
+                    // Non-specializable items are always projectable.
                     true
                 } else {
                     // Only reveal a specializable default if we're past type-checking
diff --git a/src/librustc_trait_selection/traits/specialize/mod.rs b/src/librustc_trait_selection/traits/specialize/mod.rs
index f98d8f2fabf..fabd8c89b72 100644
--- a/src/librustc_trait_selection/traits/specialize/mod.rs
+++ b/src/librustc_trait_selection/traits/specialize/mod.rs
@@ -20,7 +20,7 @@ use rustc_errors::struct_span_err;
 use rustc_hir::def_id::DefId;
 use rustc_middle::lint::LintDiagnosticBuilder;
 use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
-use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, TyCtxt};
 use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
 use rustc_session::lint::builtin::ORDER_DEPENDENT_TRAIT_OBJECTS;
 use rustc_span::DUMMY_SP;
@@ -112,48 +112,6 @@ pub fn translate_substs<'a, 'tcx>(
     source_substs.rebase_onto(infcx.tcx, source_impl, target_substs)
 }
 
-/// Given a selected impl described by `impl_data`, returns the
-/// definition and substitutions for the method with the name `name`
-/// the kind `kind`, and trait method substitutions `substs`, in
-/// that impl, a less specialized impl, or the trait default,
-/// whichever applies.
-pub fn find_associated_item<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    item: &ty::AssocItem,
-    substs: SubstsRef<'tcx>,
-    impl_data: &super::VtableImplData<'tcx, ()>,
-) -> (DefId, SubstsRef<'tcx>) {
-    debug!("find_associated_item({:?}, {:?}, {:?}, {:?})", param_env, item, substs, impl_data);
-    assert!(!substs.needs_infer());
-
-    let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
-    let trait_def = tcx.trait_def(trait_def_id);
-
-    if let Ok(ancestors) = trait_def.ancestors(tcx, impl_data.impl_def_id) {
-        match ancestors.leaf_def(tcx, item.ident, item.kind) {
-            Some(node_item) => {
-                let substs = tcx.infer_ctxt().enter(|infcx| {
-                    let param_env = param_env.with_reveal_all();
-                    let substs = substs.rebase_onto(tcx, trait_def_id, impl_data.substs);
-                    let substs = translate_substs(
-                        &infcx,
-                        param_env,
-                        impl_data.impl_def_id,
-                        substs,
-                        node_item.defining_node,
-                    );
-                    infcx.tcx.erase_regions(&substs)
-                });
-                (node_item.item.def_id, substs)
-            }
-            None => bug!("{:?} not found in {:?}", item, impl_data.impl_def_id),
-        }
-    } else {
-        (item.def_id, substs)
-    }
-}
-
 /// Is `impl1` a specialization of `impl2`?
 ///
 /// Specialization is determined by the sets of types to which the impls apply;
diff --git a/src/librustc_ty/instance.rs b/src/librustc_ty/instance.rs
index 447c49e40fb..2b7d7e7e637 100644
--- a/src/librustc_ty/instance.rs
+++ b/src/librustc_ty/instance.rs
@@ -1,9 +1,11 @@
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, Instance, TyCtxt, TypeFoldable};
+use rustc_infer::infer::TyCtxtInferExt;
 use rustc_span::sym;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
+use traits::{Reveal, translate_substs};
 
 use log::debug;
 
@@ -82,21 +84,49 @@ fn resolve_associated_item<'tcx>(
     // the actual function:
     match vtbl {
         traits::VtableImpl(impl_data) => {
-            let (def_id, substs) =
-                traits::find_associated_item(tcx, param_env, trait_item, rcvr_substs, &impl_data);
-
-            let resolved_item = tcx.associated_item(def_id);
+            debug!(
+                "resolving VtableImpl: {:?}, {:?}, {:?}, {:?}",
+                param_env, trait_item, rcvr_substs, impl_data
+            );
+            assert!(!rcvr_substs.needs_infer());
+
+            let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
+            let trait_def = tcx.trait_def(trait_def_id);
+            let leaf_def = trait_def
+                .ancestors(tcx, impl_data.impl_def_id)
+                .ok()?
+                .leaf_def(tcx, trait_item.ident, trait_item.kind)
+                .unwrap_or_else(|| {
+                    bug!("{:?} not found in {:?}", trait_item, impl_data.impl_def_id);
+                });
+            let def_id = leaf_def.item.def_id;
+
+            let substs = tcx.infer_ctxt().enter(|infcx| {
+                let param_env = param_env.with_reveal_all();
+                let substs = rcvr_substs.rebase_onto(tcx, trait_def_id, impl_data.substs);
+                let substs = translate_substs(
+                    &infcx,
+                    param_env,
+                    impl_data.impl_def_id,
+                    substs,
+                    leaf_def.defining_node,
+                );
+                infcx.tcx.erase_regions(&substs)
+            });
 
             // Since this is a trait item, we need to see if the item is either a trait default item
             // or a specialization because we can't resolve those unless we can `Reveal::All`.
             // NOTE: This should be kept in sync with the similar code in
             // `rustc_middle::traits::project::assemble_candidates_from_impls()`.
-            let eligible = if !resolved_item.defaultness.is_default() {
+            let eligible = if leaf_def.is_final() {
+                // Non-specializable items are always projectable.
                 true
-            } else if param_env.reveal == traits::Reveal::All {
-                !trait_ref.needs_subst()
             } else {
-                false
+                // Only reveal a specializable default if we're past type-checking
+                // and the obligation is monomorphic, otherwise passes such as
+                // transmute checking and polymorphic MIR optimizations could
+                // get a result which isn't correct for all monomorphizations.
+                if param_env.reveal == Reveal::All { !trait_ref.needs_subst() } else { false }
             };
 
             if !eligible {