Add supertrait item shadowing for type-level path resolution

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -405,9 +405,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
     pub(super) fn report_ambiguous_assoc_item(
         &self,
-        bound1: ty::PolyTraitRef<'tcx>,
-        bound2: ty::PolyTraitRef<'tcx>,
-        matching_candidates: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
+        matching_candidates: &[ty::PolyTraitRef<'tcx>],
         qself: AssocItemQSelf,
         assoc_tag: ty::AssocTag,
         assoc_ident: Ident,
@@ -439,7 +437,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // predicates!).
         // FIXME: Turn this into a structured, translatable & more actionable suggestion.
         let mut where_bounds = vec![];
-        for bound in [bound1, bound2].into_iter().chain(matching_candidates) {
+        for &bound in matching_candidates {
             let bound_id = bound.def_id();
             let assoc_item = tcx.associated_items(bound_id).find_by_ident_and_kind(
                 tcx,

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -24,6 +24,7 @@ use std::{assert_matches, slice};
 use rustc_abi::FIRST_VARIANT;
 use rustc_ast::LitKind;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
+use rustc_data_structures::sso::SsoHashSet;
 use rustc_errors::codes::*;
 use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, ErrorGuaranteed, FatalError, StashKey,
@@ -1262,6 +1263,74 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         )
     }
 
+    /// When there are multiple traits which contain an identically named
+    /// associated item, this function eliminates any traits which are a
+    /// supertrait of another candidate trait.
+    ///
+    /// This is the type-level analogue of
+    /// `rustc_hir_typeck::method::probe::ProbeContext::collapse_candidates_to_subtrait_pick`;
+    /// keep both implementations in sync.
+    ///
+    /// This implements RFC #3624.
+    fn collapse_candidates_to_subtrait_pick(
+        &self,
+        matching_candidates: &[ty::PolyTraitRef<'tcx>],
+    ) -> Option<ty::PolyTraitRef<'tcx>> {
+        if !self.tcx().features().supertrait_item_shadowing() {
+            return None;
+        }
+
+        let mut child_trait = matching_candidates[0];
+        let mut supertraits: SsoHashSet<_> =
+            traits::supertrait_def_ids(self.tcx(), child_trait.def_id()).collect();
+
+        let mut remaining_candidates: Vec<_> = matching_candidates[1..].iter().copied().collect();
+        while !remaining_candidates.is_empty() {
+            let mut made_progress = false;
+            let mut next_round = vec![];
+
+            for remaining_trait in remaining_candidates {
+                if supertraits.contains(&remaining_trait.def_id()) {
+                    made_progress = true;
+                    continue;
+                }
+
+                // This candidate is not a supertrait of the `child_trait`.
+                // Check if it's a subtrait of the `child_trait`, instead.
+                // If it is, then it must have been a subtrait of every
+                // other pick we've eliminated at this point. It will
+                // take over at this point.
+                let remaining_trait_supertraits: SsoHashSet<_> =
+                    traits::supertrait_def_ids(self.tcx(), remaining_trait.def_id()).collect();
+                if remaining_trait_supertraits.contains(&child_trait.def_id()) {
+                    child_trait = remaining_trait;
+                    supertraits = remaining_trait_supertraits;
+                    made_progress = true;
+                    continue;
+                }
+
+                // Neither `child_trait` or the current candidate are
+                // supertraits of each other.
+                // Don't bail here, since we may be comparing two supertraits
+                // of a common subtrait. These two supertraits won't be related
+                // at all, but we will pick them up next round when we find their
+                // child as we continue iterating in this round.
+                next_round.push(remaining_trait);
+            }
+
+            if made_progress {
+                // If we've made progress, iterate again.
+                remaining_candidates = next_round;
+            } else {
+                // Otherwise, we must have at least two candidates which
+                // are not related to each other at all.
+                return None;
+            }
+        }
+
+        Some(child_trait)
+    }
+
     /// Search for a single trait bound whose trait defines the associated item given by
     /// `assoc_ident`.
     ///
@@ -1296,10 +1365,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         };
 
         if let Some(bound2) = matching_candidates.next() {
+            let all_matching_candidates: Vec<_> =
+                [bound1, bound2].into_iter().chain(matching_candidates).collect();
+            if let Some(bound) = self.collapse_candidates_to_subtrait_pick(&all_matching_candidates)
+            {
+                return Ok(bound);
+            }
+
             return Err(self.report_ambiguous_assoc_item(
-                bound1,
-                bound2,
-                matching_candidates,
+                &all_matching_candidates,
                 qself,
                 assoc_tag,
                 assoc_ident,

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -2359,6 +2359,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     /// multiple conflicting picks if there is one pick whose trait container is a subtrait
     /// of the trait containers of all of the other picks.
     ///
+    /// This is the method-probe analogue of
+    /// `rustc_hir_analysis::hir_ty_lowering::HirTyLowerer::collapse_candidates_to_subtrait_pick`;
+    /// keep both implementations in sync.
+    ///
     /// This implements RFC #3624.
     fn collapse_candidates_to_subtrait_pick(
         &self,

tests/ui/README.md

@@ -1341,6 +1341,10 @@ Generic collection of tests for suggestions, when no more specific directories a
 
 **FIXME**: Some overlap with `tests/ui/did_you_mean/`, that directory should probably be moved under here.
 
+## `tests/ui/supertrait-shadowing/`
+
+Tests for supertrait item shadowing (RFC 3624).
+
 ## `tests/ui/svh/`: Strict Version Hash
 
 Tests on the *Strict Version Hash* (SVH, also known as the "crate hash").