trait_sel: prefer only nested alias bounds

davidtwco · davidtwco · commit 306cf28083c9 · 2025-10-13T14:17:51.000+01:00
diff --git a/Cargo.lock b/Cargo.lock
@@ -590,7 +590,7 @@ dependencies = [
  "serde_json",
  "tempfile",
  "termize",
- "toml 0.9.7",
+ "toml 0.9.8",
  "ui_test",
  "walkdir",
 ]
@@ -632,7 +632,7 @@ dependencies = [
  "regex-syntax 0.8.5",
  "semver",
  "serde",
- "toml 0.9.7",
+ "toml 0.9.8",
  "unicode-normalization",
  "unicode-script",
  "url",
@@ -1164,7 +1164,7 @@ dependencies = [
  "libc",
  "option-ext",
  "redox_users 0.5.2",
- "windows-sys 0.60.2",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]
@@ -2374,7 +2374,7 @@ version = "0.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "536bfad37a309d62069485248eeaba1e8d9853aaf951caaeaed0585a95346f08"
 dependencies = [
- "windows-sys 0.60.2",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]
@@ -5097,9 +5097,9 @@ dependencies = [
 
 [[package]]
 name = "serde_spanned"
-version = "1.0.2"
+version = "1.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5417783452c2be558477e104686f7de5dae53dba813c28435e0e70f82d9b04ee"
+checksum = "e24345aa0fe688594e73770a5f6d1b216508b4f93484c0026d521acd30134392"
 dependencies = [
  "serde_core",
 ]
@@ -5585,14 +5585,14 @@ dependencies = [
 
 [[package]]
 name = "toml"
-version = "0.9.7"
+version = "0.9.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "00e5e5d9bf2475ac9d4f0d9edab68cc573dc2fd644b0dba36b0c30a92dd9eaa0"
+checksum = "f0dc8b1fb61449e27716ec0e1bdf0f6b8f3e8f6b05391e8497b8b6d7804ea6d8"
 dependencies = [
  "indexmap",
  "serde_core",
- "serde_spanned 1.0.2",
- "toml_datetime 0.7.2",
+ "serde_spanned 1.0.3",
+ "toml_datetime 0.7.3",
  "toml_parser",
  "toml_writer",
  "winnow 0.7.13",
@@ -5609,9 +5609,9 @@ dependencies = [
 
 [[package]]
 name = "toml_datetime"
-version = "0.7.2"
+version = "0.7.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32f1085dec27c2b6632b04c80b3bb1b4300d6495d1e129693bdda7d91e72eec1"
+checksum = "f2cdb639ebbc97961c51720f858597f7f24c4fc295327923af55b74c3c724533"
 dependencies = [
  "serde_core",
 ]
@@ -5645,9 +5645,9 @@ dependencies = [
 
 [[package]]
 name = "toml_parser"
-version = "1.0.3"
+version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4cf893c33be71572e0e9aa6dd15e6677937abd686b066eac3f8cd3531688a627"
+checksum = "c0cbe268d35bdb4bb5a56a2de88d0ad0eb70af5384a99d648cd4b3d04039800e"
 dependencies = [
  "winnow 0.7.13",
 ]
@@ -5660,9 +5660,9 @@ checksum = "5d99f8c9a7727884afe522e9bd5edbfc91a3312b36a77b5fb8926e4c31a41801"
 
 [[package]]
 name = "toml_writer"
-version = "1.0.3"
+version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d163a63c116ce562a22cda521fcc4d79152e7aba014456fb5eb442f6d6a10109"
+checksum = "df8b2b54733674ad286d16267dcfc7a71ed5c776e4ac7aa3c3e2561f7c637bf2"
 
 [[package]]
 name = "tracing"
diff --git a/compiler/rustc_middle/src/traits/select.rs b/compiler/rustc_middle/src/traits/select.rs
@@ -116,8 +116,9 @@ pub enum SelectionCandidate<'tcx> {
 
     /// This is a trait matching with a projected type as `Self`, and we found
     /// an applicable bound in the trait definition. The `usize` is an index
-    /// into the list returned by `tcx.item_bounds`.
-    ProjectionCandidate(usize),
+    /// into the list returned by `tcx.item_bounds` and the `bool` is whether
+    /// this is candidate from recursion on the self type of a projection.
+    ProjectionCandidate(usize, bool),
 
     /// Implementation of a `Fn`-family trait by one of the anonymous types
     /// generated for an `||` expression.
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -9,7 +9,7 @@ use derive_where::derive_where;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::SolverTraitLangItem;
 use rustc_type_ir::search_graph::CandidateHeadUsages;
-use rustc_type_ir::solve::SizedTraitKind;
+use rustc_type_ir::solve::{AliasBoundSource, SizedTraitKind};
 use rustc_type_ir::{
     self as ty, Interner, TypeFlags, TypeFoldable, TypeFolder, TypeSuperFoldable,
     TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor, TypingMode, Upcast,
@@ -451,7 +451,7 @@ where
                         matches!(
                             c.source,
                             CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)
-                                | CandidateSource::AliasBound
+                                | CandidateSource::AliasBound(_)
                         ) && has_no_inference_or_external_constraints(c.result)
                     })
                 {
@@ -651,6 +651,7 @@ where
                 goal,
                 candidates,
                 AliasBoundKind::SelfBounds,
+                AliasBoundSource::NonNested,
             );
         });
     }
@@ -670,6 +671,7 @@ where
         goal: Goal<I, G>,
         candidates: &mut Vec<Candidate<I>>,
         consider_self_bounds: AliasBoundKind,
+        source: AliasBoundSource,
     ) {
         let (kind, alias_ty) = match self_ty.kind() {
             ty::Bool
@@ -711,7 +713,7 @@ where
                     self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                 {
                     candidates.push(Candidate {
-                        source: CandidateSource::AliasBound,
+                        source: CandidateSource::AliasBound(source),
                         result,
                         head_usages: CandidateHeadUsages::default(),
                     });
@@ -735,7 +737,7 @@ where
                 {
                     candidates.extend(G::probe_and_consider_implied_clause(
                         self,
-                        CandidateSource::AliasBound,
+                        CandidateSource::AliasBound(source),
                         goal,
                         assumption,
                         [],
@@ -750,7 +752,7 @@ where
                 {
                     candidates.extend(G::probe_and_consider_implied_clause(
                         self,
-                        CandidateSource::AliasBound,
+                        CandidateSource::AliasBound(source),
                         goal,
                         assumption,
                         [],
@@ -772,6 +774,7 @@ where
                 goal,
                 candidates,
                 AliasBoundKind::NonSelfBounds,
+                AliasBoundSource::Nested,
             ),
             Err(NoSolution) => {}
         }
@@ -1030,7 +1033,7 @@ where
                     item_bound.fold_with(&mut ReplaceOpaque { cx: self.cx(), alias_ty, self_ty });
                 candidates.extend(G::probe_and_match_goal_against_assumption(
                     self,
-                    CandidateSource::AliasBound,
+                    CandidateSource::AliasBound(AliasBoundSource::NonNested),
                     goal,
                     assumption,
                     |ecx| {
diff --git a/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs b/compiler/rustc_next_trait_solver/src/solve/effect_goals.rs
@@ -4,8 +4,8 @@
 use rustc_type_ir::fast_reject::DeepRejectCtxt;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::SolverTraitLangItem;
-use rustc_type_ir::solve::SizedTraitKind;
 use rustc_type_ir::solve::inspect::ProbeKind;
+use rustc_type_ir::solve::{AliasBoundSource, SizedTraitKind};
 use rustc_type_ir::{self as ty, Interner, TypingMode, elaborate};
 use tracing::instrument;
 
@@ -96,7 +96,7 @@ where
         ) {
             candidates.extend(Self::probe_and_match_goal_against_assumption(
                 ecx,
-                CandidateSource::AliasBound,
+                CandidateSource::AliasBound(AliasBoundSource::NonNested),
                 goal,
                 clause,
                 |ecx| {
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -4,7 +4,9 @@ use rustc_type_ir::data_structures::IndexSet;
 use rustc_type_ir::fast_reject::DeepRejectCtxt;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::SolverTraitLangItem;
-use rustc_type_ir::solve::{CandidatePreferenceMode, CanonicalResponse, SizedTraitKind};
+use rustc_type_ir::solve::{
+    AliasBoundSource, CandidatePreferenceMode, CanonicalResponse, SizedTraitKind,
+};
 use rustc_type_ir::{
     self as ty, Interner, Movability, PredicatePolarity, TraitPredicate, TraitRef,
     TypeVisitableExt as _, TypingMode, Upcast as _, elaborate,
@@ -1381,22 +1383,23 @@ where
             return Ok((candidate.result, Some(TraitGoalProvenVia::Misc)));
         }
 
-        let potential_alias_bound_response =
-            candidates.iter().any(|c| matches!(c.source, CandidateSource::AliasBound)).then(|| {
-                let alias_bounds: Vec<_> = candidates
-                    .extract_if(.., |c| matches!(c.source, CandidateSource::AliasBound))
-                    .collect();
-                if let Some((response, _)) = self.try_merge_candidates(&alias_bounds) {
-                    (response, Some(TraitGoalProvenVia::AliasBound))
-                } else {
-                    (self.bail_with_ambiguity(&alias_bounds), None)
-                }
-            });
-
+        // Extract non-nested alias bound candidates, will be preferred over where bounds if
+        // `candidate_preference_mode == CandidatePreferenceMode::Marker`.
         if matches!(candidate_preference_mode, CandidatePreferenceMode::Marker)
-            && let Some(alias_bound_response) = potential_alias_bound_response
+            && candidates.iter().any(|c| {
+                matches!(c.source, CandidateSource::AliasBound(AliasBoundSource::NonNested))
+            })
         {
-            return Ok(alias_bound_response);
+            let alias_bounds: Vec<_> = candidates
+                .extract_if(.., |c| {
+                    matches!(c.source, CandidateSource::AliasBound(AliasBoundSource::NonNested))
+                })
+                .collect();
+            return if let Some((response, _)) = self.try_merge_candidates(&alias_bounds) {
+                Ok((response, Some(TraitGoalProvenVia::AliasBound)))
+            } else {
+                Ok((self.bail_with_ambiguity(&alias_bounds), None))
+            };
         }
 
         // If there are non-global where-bounds, prefer where-bounds
@@ -1446,8 +1449,16 @@ where
             };
         }
 
-        if let Some(response) = potential_alias_bound_response {
-            return Ok(response);
+        // Next, prefer any alias bound (nested or otherwise)..
+        if candidates.iter().any(|c| matches!(c.source, CandidateSource::AliasBound(_))) {
+            let alias_bounds: Vec<_> = candidates
+                .extract_if(.., |c| matches!(c.source, CandidateSource::AliasBound(_)))
+                .collect();
+            return if let Some((response, _)) = self.try_merge_candidates(&alias_bounds) {
+                Ok((response, Some(TraitGoalProvenVia::AliasBound)))
+            } else {
+                Ok((self.bail_with_ambiguity(&alias_bounds), None))
+            };
         }
 
         self.filter_specialized_impls(AllowInferenceConstraints::No, &mut candidates);
diff --git a/compiler/rustc_trait_selection/src/solve/select.rs b/compiler/rustc_trait_selection/src/solve/select.rs
@@ -126,7 +126,9 @@ fn candidate_should_be_dropped_in_favor_of<'tcx>(
         // Prefer dyn candidates over non-dyn candidates. This is necessary to
         // handle the unsoundness between `impl<T: ?Sized> Any for T` and `dyn Any: Any`.
         (
-            CandidateSource::Impl(_) | CandidateSource::ParamEnv(_) | CandidateSource::AliasBound,
+            CandidateSource::Impl(_)
+            | CandidateSource::ParamEnv(_)
+            | CandidateSource::AliasBound(_),
             CandidateSource::BuiltinImpl(BuiltinImplSource::Object { .. }),
         ) => true,
 
@@ -175,7 +177,9 @@ fn to_selection<'tcx>(
                 })
             }
             CandidateSource::BuiltinImpl(builtin) => ImplSource::Builtin(builtin, nested),
-            CandidateSource::ParamEnv(_) | CandidateSource::AliasBound => ImplSource::Param(nested),
+            CandidateSource::ParamEnv(_) | CandidateSource::AliasBound(_) => {
+                ImplSource::Param(nested)
+            }
             CandidateSource::CoherenceUnknowable => {
                 span_bug!(span, "didn't expect to select an unknowable candidate")
             }
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -741,7 +741,7 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
     let mut ambiguous = false;
     let _ = selcx.for_each_item_bound(
         obligation.predicate.self_ty(),
-        |selcx, clause, _| {
+        |selcx, clause, _, _| {
             let Some(clause) = clause.as_projection_clause() else {
                 return ControlFlow::Continue(());
             };
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -208,7 +208,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             let mut distinct_normalized_bounds = FxHashSet::default();
             let _ = self.for_each_item_bound::<!>(
                 placeholder_trait_predicate.self_ty(),
-                |selcx, bound, idx| {
+                |selcx, bound, idx, nested| {
                     let Some(bound) = bound.as_trait_clause() else {
                         return ControlFlow::Continue(());
                     };
@@ -224,18 +224,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         );
 
                         // We checked the polarity already
+                        debug!(?idx, ?nested, ?bound);
                         match selcx.match_normalize_trait_ref(
                             obligation,
                             placeholder_trait_predicate.trait_ref,
                             bound.map_bound(|pred| pred.trait_ref),
                         ) {
                             Ok(None) => {
-                                candidates.vec.push(ProjectionCandidate(idx));
+                                candidates.vec.push(ProjectionCandidate(idx, nested));
                             }
                             Ok(Some(normalized_trait))
                                 if distinct_normalized_bounds.insert(normalized_trait) =>
                             {
-                                candidates.vec.push(ProjectionCandidate(idx));
+                                candidates.vec.push(ProjectionCandidate(idx, nested));
                             }
                             _ => {}
                         }
@@ -825,7 +826,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
 
                 ty::Alias(ty::Opaque, alias) => {
-                    if candidates.vec.iter().any(|c| matches!(c, ProjectionCandidate(_))) {
+                    if candidates.vec.iter().any(|c| matches!(c, ProjectionCandidate(..))) {
                         // We do not generate an auto impl candidate for `impl Trait`s which already
                         // reference our auto trait.
                         //
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -67,7 +67,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)
             }
 
-            ProjectionCandidate(idx) => {
+            ProjectionCandidate(idx, _) => {
                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
                 ImplSource::Param(obligations)
             }
@@ -150,7 +150,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let candidate_predicate = self
             .for_each_item_bound(
                 placeholder_self_ty,
-                |_, clause, clause_idx| {
+                |_, clause, clause_idx, _| {
                     if clause_idx == idx {
                         ControlFlow::Break(clause)
                     } else {
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
diff --git a/compiler/rustc_type_ir/src/solve/mod.rs b/compiler/rustc_type_ir/src/solve/mod.rs
diff --git a/tests/ui/sized-hierarchy/prefer-nested-alias-bound.rs b/tests/ui/sized-hierarchy/prefer-nested-alias-bound.rs
