trait_sel: sizedness goals prefer alias candidates

For sizedness, default and auto trait predicates, now prefer non-param
candidates if any exist. As these traits do not have generic parameters,
it never makes sense to prefer an non-alias candidate, as there can
never be a more permissive candidate.

Author: davidtwco

compiler/rustc_middle/src/ty/context.rs

@@ -495,6 +495,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.is_default_trait(def_id)
     }
 
+    fn is_sizedness_trait(self, def_id: DefId) -> bool {
+        self.is_sizedness_trait(def_id)
+    }
+
     fn as_lang_item(self, def_id: DefId) -> Option<TraitSolverLangItem> {
         lang_item_to_trait_lang_item(self.lang_items().from_def_id(def_id)?)
     }
@@ -1652,6 +1656,10 @@ impl<'tcx> TyCtxt<'tcx> {
             .any(|&default_trait| self.lang_items().get(default_trait) == Some(def_id))
     }
 
+    pub fn is_sizedness_trait(self, def_id: DefId) -> bool {
+        matches!(self.as_lang_item(def_id), Some(LangItem::Sized | LangItem::MetaSized))
+    }
+
     /// Returns a range of the start/end indices specified with the
     /// `rustc_layout_scalar_valid_range` attribute.
     // FIXME(eddyb) this is an awkward spot for this method, maybe move it?

compiler/rustc_middle/src/ty/mod.rs

@@ -59,7 +59,7 @@ pub use rustc_type_ir::fast_reject::DeepRejectCtxt;
 )]
 use rustc_type_ir::inherent;
 pub use rustc_type_ir::relate::VarianceDiagInfo;
-pub use rustc_type_ir::solve::SizedTraitKind;
+pub use rustc_type_ir::solve::{CandidatePreferenceMode, SizedTraitKind};
 pub use rustc_type_ir::*;
 #[allow(hidden_glob_reexports, unused_imports)]
 use rustc_type_ir::{InferCtxtLike, Interner};

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -4,7 +4,7 @@ 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::TraitSolverLangItem;
-use rustc_type_ir::solve::{CanonicalResponse, SizedTraitKind};
+use rustc_type_ir::solve::{CandidatePreferenceMode, CanonicalResponse, SizedTraitKind};
 use rustc_type_ir::{
     self as ty, Interner, Movability, TraitPredicate, TraitRef, TypeVisitableExt as _, TypingMode,
     Upcast as _, elaborate,
@@ -1341,6 +1341,7 @@ where
     #[instrument(level = "debug", skip(self), ret)]
     pub(super) fn merge_trait_candidates(
         &mut self,
+        candidate_preference_mode: CandidatePreferenceMode,
         mut candidates: Vec<Candidate<I>>,
     ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
         if let TypingMode::Coherence = self.typing_mode() {
@@ -1366,6 +1367,26 @@ 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
+                    .iter()
+                    .filter(|c| matches!(c.source, CandidateSource::AliasBound))
+                    .map(|c| c.result)
+                    .collect();
+                if let Some(response) = self.try_merge_responses(&alias_bounds) {
+                    (response, Some(TraitGoalProvenVia::AliasBound))
+                } else {
+                    (self.bail_with_ambiguity(&alias_bounds), None)
+                }
+            });
+
+        if matches!(candidate_preference_mode, CandidatePreferenceMode::Marker)
+            && let Some(alias_bound_response) = potential_alias_bound_response
+        {
+            return Ok(alias_bound_response);
+        }
+
         // If there are non-global where-bounds, prefer where-bounds
         // (including global ones) over everything else.
         let has_non_global_where_bounds = candidates
@@ -1384,17 +1405,8 @@ where
             };
         }
 
-        if candidates.iter().any(|c| matches!(c.source, CandidateSource::AliasBound)) {
-            let alias_bounds: Vec<_> = candidates
-                .iter()
-                .filter(|c| matches!(c.source, CandidateSource::AliasBound))
-                .map(|c| c.result)
-                .collect();
-            return if let Some(response) = self.try_merge_responses(&alias_bounds) {
-                Ok((response, Some(TraitGoalProvenVia::AliasBound)))
-            } else {
-                Ok((self.bail_with_ambiguity(&alias_bounds), None))
-            };
+        if let Some(response) = potential_alias_bound_response {
+            return Ok(response);
         }
 
         self.filter_specialized_impls(AllowInferenceConstraints::No, &mut candidates);
@@ -1429,7 +1441,10 @@ where
         goal: Goal<I, TraitPredicate<I>>,
     ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
         let candidates = self.assemble_and_evaluate_candidates(goal, AssembleCandidatesFrom::All);
-        self.merge_trait_candidates(candidates)
+
+        let candidate_preference_mode =
+            CandidatePreferenceMode::compute(self.cx(), goal.predicate.def_id());
+        self.merge_trait_candidates(candidate_preference_mode, candidates)
     }
 
     fn try_stall_coroutine_witness(

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -28,8 +28,9 @@ use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::TypeErrorToStringExt;
 use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
 use rustc_middle::ty::{
-    self, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate, SizedTraitKind, Ty, TyCtxt,
-    TypeFoldable, TypeVisitableExt, TypingMode, Upcast, elaborate, may_use_unstable_feature,
+    self, CandidatePreferenceMode, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate,
+    SizedTraitKind, Ty, TyCtxt, TypeFoldable, TypeVisitableExt, TypingMode, Upcast, elaborate,
+    may_use_unstable_feature,
 };
 use rustc_span::{Symbol, sym};
 use tracing::{debug, instrument, trace};
@@ -474,7 +475,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         } else {
             let has_non_region_infer = stack.obligation.predicate.has_non_region_infer();
-            if let Some(candidate) = self.winnow_candidates(has_non_region_infer, candidates) {
+            let candidate_preference_mode =
+                CandidatePreferenceMode::compute(self.tcx(), stack.obligation.predicate.def_id());
+            if let Some(candidate) =
+                self.winnow_candidates(has_non_region_infer, candidate_preference_mode, candidates)
+            {
                 self.filter_reservation_impls(candidate)
             } else {
                 Ok(None)
@@ -1824,6 +1829,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
     fn winnow_candidates(
         &mut self,
         has_non_region_infer: bool,
+        candidate_preference_mode: CandidatePreferenceMode,
         mut candidates: Vec<EvaluatedCandidate<'tcx>>,
     ) -> Option<SelectionCandidate<'tcx>> {
         if candidates.len() == 1 {
@@ -1877,6 +1883,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             break;
         }
 
+        let alias_bound = candidates
+            .iter()
+            .filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
+            .try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
+
+        if matches!(candidate_preference_mode, CandidatePreferenceMode::Marker) {
+            match alias_bound {
+                Some(Some(index)) => return Some(ProjectionCandidate(index)),
+                Some(None) => {}
+                None => return None,
+            }
+        }
+
         // The next highest priority is for non-global where-bounds. However, while we don't
         // prefer global where-clauses here, we do bail with ambiguity when encountering both
         // a global and a non-global where-clause.
@@ -1910,10 +1929,6 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         // fairly arbitrary but once again necessary for backwards compatibility.
         // If there are multiple applicable candidates which don't affect type inference,
         // choose the one with the lowest index.
-        let alias_bound = candidates
-            .iter()
-            .filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None })
-            .try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) });
         match alias_bound {
             Some(Some(index)) => return Some(ProjectionCandidate(index)),
             Some(None) => {}

compiler/rustc_type_ir/src/interner.rs

@@ -308,6 +308,8 @@ pub trait Interner:
 
     fn is_default_trait(self, def_id: Self::DefId) -> bool;
 
+    fn is_sizedness_trait(self, def_id: Self::DefId) -> bool;
+
     fn as_lang_item(self, def_id: Self::DefId) -> Option<TraitSolverLangItem>;
 
     fn associated_type_def_ids(self, def_id: Self::DefId) -> impl IntoIterator<Item = Self::DefId>;

compiler/rustc_type_ir/src/solve/mod.rs

@@ -125,6 +125,30 @@ pub struct PredefinedOpaquesData<I: Interner> {
     pub opaque_types: Vec<(ty::OpaqueTypeKey<I>, I::Ty)>,
 }
 
+/// Which trait candidates should be preferred over other candidates? By default, prefer where
+/// bounds over alias bounds. For marker traits, prefer alias bounds over where bounds.
+#[derive(Clone, Copy, Debug)]
+pub enum CandidatePreferenceMode {
+    /// Prefers where bounds over alias bounds
+    Default,
+    /// Prefers alias bounds over where bounds
+    Marker,
+}
+
+impl CandidatePreferenceMode {
+    /// Given `trait_def_id`, which candidate preference mode should be used?
+    pub fn compute<I: Interner>(cx: I, trait_def_id: I::DefId) -> CandidatePreferenceMode {
+        let is_sizedness_or_auto_or_default_goal = cx.is_sizedness_trait(trait_def_id)
+            || cx.trait_is_auto(trait_def_id)
+            || cx.is_default_trait(trait_def_id);
+        if is_sizedness_or_auto_or_default_goal {
+            CandidatePreferenceMode::Marker
+        } else {
+            CandidatePreferenceMode::Default
+        }
+    }
+}
+
 /// Possible ways the given goal can be proven.
 #[derive_where(Clone, Copy, Hash, PartialEq, Eq, Debug; I: Interner)]
 pub enum CandidateSource<I: Interner> {