new solver: prefer trivial builtin impls over where-clauses

for now, only builtin `Sized` impls are tracked as being `Trivial`

1 files changed, 26 insertions, 2 deletions

diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index f4d7c3ce76c..4faa243c02a 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -8,6 +8,7 @@ use rustc_type_ir::lang_items::TraitSolverLangItem;
 use rustc_type_ir::solve::CanonicalResponse;
 use rustc_type_ir::visit::TypeVisitableExt as _;
 use rustc_type_ir::{self as ty, Interner, TraitPredicate, TypingMode, Upcast as _, elaborate};
+use smallvec::SmallVec;
 use tracing::{instrument, trace};
 
 use crate::delegate::SolverDelegate;
@@ -225,7 +226,7 @@ where
         }
 
         ecx.probe_and_evaluate_goal_for_constituent_tys(
-            CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
+            CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial),
             goal,
             structural_traits::instantiate_constituent_tys_for_sized_trait,
         )
@@ -1194,7 +1195,30 @@ where
             };
         }
 
-        // FIXME: prefer trivial builtin impls
+        // We prefer trivial builtin candidates, i.e. builtin impls without any
+        // nested requirements, over all others. This is a fix for #53123 and
+        // prevents where-bounds from accidentally extending the lifetime of a
+        // variable.
+        if candidates
+            .iter()
+            .any(|c| matches!(c.source, CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial)))
+        {
+            let trivial_builtin_impls: SmallVec<[_; 1]> = candidates
+                .iter()
+                .filter(|c| {
+                    matches!(c.source, CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial))
+                })
+                .map(|c| c.result)
+                .collect();
+            // There should only ever be a single trivial builtin candidate
+            // as they would otherwise overlap.
+            assert_eq!(trivial_builtin_impls.len(), 1);
+            return if let Some(response) = self.try_merge_responses(&trivial_builtin_impls) {
+                Ok((response, Some(TraitGoalProvenVia::Misc)))
+            } else {
+                Ok((self.bail_with_ambiguity(&trivial_builtin_impls), None))
+            };
+        }
 
         // If there are non-global where-bounds, prefer where-bounds
         // (including global ones) over everything else.