From 4acee4db3dcd03d5a35443e6c5fc0488baf10c67 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sun, 18 May 2025 12:23:05 +0000
Subject: [PATCH] Only assemble impl candidates if there are no candidates that
 would have shadowed it

---
 .../src/solve/assembly/mod.rs                   | 17 ++++++++++++++++-
 ...ained-projection-normalization-2.next.stderr | 12 ++++++------
 ...trained-projection-normalization.next.stderr | 12 ++++++------
 3 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
index c281ef216c788..0993054a14b9d 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -394,9 +394,24 @@ where
 
         match assemble_from {
             AssembleCandidatesFrom::All => {
-                self.assemble_impl_candidates(goal, &mut candidates);
                 self.assemble_builtin_impl_candidates(goal, &mut candidates);
+
+                // UwU
+                if !matches!(self.typing_mode(), TypingMode::Coherence) {
+                    if candidates.iter().any(|cand| {
+                        matches!(
+                            cand.source,
+                            CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)
+                                | CandidateSource::AliasBound
+                                | CandidateSource::BuiltinImpl(BuiltinImplSource::Trivial)
+                        )
+                    }) {
+                        return candidates;
+                    }
+                }
+
                 self.assemble_object_bound_candidates(goal, &mut candidates);
+                self.assemble_impl_candidates(goal, &mut candidates);
             }
             AssembleCandidatesFrom::EnvAndBounds => {}
         }
diff --git a/tests/ui/traits/unconstrained-projection-normalization-2.next.stderr b/tests/ui/traits/unconstrained-projection-normalization-2.next.stderr
index 2da655afa935c..b79a3ff7df07a 100644
--- a/tests/ui/traits/unconstrained-projection-normalization-2.next.stderr
+++ b/tests/ui/traits/unconstrained-projection-normalization-2.next.stderr
@@ -1,9 +1,3 @@
-error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
-  --> $DIR/unconstrained-projection-normalization-2.rs:14:6
-   |
-LL | impl<T: ?Sized> Every for Thing {
-   |      ^ unconstrained type parameter
-
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/unconstrained-projection-normalization-2.rs:16:18
    |
@@ -28,6 +22,12 @@ help: consider relaxing the implicit `Sized` restriction
 LL |     type Assoc: ?Sized;
    |               ++++++++
 
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unconstrained-projection-normalization-2.rs:14:6
+   |
+LL | impl<T: ?Sized> Every for Thing {
+   |      ^ unconstrained type parameter
+
 error[E0282]: type annotations needed
   --> $DIR/unconstrained-projection-normalization-2.rs:20:11
    |
diff --git a/tests/ui/traits/unconstrained-projection-normalization.next.stderr b/tests/ui/traits/unconstrained-projection-normalization.next.stderr
index c52e8dd68aa87..fade0b46f6691 100644
--- a/tests/ui/traits/unconstrained-projection-normalization.next.stderr
+++ b/tests/ui/traits/unconstrained-projection-normalization.next.stderr
@@ -1,9 +1,3 @@
-error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
-  --> $DIR/unconstrained-projection-normalization.rs:13:6
-   |
-LL | impl<T: ?Sized> Every for Thing {
-   |      ^ unconstrained type parameter
-
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/unconstrained-projection-normalization.rs:15:18
    |
@@ -28,6 +22,12 @@ help: consider relaxing the implicit `Sized` restriction
 LL |     type Assoc: ?Sized;
    |               ++++++++
 
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unconstrained-projection-normalization.rs:13:6
+   |
+LL | impl<T: ?Sized> Every for Thing {
+   |      ^ unconstrained type parameter
+
 error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0207, E0277.