Auto merge of #123471 - compiler-errors:match_projection_projections, r=oli-obk

Check def id before calling `match_projection_projections`

When I "inlined" `assemble_candidates_from_predicates` into `for_each_item_bound` in #120584, I forgot to copy over the check that actually made sure the def id of the candidate was equal to the def id of the obligation. This means that we normalize goal a bit too often even if it's not productive to do so.

This PR adds that def id check back.
Fixes #123448

2 files changed, 41 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 9246a41a2bc..56b25b0fe6c 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -793,6 +793,9 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
             let Some(clause) = clause.as_projection_clause() else {
                 return ControlFlow::Continue(());
             };
+            if clause.projection_def_id() != obligation.predicate.def_id {
+                return ControlFlow::Continue(());
+            }
 
             let is_match =
                 selcx.infcx.probe(|_| selcx.match_projection_projections(obligation, clause, true));
diff --git a/tests/ui/traits/make-sure-to-filter-projections-by-def-id.rs b/tests/ui/traits/make-sure-to-filter-projections-by-def-id.rs
new file mode 100644
index 00000000000..27a3aad733c
--- /dev/null
+++ b/tests/ui/traits/make-sure-to-filter-projections-by-def-id.rs
@@ -0,0 +1,38 @@
+//@ check-pass
+
+#![recursion_limit = "1024"]
+// Really high recursion limit ^
+
+// Test that ensures we're filtering projections by def id before matching
+// them in `match_projection_projections`.
+
+use std::ops::{Add, Sub};
+
+pub trait Scalar {}
+
+pub trait VectorCommon: Sized {
+    type T: Scalar;
+}
+
+pub trait VectorOpsByValue<Rhs = Self, Output = Self>:
+    VectorCommon + Add<Rhs, Output = Output> + Sub<Rhs, Output = Output>
+{
+}
+
+pub trait VectorView<'a>:
+    VectorOpsByValue<Self, Self::Owned> + VectorOpsByValue<Self::Owned, Self::Owned>
+{
+    type Owned;
+}
+
+pub trait Vector: VectorOpsByValue<Self> + for<'a> VectorOpsByValue<Self::View<'a>> {
+    type View<'a>: VectorView<'a, T = Self::T, Owned = Self>
+    where
+        Self: 'a;
+}
+
+pub trait MatrixCommon {
+    type V: Vector;
+}
+
+fn main() {}