From 4a8cfe9d14f479ebc2e156f08ac33e8b4ed0d08a Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 12 Oct 2022 05:10:29 +0000
Subject: [PATCH] Sort elaborated existential predicates in object_ty_for_trait

---
 .../src/traits/object_safety.rs               | 29 ++++++++++++-------
 src/test/ui/object-safety/issue-102933.rs     | 25 ++++++++++++++++
 2 files changed, 43 insertions(+), 11 deletions(-)
 create mode 100644 src/test/ui/object-safety/issue-102933.rs

diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 1afa04007b8..545524f63a7 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -581,17 +581,24 @@ fn object_ty_for_trait<'tcx>(
     });
     debug!(?trait_predicate);
 
-    let elaborated_predicates = elaborate_trait_ref(tcx, trait_ref).filter_map(|obligation| {
-        debug!(?obligation);
-        let pred = obligation.predicate.to_opt_poly_projection_pred()?;
-        Some(pred.map_bound(|p| {
-            ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
-                item_def_id: p.projection_ty.item_def_id,
-                substs: p.projection_ty.substs,
-                term: p.term,
-            })
-        }))
-    });
+    let mut elaborated_predicates: Vec<_> = elaborate_trait_ref(tcx, trait_ref)
+        .filter_map(|obligation| {
+            debug!(?obligation);
+            let pred = obligation.predicate.to_opt_poly_projection_pred()?;
+            Some(pred.map_bound(|p| {
+                ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
+                    item_def_id: p.projection_ty.item_def_id,
+                    substs: p.projection_ty.substs,
+                    term: p.term,
+                })
+            }))
+        })
+        .collect();
+    // NOTE: Since #37965, the existential predicates list has depended on the
+    // list of predicates to be sorted. This is mostly to enforce that the primary
+    // predicate comes first.
+    elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
+    elaborated_predicates.dedup();
 
     let existential_predicates = tcx
         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(elaborated_predicates));
diff --git a/src/test/ui/object-safety/issue-102933.rs b/src/test/ui/object-safety/issue-102933.rs
new file mode 100644
index 00000000000..843391cffb2
--- /dev/null
+++ b/src/test/ui/object-safety/issue-102933.rs
@@ -0,0 +1,25 @@
+// check-pass
+
+use std::future::Future;
+
+pub trait Service {
+    type Response;
+    type Future: Future<Output = Self::Response>;
+}
+
+pub trait A1: Service<Response = i32> {}
+
+pub trait A2: Service<Future = Box<dyn Future<Output = i32>>> + A1 {
+    fn foo(&self) {}
+}
+
+pub trait B1: Service<Future = Box<dyn Future<Output = i32>>> {}
+
+pub trait B2: Service<Response = i32> + B1 {
+    fn foo(&self) {}
+}
+
+fn main() {
+    let x: &dyn A2 = todo!();
+    let x: &dyn B2 = todo!();
+}