From 73cc2fc79172084f10590f6ad434bf0a480e7773 Mon Sep 17 00:00:00 2001
From: alecmocatta <alec@mocatta.net>
Date: Tue, 5 Nov 2019 13:12:28 +0000
Subject: [PATCH] allow auto trait predicates in where clauses

---
 src/librustc/traits/object_safety.rs | 31 ++++++++++++++++++----------
 src/test/ui/issues/issue-50781-1.rs  | 15 ++++++++++++++
 2 files changed, 35 insertions(+), 11 deletions(-)
 create mode 100644 src/test/ui/issues/issue-50781-1.rs

diff --git a/src/librustc/traits/object_safety.rs b/src/librustc/traits/object_safety.rs
index 8ded1417ee570..eb55730d276b7 100644
--- a/src/librustc/traits/object_safety.rs
+++ b/src/librustc/traits/object_safety.rs
@@ -331,17 +331,26 @@ impl<'tcx> TyCtxt<'tcx> {
             return Some(MethodViolationCode::Generic);
         }
 
-        if self.predicates_of(method.def_id).predicates.iter()
-                // A trait object can't claim to live more than the concrete type,
-                // so outlives predicates will always hold.
-                .cloned()
-                .filter(|(p, _)| p.to_opt_type_outlives().is_none())
-                .collect::<Vec<_>>()
-                // Do a shallow visit so that `contains_illegal_self_type_reference`
-                // may apply it's custom visiting.
-                .visit_tys_shallow(|t| {
-                    self.contains_illegal_self_type_reference(trait_def_id, t)
-                }) {
+        if self
+            .predicates_of(method.def_id)
+            .predicates
+            .iter()
+            .cloned()
+            // A trait object can't claim to live more than the concrete type,
+            // so outlives predicates will always hold. Auto trait predicates
+            // are allowed as they don't affect vtables thus can't introduce
+            // unsoundness.
+            .filter(|(p, _)| {
+                p.to_opt_type_outlives().is_none()
+                    && p.to_opt_poly_trait_ref()
+                        .map_or(true, |trait_ref| !self.trait_is_auto(trait_ref.def_id()))
+            })
+            .collect::<Vec<_>>()
+            // Do a shallow visit so that `contains_illegal_self_type_reference`
+            // may apply it's custom visiting.
+            .visit_tys_shallow(|t| {
+                self.contains_illegal_self_type_reference(trait_def_id, t)
+            }) {
             return Some(MethodViolationCode::WhereClauseReferencesSelf);
         }
 
diff --git a/src/test/ui/issues/issue-50781-1.rs b/src/test/ui/issues/issue-50781-1.rs
new file mode 100644
index 0000000000000..538a343493cdb
--- /dev/null
+++ b/src/test/ui/issues/issue-50781-1.rs
@@ -0,0 +1,15 @@
+// run-pass
+
+#![deny(where_clauses_object_safety)]
+
+trait Y {
+    fn foo(&self) where Self: Send {}
+    fn bar(&self) where Self: Send + Sync {}
+}
+
+impl Y for () {}
+
+fn main() {
+    <dyn Y + Send as Y>::foo(&());
+    <dyn Y + Send + Sync as Y>::bar(&());
+}