Don't require associated types with Self: Sized bounds in dyn Trait objects

Author: oli-obk

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1131,6 +1131,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
         }
 
+        for def_ids in associated_types.values_mut() {
+            for def_id in def_ids.clone() {
+                // If the associated type has a `where Self: Sized` bound, we do not need to constrain the associated
+                // type in the `dyn Trait`.
+                if tcx.generics_require_sized_self(def_id) {
+                    def_ids.remove(&def_id);
+                }
+            }
+        }
+
         self.complain_about_missing_associated_types(
             associated_types,
             potential_assoc_types,

compiler/rustc_middle/src/query/mod.rs

@@ -2197,6 +2197,10 @@ rustc_queries! {
         desc { "getting cfg-ed out item names" }
         separate_provide_extern
     }
+
+    query generics_require_sized_self(def_id: DefId) -> bool {
+        desc { "check whether the item has a `where Self: Sized` bound" }
+    }
 }
 
 rustc_query_append! { define_callbacks! }

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -922,5 +922,10 @@ pub fn contains_illegal_impl_trait_in_trait<'tcx>(
 }
 
 pub fn provide(providers: &mut Providers) {
-    *providers = Providers { object_safety_violations, check_is_object_safe, ..*providers };
+    *providers = Providers {
+        object_safety_violations,
+        check_is_object_safe,
+        generics_require_sized_self,
+        ..*providers
+    };
 }

tests/ui/object-safety/assoc_type_bounds_sized.rs

@@ -1,9 +1,11 @@
+// check-pass
+
 trait Foo {
     type Bar
     where
         Self: Sized;
 }
 
-fn foo(_: &dyn Foo) {} //~ ERROR: the value of the associated type `Bar` (from trait `Foo`) must be specified
+fn foo(_: &dyn Foo) {}
 
 fn main() {}