Add `sub_types` docs

diff --git a/compiler/rustc_mir/src/borrow_check/type_check/mod.rs b/compiler/rustc_mir/src/borrow_check/type_check/mod.rs
index b4fe3313e8a1ed15e9ca2eb7a17092f584dfe728..d6c3ba61731c64728ab83736a40c729b5c876fab 100644 (file)
--- a/compiler/rustc_mir/src/borrow_check/type_check/mod.rs
+++ b/compiler/rustc_mir/src/borrow_check/type_check/mod.rs
@@ -1119,6 +1119,7 @@ fn relate_types(
         )
     }
 
+    /// Try to relate `sub <: sup`
     fn sub_types(
         &mut self,
         sub: Ty<'tcx>,