diff --git a/compiler/rustc_data_structures/src/flock.rs b/compiler/rustc_data_structures/src/flock.rs
index 4f5d8d7ea48..293ef4caac4 100644
--- a/compiler/rustc_data_structures/src/flock.rs
+++ b/compiler/rustc_data_structures/src/flock.rs
@@ -222,6 +222,10 @@ cfg_if! {
                 let msg = "file locks not supported on this platform";
                 Err(io::Error::new(io::ErrorKind::Other, msg))
             }
+
+            pub fn error_unsupported(_err: &io::Error) -> bool {
+                true
+            }
         }
     }
 }