diff --git a/crates/engine/src/autosave.rs b/crates/engine/src/autosave.rs
index 09a63f4f..7c6c7c54 100644
--- a/crates/engine/src/autosave.rs
+++ b/crates/engine/src/autosave.rs
@@ -186,6 +186,7 @@ impl Autosave {
         let mut n = 1;
         loop {
             // Not recursive, so a folder that already exists is reported and the next name tried.
+            #[cfg_attr(not(unix), allow(unused_mut))]
             let mut builder = std::fs::DirBuilder::new();
             #[cfg(unix)]
             std::os::unix::fs::DirBuilderExt::mode(&mut builder, 0o700);