Add comment to unsafe use.
@@ -26,6 +26,8 @@ macro_rules! concat_keys {
let capacity = concat_keys!(@capacity $($key),+);
let mut buf = Vec::with_capacity(capacity);
+ // Unsafe `set_len` here is safe because we never read from `buf`
+ // before we write all elements to it.
#[allow(unsafe_code)]
unsafe {
buf.set_len(capacity);
