diff --git a/src/main.rs b/src/main.rs
index 6632de7..53339c1 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -301,7 +301,7 @@ fn parse_files(text: &str) -> anyhow::Result<BTreeMap<String, String>> {
     // opening, and a new FILE closes whatever was open.
     let mut files = BTreeMap::new();
     let mut current: Option<(String, String)> = None;
-    let mut close = |current: &mut Option<(String, String)>, files: &mut BTreeMap<String, String>| {
+    let close = |current: &mut Option<(String, String)>, files: &mut BTreeMap<String, String>| {
         if let Some((path, body)) = current.take() {
             files.insert(path, body);
         }