fix(list_sync): make sure the public interface allows using the Item type

Before this change the public interface wouldn't allow downstream users
to use the actual items since it wasn't communicated on the function
signature.

Previously a user would run into erorrs like this:
> no field `key` on type `<impl std::iter::Iterator as std::iter::Iterator>::Item`

src/ls.rs

@@ -1,9 +1,29 @@
 //! Functions for iterating over the cache.
+use anyhow::Result;
 use std::path::Path;
 
 use crate::index;
 
 /// Returns a synchronous iterator that lists all cache index entries.
-pub fn list_sync<P: AsRef<Path>>(cache: P) -> impl Iterator {
+pub fn list_sync<P: AsRef<Path>>(cache: P) -> impl Iterator<Item = Result<index::Metadata>> {
     index::ls(cache.as_ref())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile;
+
+    #[test]
+    fn test_list_sync() {
+        // check that the public interface to list elements can actually use the
+        // Iterator::Item
+        let tmp = tempfile::tempdir().unwrap();
+        let dir = tmp.path().to_owned();
+
+        assert!(list_sync(dir)
+            .map(|x| Ok(x?.key))
+            .collect::<Result<Vec<_>>>()
+            .is_err())
+    }
+}