1 files changed, 19 insertions, 0 deletions

diff --git a/crates/ra_hir/src/util.rs b/crates/ra_hir/src/util.rs
new file mode 100644
index 000000000..46f423c91
--- /dev/null
+++ b/crates/ra_hir/src/util.rs
@@ -0,0 +1,19 @@
+//! Internal utility functions.
+
+use std::sync::Arc;
+
+/// Helper for mutating `Arc<[T]>` (i.e. `Arc::make_mut` for Arc slices).
+/// The underlying values are cloned if there are other strong references.
+pub(crate) fn make_mut_arc_slice<T: Clone, R>(
+    a: &mut Arc<[T]>,
+    f: impl FnOnce(&mut [T]) -> R,
+) -> R {
+    if let Some(s) = Arc::get_mut(a) {
+        f(s)
+    } else {
+        let mut v = a.to_vec();
+        let r = f(&mut v);
+        *a = Arc::from(v);
+        r
+    }
+}