Generalize `{Rc,Arc}::make_mut()` to unsized types.

This requires introducing a new internal type `RcUninit` (and
`ArcUninit`), which can own an `RcBox<T>` without requiring it to be
initialized, sized, or a slice. This is similar to `UniqueRc`, but
`UniqueRc` doesn't support the allocator parameter, and there is no
`UniqueArc`.

1 files changed, 18 insertions, 0 deletions

diff --git a/library/alloc/tests/arc.rs b/library/alloc/tests/arc.rs
index d564a30b103..c37a80dca95 100644
--- a/library/alloc/tests/arc.rs
+++ b/library/alloc/tests/arc.rs
@@ -209,3 +209,21 @@ fn weak_may_dangle() {
     // `val` dropped here while still borrowed
     // borrow might be used here, when `val` is dropped and runs the `Drop` code for type `std::sync::Weak`
 }
+
+/// This is similar to the doc-test for `Arc::make_mut()`, but on an unsized type (slice).
+#[test]
+fn make_mut_unsized() {
+    use alloc::sync::Arc;
+
+    let mut data: Arc<[i32]> = Arc::new([10, 20, 30]);
+
+    Arc::make_mut(&mut data)[0] += 1; // Won't clone anything
+    let mut other_data = Arc::clone(&data); // Won't clone inner data
+    Arc::make_mut(&mut data)[1] += 1; // Clones inner data
+    Arc::make_mut(&mut data)[2] += 1; // Won't clone anything
+    Arc::make_mut(&mut other_data)[0] *= 10; // Won't clone anything
+
+    // Now `data` and `other_data` point to different allocations.
+    assert_eq!(*data, [11, 21, 31]);
+    assert_eq!(*other_data, [110, 20, 30]);
+}