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/src/rc/tests.rs b/library/alloc/src/rc/tests.rs
index 0f09be7721f..5e2e4beb94a 100644
--- a/library/alloc/src/rc/tests.rs
+++ b/library/alloc/src/rc/tests.rs
@@ -316,6 +316,24 @@ fn test_cowrc_clone_weak() {
     assert!(cow1_weak.upgrade().is_none());
 }
 
+/// This is similar to the doc-test for `Rc::make_mut()`, but on an unsized type (slice).
+#[test]
+fn test_cowrc_unsized() {
+    use std::rc::Rc;
+
+    let mut data: Rc<[i32]> = Rc::new([10, 20, 30]);
+
+    Rc::make_mut(&mut data)[0] += 1; // Won't clone anything
+    let mut other_data = Rc::clone(&data); // Won't clone inner data
+    Rc::make_mut(&mut data)[1] += 1; // Clones inner data
+    Rc::make_mut(&mut data)[2] += 1; // Won't clone anything
+    Rc::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]);
+}
+
 #[test]
 fn test_show() {
     let foo = Rc::new(75);