Auto merge of #104205 - clubby789:grow-rc, r=thomcc

Attempt to reuse `Vec<T>` backing storage for `Rc/Arc<[T]>`

If a `Vec<T>` has sufficient capacity to store the inner `RcBox<[T]>`, we can just reuse the existing allocation and shift the elements up, instead of making a new allocation.

library/alloc/tests/arc.rs

@@ -210,3 +210,18 @@ 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`
 }
+
+#[test]
+fn arc_from_vec_opt() {
+    let mut v = Vec::with_capacity(64);
+    v.push(0usize);
+    let addr = v.as_ptr().cast::<u8>();
+    let arc: Arc<[_]> = v.into();
+    unsafe {
+        assert_eq!(
+            arc.as_ptr().cast::<u8>().offset_from(addr),
+            (std::mem::size_of::<usize>() * 2) as isize,
+            "Vector allocation not reused"
+        );
+    }
+}

library/alloc/tests/rc.rs

@@ -206,3 +206,18 @@ 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::rc::Weak`
 }
+
+#[test]
+fn rc_from_vec_opt() {
+    let mut v = Vec::with_capacity(64);
+    v.push(0usize);
+    let addr = v.as_ptr().cast::<u8>();
+    let rc: Rc<[_]> = v.into();
+    unsafe {
+        assert_eq!(
+            rc.as_ptr().cast::<u8>().offset_from(addr),
+            (std::mem::size_of::<usize>() * 2) as isize,
+            "Vector allocation not reused"
+        );
+    }
+}