fn from_slice(v: &[T]) -> Self {
        unsafe { Arc::copy_from_slice(v) }
    }
Makes a mutable reference into the given `Arc`.

If there are other `Arc` or [`Weak`] pointers to the same allocation,
then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
to ensure unique ownership. This is also referred to as clone-on-write.

Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
any remaining `Weak` pointers.

See also [`get_mut`][get_mut], which will fail rather than cloning.

[clone]: Clone:
benchmarks/haystacks/code/rust-library.rs