```rust
trait Storage {
    type Mem<T>: Deref<Target = [T]> + DerefMut;
}

enum Inline<const N: usize> {}
impl<const N: usize> Storage for Inline<N> {
    type Mem<T> = [T; N];
}

enum Borrowed<'a> {}
impl<'a> Storage for Borrowed<'a> {
    type Mem<T> = &'a mut [T];
}

enum Heap {}
impl Storage for Heap {
    type Mem<T> = Box<[T]>;
}

struct Vec<T, S: Storage> {
    mem: S::Mem<T>,
    len: usize,
}