"Small vector" optimization for Rust: store up to a small number of items on the stack
```rust use smallvec::{SmallVec, smallvec};
// This SmallVec can hold up to 4 items on the stack: let mut v: SmallVec<[i32; 4]> = smallvec![1, 2, 3, 4];
// It will automatically move its contents to the heap if // contains more than four items: v.push(5);
// SmallVec points to a slice, so you can use normal slice // indexing and other methods to access its contents: v[0] = v[1] + v[2]; v.sort(); ```