Non-error slicing

In Python and JavaScript, there's a function vec.slice that never throws an error:

vec.slice(x, -y)

means "remove the first x elements from the start and the first y elements from the end". Rust currently throws an error if x is greater than vec.len() - 1. It returns an empty vec when the span includes no elements: that's what you want for many use cases.

It's convenient to do Python/JavaScript like accesses as an idiomatic version of takeing and droping (in the Haskell sense) in Rust.

For clarity, here is an example implementation:

fn slice_vec<T>(vec: &[T], start: isize, end: isize) -> &[T] {
    let len = vec.len() as isize;
    let start = if start >= 0 { start } else { len + start };
    let end = if end >= 0 { end } else { len + end };
    let start = start.max(0) as usize;
    let end = end.min(len).max(start) as usize;
    &vec[start..end]
}

Start by reading the requested slicing semantics and example implementation in the issue, then review the Rust RFC process in this repository. Compare the proposed behavior with existing Rust slice, take, and drop APIs. Done means a decided RFC-level proposal that clearly specifies behavior for positive and negative bounds.