Add method for consuming a Vec and taking one element.

[canndrew]

Sometimes I want to take a single element from a Vec and throw the rest of the Vec away. I can do this using swap_remove then dropping the Vec (or letting it fall out of scope) but it would be clearer and slightly more efficient if there was a method for doing this directly.

I propose adding this method to Vec:

impl<T> Vec<T> {
    /// Take a single element of a vector and return it, consuming the
    /// vector. If `index` is invalid the original vector is returned.
    pub fn into_element(self, index: usize) -> Result<T, Vec<T>> {
        ...
    }
}

[oli-obk]

You can do vec.into_iter().nth(index), which is pretty concise already, and due to the fact that the Vec iterator is ExactSizeIterator, you'll even get O(1) on the nth operation.

[canndrew]

Aha! I didn't think of that. Although the fact that I didn't think of it is more evidence that this method could be useful.

[oli-obk]

Although the fact that I didn't think of it is more evidence that this method could be useful.

It's Rust... The first thing you should be thinking when you want something is: "can an Iterator solve this?".

[canndrew]

The first thing I think is "does this type have a method that does this?" And if it doesn't it usually should. Also I don't see what ExactSizeIterator has to do with this although I assume std::vec::IntoIter::nth has the behaviour I want.

[oli-obk]

I got confused with C++'s random access iterator

[Stebalien]

Actually, Vec's IntoIter doesn't have an optimized nth because it needs to drop the skipped values. This is totally doable (and would give a significant speedup according to a hastily run benchmark) but no one has stepped up to implement it. See: rust-lang/rust#24214