Add from_const_with_len about rust-smallvec

[mbrubeck]

I'm open to this idea. I would love it if the function could take a slice instead of an array + length, but I don't know whether that will be possible.

Unfortunately, panic! in const contexts was only stabilized in Rust 1.57, which as far as I am aware would cause an issue since currently the const_new feature only requires Rust 1.51.

We could put the new function behind its own feature gate that requires Rust 1.57.

Also note that, as soon as I have time, I will be switching the main development branch to #284 which will be the basis for smallvec 2.0, which gives us a chance to increase the minimum Rust version and make other breaking changes.

[jakergrossman]

I would love it if the function could take a slice instead of an array + length

I'm not sure how you could specify the logical capacity passing just a slice, though. Maybe I'm missing something.

Edit: Hm I guess you would do something like:

impl<T, const N: usize> SmallVec<[T; N]> {

    // ...

    pub const fn from_const_slice(items: &[T]) -> Self {
        if items.len() >= N {
            panic!("Expected length of slice to be less than or equal to array size");
        }

        SmallVec {
            // ... something here
        }
    }

    // ...

}

I guess I'm not sure how we can constly move a smaller size slice into a bigger array, since we want to move the contiguous slice into the SmallVec.