Summary

Add "panic-safe" or "total" alternatives to the existing panicking indexing syntax.

Motivation

SliceExt::get and SliceExt::get_mut can be thought as non-panicking versions of the simple indexing syntax, a[idx], and SliceExt::get_unchecked and SliceExt::get_unchecked_mut can be thought of as unsafe versions with bounds checks elided. However, there is no such equivalent for a[start..end], a[start..], or a[..end]. This RFC proposes such methods to fill the gap.

Detailed design

The get, get_mut, get_unchecked, and get_unchecked_mut will be made generic over usize as well as ranges of usize like slice's Index implementation currently is. This will allow e.g. a.get(start..end) which will behave analagously to a[start..end].

Because methods cannot be overloaded in an ad-hoc manner in the same way that traits may be implemented, we introduce a SliceIndex trait which is implemented by types which can index into a slice:

pub trait SliceIndex<T> {
    type Output: ?Sized;

    fn get(self, slice: &[T]) -> Option<&Self::Output>;
    fn get_mut(self, slice: &mut [T]) -> Option<&mut Self::Output>;
    unsafe fn get_unchecked(self, slice: &[T]) -> &Self::Output;
    unsafe fn get_mut_unchecked(self, slice: &[T]) -> &mut Self::Output;
    fn index(self, slice: &[T]) -> &Self::Output;
    fn index_mut(self, slice: &mut [T]) -> &mut Self::Output;
}

impl<T> SliceIndex<T> for usize {
    type Output = T;
    // ...
}

impl<T, R> SliceIndex<T> for R
    where R: RangeArgument<usize>
{
    type Output = [T];
    // ...
}

And then alter the Index, IndexMut, get, get_mut, get_unchecked, and get_mut_unchecked implementations to be generic over SliceIndex:

impl<T> [T] {
    pub fn get<I>(&self, idx: I) -> Option<I::Output>
        where I: SliceIndex<T>
    {
        idx.get(self)
    }

    pub fn get_mut<I>(&mut self, idx: I) -> Option<I::Output>
        where I: SliceIndex<T>
    {
        idx.get_mut(self)
    }

    pub unsafe fn get_unchecked<I>(&self, idx: I) -> I::Output
        where I: SliceIndex<T>
    {
        idx.get_unchecked(self)
    }

    pub unsafe fn get_mut_unchecked<I>(&mut self, idx: I) -> I::Output
        where I: SliceIndex<T>
    {
        idx.get_mut_unchecked(self)
    }
}

impl<T, I> Index<I> for [T]
    where I: SliceIndex<T>
{
    type Output = I::Output;

    fn index(&self, idx: I) -> &I::Output {
        idx.index(self)
    }
}

impl<T, I> IndexMut<I> for [T]
    where I: SliceIndex<T>
{
    fn index_mut(&self, idx: I) -> &mut I::Output {
        idx.index_mut(self)
    }
}

Drawbacks

The SliceIndex trait is unfortunate - it's tuned for exactly the set of methods it's used by. It only exists because inherent methods cannot be overloaded the same way that trait implementations can be. It would most likely remain unstable indefinitely.
Documentation may suffer. Rustdoc output currently explicitly shows each of the ways you can index a slice, while there will simply be a single generic implementation with this change. This may not be that bad, though. The doc block currently seems to provided the most valuable information to newcomers rather than the trait bound, and that will still be present with this change.

Alternatives

Stay as is.
A previous version of this RFC introduced new get_slice etc methods rather than overloading get etc. This avoids the utility trait but is somewhat less ergonomic.
Instead of one trait amalgamating all of the required methods, we could have one trait per method. This would open a more reasonable door to stabilizing those traits, but adds quite a lot more surface area. Replacing an unstable SliceIndex trait with a collection would be backwards compatible.

Unresolved questions

None

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