Move SliceIndex type parameter to parameters

[SOF3]

If I have a struct like this:

struct T1 {}
struct T2 {}

pub struct Aggregate {
    v1: Vec<T1>,
    v2: Vec<T2>,
}

impl Aggregate {
    pub fn compute<R>(&self, range: R) -> Option<u32> {
        for v in self.v1.get(range)? {
            output += v.compute();
        }
        for v in self.v2.get(range)? {
            output += v.compute();
        }
        Some(output)
    }
}

The type bound for R would need to be R: SliceIndex<[T1]> + SliceIndex<[T2]>, which exposes internal implementation details.

I propose moving the <T> in SliceIndex<T> to the methods and leverage GATs:

trait SliceIndex : Sized {
    type Output<T: ?Sized>: ?Sized;
    fn get<T: ?Sized>(self, slice: &T) -> Option<&Self::Output<T>>;
    fn get_mut<T: ?Sized>(self, slice: &mut T) -> Option<&mut Self::Output<T>>;
    // ...
}

[shepmaster]

How could you remove the type parameter without breaking backwards compatibility? This compiles on stable:

fn it_is_stable(_: impl std::slice::SliceIndex<i32>) {}

[SOF3]

Currently the trait is sealed, so the usage pattern is quite limited. Can we do something in an edition change to remove the unnecessary type bounds somehow? Or just accept an unused type bound? Probably wouldn't work with impl SliceIndex<[i32], Output = [i32]> though.