Add ImmutableEqVector::find_slice

[nwin]

Provide an equivalent of &str::find_str().
Closes #15853.

[lilyball]

I am inclined to say find_slice() is a sensible addition, but this is a very naive implementation. Unfortunately I'm not sure if there's a better way to search for arbitrary equatable elements. find_str() has a much better implementation but it relies on being able to search bytes. I'm not sure if the dependence on a u8 value is actually critical to the implementation, but it's possible.

FWIW find_slice() can be done in one line as

let i = haystack.windows(needle.len()).position(|v| v == needle);

Not that I'm saying that this is how it should be implemented, but I am saying that, as long as it's implemented with a naive solution, I'm not sure if there's actually a particularly compelling reason to have it. I'd much rather provide a solution that's more efficient than what people can trivially do on their own.

[nwin]

Well, looking at the implementations of the other functions this one-liner is exactly how it should be implemented. Makes me cry.

[bluss]

@nwin, the simple implementation has a worst case behaviour with complexity O(n²); there exists better algorithms, and maybe the two-way algorithm used for strings can be adapted for this. It doesn't look impossible.

[nwin]

I see the problem. The two-way algorithm needs an ordered alphabet. KMP would be fine for the general case but the implementation is a bit more complex and would profit from unsafe blocks. Dunno if that is worth to push. But as far as I see we require at least Eq for more intelligent algorithms.

[alexcrichton]

Closing due to inactivity. For now we probably shouldn't be including quadratic complexity algorithms where there exist much better solutions, but this seems like a nice function to have!