[Pre-RFC] Implicit number type widening

[RalfJung]

@scottmcm

I consider this a non-issue, because arr[usize::MAX] or let idx = -1; arr[idx as usize] will also assert at runtime. So one needs to get the value correct when indexing regardless of the type , so I don’t see any harm in allowing additional types.

Agreed. "Index into that array with this u16/u32/..." is a perfectly well-defined operation. It is partial, just like normal indexing. It could be implemented as

fn get<I>(&self, i: I) -> Option<&T> where usize: TryFrom<I> {
  self.get_with_usize(usize::try_from(i)?)
}

If either the cast or the indexing fails, we get None, otherwise we get Some. There is nothing lossy about this.

Is there any situation in which this method would not do what is intended, would lead to subtle bugs or would generally not be desirable? Off the top of my head, I don't see any.