Perhaps warn on a backwards Range

Recently I did the following:

for i in (idx..0) { 
    //stuff
}

There the type of idx was usize.

This, of course, does not exactly do what is intended (count backwards from idx to 0), and I couldn't find the bug for a while. The iterator does absolutely nothing at all, it doesn't even panic. I think it may be useful to have a warning emitted when this can be detected. I think the following two cases should be surefire:

1. Two indices (a..b) where b and a's values are known to be constant at compile time, and b < a.
2. A terminal index, min in (a..min) where min is the minimum value for the type, e.g. if a is i32 then (a..std::i32::MIN) would tickle it. If a is a usize then (a..0) would tickle it. This is because a range-exclusive Range like .. can only produce an empty iterator if the last index is min, so it's almost certainly an attempt at a backwards iterator.

There may be other provable cases, but these are the easy ones.

The issue names no files, tests, or implementation entry points; begin by reviewing its two proposed compile-time cases and the RFC process. Done means deciding whether a backwards-Range warning belongs in Rust and documenting the accepted scope and validation needed.