Casting a known NaN value to an integral type

[llogiq]

This is actually undefined behavior, and doesn't even get a warning from rustc.

[Pyriphlegethon]

Is there some list of expressions that can return NaN? The only example I found in that reddit thread was 0.0/0.0.

[llogiq]

As far as I know, 0.0/0.0, -0/0, 0/-0, -0/-0 and just about every float operation that has at least one NaN operand, e.g.

let z = foo();
let x = 0/-0;
let y = x + 1 * z - 3;
y as isize; //~ERROR: Casting a known NaN value to an integral type.

So this involves keeping track of known NaNs. It's similar to what consts does (complete with path chasing), but we don't need the whole expression to be constant, one NaN is sufficient.

[Ryan1729]

As of 1.45, This is no longer UB and NaN in particular is sent to 0.

Might still be worth linting, though, given it's still likely to be a bug, and older version of rust will still see use for a while yet.