Warn for unmatchable NaN case.

[cscott]

Rust already warns for unmatchable cases in a match expression. Since NaN != NaN, any use of NaN (f64, float, other?) in a match pattern should generate the same unmatchable case warning.

If the reader is curious, the proper way to match NaN is something like:

match x {
    x if f64::isNaN(x) => { ... }
...
}

For bonus points, the unmatchable case warning for NaN could mention this: "use isNaN in a guard expression instead".

[emillon]

Hi,

I'm not sure what the "bad" way to match NaN would be. Since NaN is not a variant (it's just a constant defined as 0.0/0.0), matching against it (NaN => ...) binds a new variable that shadows NaN. It's an irrefutable pattern so it will definitely be noticed. I may have overlooked something though.

[huonw]

@emillon it is a static, which are valid in patterns.

[emillon]

Oops, thanks the clarification. So the test case is:

use std::float::NaN;

fn main() {
    let x = NaN;
    let isnan = match(x) {
        NaN => true,
        _ => false,
    };
    println (fmt!("%b", isnan))
}

Interesting project to dive in the compiler, let's see how it goes.

[cscott]

Ideally nested uses of NaN will trigger the same error, ie:

   match vector {
      (NaN, _) => true,
      _ => false
    }