NaN behaviour of is_sign_positive and is_sign_negative is not like for primitive types

[tspiteri]

extern crate half;
use half::f16;
use std::f32;
use std::mem;
fn main() {
    let nan32 = f32::NAN;
    println!(
        "nan32.is_sign_positive() == nan32.is_sign_negative() -> {}",
        nan32.is_sign_positive() == nan32.is_sign_negative()
    );
    let unan32: u32 = unsafe { mem::transmute(nan32) };
    let negated32: f32 = unsafe { mem::transmute(unan32 ^ 0x8000_0000) };
    println!(
        "nan32.is_sign_positive() == negated32.is_sign_positive() -> {}",
        nan32.is_sign_positive() == negated32.is_sign_positive()
    );

    let nan16 = half::consts::NAN;
    println!(
        "nan16.is_sign_positive() == nan16.is_sign_negative() -> {}",
        nan16.is_sign_positive() == nan16.is_sign_negative()
    );
    let unan16: u16 = unsafe { mem::transmute(nan16) };
    let negated16: f16 = unsafe { mem::transmute(unan16 ^ 0x8000) };
    println!(
        "nan16.is_sign_positive() == negated16.is_sign_positive() -> {}",
        nan16.is_sign_positive() == negated16.is_sign_positive()
    );
}

Output:

nan32.is_sign_positive() == nan32.is_sign_negative() -> false
nan32.is_sign_positive() == negated32.is_sign_positive() -> false
nan16.is_sign_positive() == nan16.is_sign_negative() -> true
nan16.is_sign_positive() == negated16.is_sign_positive() -> true

Note that in Rust version 1.20.0, rust-lang/rust#42431, the behaviour of is_sign_positive and is_sign_negative for NaN was updated to conform to IEEE754.