<f64 as FromStr> produces incorrect results in some cases

[JRoush]

In at least one case (and presumably more), FromStr::from_str() produces a result that is off by one ulp.

Example:

pub fn main() {
    use std::str::FromStr;    
    let literal: f64 = 0.123f64;
    let fromstr: f64 = FromStr::from_str("0.123").unwrap();
    println!("Literal: {:16X}", as_int(literal)); // Literal: 3FBF7CED916872B0
    println!("FromStr: {:16X}", as_int(fromstr)); // FromStr: 3FBF7CED916872B1
    println!("Equal: {}", literal == fromstr);    // Equal: false
}

pub fn as_int(f: f64) -> u64 {
    unsafe{ std::mem::transmute::<f64,u64>(f) }
}

Here the compiler produces the correct result for the literal value but the runtime parsing function produces a different, incorrect result. The difference is tiny, but of course it is enough to cause problems :)

For the record, here is the output of rustc --version --verbose on my system:

rustc 1.0.0-nightly (30e1f9a1c 2015-03-14) (built 2015-03-15)
binary: rustc
commit-hash: 30e1f9a1c2bf7134135800bc9afd082773defadc
commit-date: 2015-03-14
build-date: 2015-03-15
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly

[lifthrasiir]

I assume this is a duplicate of #7648. We have really hairy decimal to floating point routine right now.

[JRoush]

@lifthrasiir Whoops, looks like you are right.

[steveklabnik]

closing as duplicate