".".parse() returns Ok(0.0)

[hanna-kruppe]

This program prints Ok(0) where I would expect an Err(..):

fn main() {
    println!("{:?}", ".".parse::<f64>());
}

If I remember correctly, this behavior has been around since pre-1.0 but it would be good if someone who has multirust available could verify that (at least for 1.0 final if earlier builds are harder to get).

Unfortunately even if we all agree that this behavior is wrong, it may not be worth the potential breakage to fix. For example, recent changes to the integer FromStr format (allowing leading plus) did, surprisingly, break real code. It's quite possible that fixing this wart is not worth the risk.

[bluss]

Travis testing says it's present in all versions since 1.0.0 (builds) (code)

[Stebalien]

I'd rather not go down the PHP route and document bugs as features, even minor ones like this. Can we get a crater run? Can't be cratered. Crap.

[alexcrichton]

triage: I-nominated

[alexcrichton]

The libs team discussed this during triage yesterday and the conclusion is that this is pretty reasonable to include in the future as a bugfix, so P-medium for now.

[hanna-kruppe]

This is an ideal E-easy bug, and although it's quite small I can mentor for this if desired. So, someone please tag this E-easy E-mentor.

Details for someone looking to fix this bug:
You'll have to find the correct code path in the parse_decimal function in src/libcore/num/dec2flt/parse.rs and change it to return an error. There is a whole if dedicated to this case, but perhaps there is a more elegant way to rewrite the surrounding code. There's also a test in libcoretest that expect the current (wrong) behavior, it's probably best to change this test to expect an error instead.