formalize FromStr syntax

[flying-sheep]

AFAIK there’s no grammar specified anywhere that describes how FromStr/parse works.

E.g. some didn’t want f64::from_str(".") to work (#30344) and I wonder why i32::from_str("1_000") does not work.

[hanna-kruppe]

At least for float parsing there are comments on the impl FromStr but they are incomplete and probably not rendered by rustdoc.

[frewsxcv]

probably not rendered by rustdoc.

The comments are now rendered by rustdoc: https://doc.rust-lang.org/std/primitive.f32.html (scroll down to FromStr). Anyone have suggestions for what to add to those docs? Do we even have a grammar for parsing floats?

[leoschwarz]

I didn't find a grammar for FromStr for floats however as most likely it would be a bad idea to change the parsing now, it could just be derived from the code and officially documented so users don't have to figure these things out themselves.

Maybe as grammar language what was used for the Rust grammar specification could be used, i.e. maybe link that and provide an expression for the valid numbers? Specifically I think it won't be too far away from this.

[flying-sheep]

it would be a bad idea to change the parsing now

i’d like parsing to be more lenient and accept underscores, i guess you only meant parsing shouldn’t get stricter?

[steveklabnik]

This is a special case of #31821, so closing as part of that.