Literal valid f64 cannot be parsed via f64::from_str

[tisonkun]

I tried this code:

use std::str::FromStr;

fn main() {
    let x = 1_000_000_.3_e+____4;
    let res = f64::from_str("1_000_000_.3_e+____4");
    println!("{:?}", x);
    println!("{:?}", res);
}

I expected to see this happen:

10000003000.0
Ok(10000003000.0)

Instead, this happened:

10000003000.0
Err(ParseFloatError { kind: Invalid })

Meta

rustc --version --verbose:

rustc 1.92.0-nightly (fa3155a64 2025-09-30)
binary: rustc
commit-hash: fa3155a644dd62e865825087b403646be01d4cef
commit-date: 2025-09-30
host: aarch64-apple-darwin
release: 1.92.0-nightly
LLVM version: 21.1.2

Backtrace

<backtrace>

[bjorn3]

https://doc.rust-lang.org/stable/std/primitive.f64.html#method.from_str documents what strings can be parsed to floats. Underscores are not allowed as input.

[lukaslueg]

You can use syn to parse Rust's syntax into a float

fn main() {
    let x = 1_000_000.3_e+____4;
    let res: Result<f64, syn::Error> = syn::parse_str::<syn::LitFloat>("1_000_000.3_e+____4")
        .and_then(|lf| lf.base10_parse());
    println!("{:?}", x);
    println!("{:?}", res);
}

10000003000.0
Ok(10000003000.0)

[tisonkun]

Thanks for your information!

Closed as information delivered. I'm fine with this situation then.