I have been working on a trait that I want to implement for the f128 type and I want it to compile if and only if the f128 feature flag is set. I can't use a crate feature as I don't want the f128 type to not have my trait implemented. I want it to look something like this:

pub trait MyTrait {
    fn my_method(&self) -> MyType;
}

#[cfg(feature = "f128")]
impl MyTrait for f128 {
    fn my_method(&self) -> MyType {
        // ...
    }
}

However this does not work, I found out that the f128 feature is a compiler feature and doesn't work with the #[cfg(...)] attribute macro. I tried some workarounds however they proved effective. I first thought I could construct a proc_macro_attribute that would function similarly to the cfg one, however I was not able to make it work and most similar solutions online (like this one) used the build.rs instead of a custom attribute macro so I tried using that.

Turning to generative ai for help it told me to make a build.rs script using the autocfg crate however it hallucinated a function that don't exist (it told me to use the AutoCfg::probe_feature method which doesn't exist) and upon correction it gave me more hacky solutions which I've learned mean that the model is at a dead end. But I looked into the crate and found the AutoCfg::emit_has_type method that I tried:

// build.rs
fn main() {
    let ac = autocfg::new();

    ac.emit_has_type("f128");
}

// main.rs
fn main() {
    dbg!(cfg!(has_f128));
}

however whether or not I run this with #![feature(f128)] I get the result

cfg!(has_f128) = false

The last thing I tried was I stumbled across the cfg_rust_features crate which gave me hope. So I tried

// build.rs
fn main() {
    let of_interest = ["f128"];
    cfg_rust_features::emit!(of_interest).unwrap();
}

However I get the error:

thread 'main' (786015) panicked at build.rs:5:43:
called Result::unwrap() on an Err value: UnsupportedFeatureTodoError("To request support for feature "f128", open an issue at: GitHub - DerickEddington/cfg_rust_features: Set cfg options according to Rust compiler, language, and library features. · GitHub")

Which lead me to inspect the crate source code where I found that it hasn't been updated in 2 years and that the functionality (here) was mostly a list containing named features and a script that autocfg can run to detect the feature.

I don't really know where to go from here, I am pretty familiar with most of the rust programming language however, when it comes to meta-programming I am still rather new to it, as the documentation / support is more sparse. I have done procedural macros but not much past that.

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