FEATURES_LOWERCASE contains features that do not exists

[barakugav]

The FEATURES_LOWERCASE static variable is created by searching for env variables with prefix CARGO_FEATURE_, and parsing the suffixes to extract features names.
Its correct that every feature has such env variable, but not each such env variable is a real enabled feature.
For example, when I compile my executable I get these additional features that are not part of my crate:

"libc", "numpy", "pyo3"

I can't say why these environment variables were on, but its not due to exports in bashrc or something like that, probably env variables manipulations by my dependencies build scripts.
Anyway, I think a better approach is to parse Cargo.toml and extract all of the possible features of the current built crate, and than check if there is an env variable called format!(CARGO_FEATURE_{}", feature.to_uppercase().replace("-", "_")).

This is how I do it in my build.rs:

fn enabled_features() -> Vec<String> {
    let cargo_toml = Path::new(&env!("CARGO_MANIFEST_DIR")).join("Cargo.toml");
    fs::read_to_string(cargo_toml)
        .expect("Failed to read Cargo.toml")
        .parse::<toml::Table>()
        .expect("Failed to parse Cargo.toml")
        .into_iter()
        .flat_map(|features| {
            features
                .as_table()
                .expect("features must be a table")
                .keys()
                .filter(|k| *k != "default")
                .cloned()
        })
        .filter(|f| {
            env::var(format!(
                "CARGO_FEATURE_{}",
                f.to_uppercase().replace("-", "_")
            ))
            .is_ok()
        })
        .collect()
}