careful.rs

#![allow(unknown_lints)]

#[track_caller]
pub fn f() {
    // ... dreaming about IO in const fns
    // * https://github.com/rust-lang/const-eval/issues/44
    // * could maybe use include_bytes
    let loc = std::panic::Location::caller();
    let contents = std::fs::read_to_string(loc.file()).unwrap();

    let mut lines: Vec<String> = Vec::new();
    for line in contents.lines().take(loc.line() as usize) {
        lines.push(line.to_string());
    }

    lines.last_mut().unwrap().truncate(loc.column() as usize);

    let src: String = lines.join("\n");
    let rev = src.chars().rev();

    let rev = rev.skip_while(|c| c.is_alphanumeric() || c.is_whitespace());
    let magic_attr = "[warn(be_careful)]";

    let mut maybe_attr: String = rev.take_while(|&c| c != '#').collect();
    maybe_attr.retain(|c| !c.is_whitespace());

    if !maybe_attr.chars().eq(magic_attr.chars().rev()) {
        // todo function name
        panic!("to call this function you must specify the `#[warn(be_careful)]` \
               attribute to ensure you are upholding the neccessary invariants");
    }
}

#[cfg(test)]
mod tests {
    #![allow(unknown_lints)]

    use super::*;
    #[test]
    fn jammed_together() {
        #[warn(be_careful)]f()
    }

    #[test]
    fn plain_attr() {
        #[warn(be_careful)] f() // hi there
    }

    #[test]
    #[should_panic(expected = "you must specify the `#[warn(be_careful)]` attribute")]
    fn bad_attribute() {
        #[warn(dont_be_careful)] f()
    }

    #[test]
    #[should_panic(expected = "you must specify the `#[warn(be_careful)]` attribute")]
    fn no_attribute() { f() }

    #[test]
    fn whitespace_ok() {
        #[warn (be_careful  )   ] f()
    }
}