build.rs

use std::{
    env,
    error::Error,
    fmt,
    fs::File,
    io::{BufWriter, Write},
    path::Path,
};

const RUSTC_MAX_ALIGN_EXP: u16 = 29;

fn main() -> Result<(), Box<dyn Error>> {
    println!("cargo::rerun-if-changed=build.rs");

    let ptr_width: u16 = env::var("CARGO_CFG_TARGET_POINTER_WIDTH")?
        .parse::<u64>()?
        .try_into()?;

    // -2 because align must be less than isize::MAX,
    // and pow(2, ptr_width - 1) = isize::MAX + 1.
    let max_align_exp = (ptr_width - 2).min(RUSTC_MAX_ALIGN_EXP);

    let out_dir = env::var_os("OUT_DIR").ok_or(env::VarError::NotPresent)?;
    let out_dir = Path::new(&out_dir);

    let mut f = BufWriter::new(File::create(out_dir.join("for_each_align.rs"))?);
    write_for_each_align_macro(&mut f, max_align_exp)?;

    Ok(())
}

fn write_for_each_align_macro(f: &mut impl Write, max_align_exp: u16) -> std::io::Result<()> {
    write!(f, "macro_rules! for_each_align {{")?;
    write!(f, "{}($($mac:tt)*) => {{", IndentLn(1))?;
    write!(f, "{}$($mac)*! {{", IndentLn(2))?;

    let ln = IndentLn(3);

    // yield all pow(2, i) for i in 0..=max_align_exp
    for i in 0..=max_align_exp {
        // pow(2, i) = 0b10...0 in binary, with i zeros
        write!(f, "{ln}0b1")?;
        for _ in 0..i {
            write!(f, "0")?;
        }
    }

    for i in (0..=2).rev() {
        write!(f, "{}", IndentLn(i))?;
        write!(f, "}}")?;
    }
    Ok(())
}
struct IndentLn(u8);
impl std::fmt::Display for IndentLn {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        writeln!(f)?;
        for _ in 0..self.0 {
            write!(f, "    ")?;
        }
        Ok(())
    }
}