A build.rs script that filters out directives for building capnp files with non-Rust targets.

capnp-build.rs

/*
Go's Cap'n Proto code generator needs some extra directives, but I don't want to require that
my Rust build environment has the relevant Go include files available. The script below filters 
out the Go directives before sending the resulting files to the compiler to make this work.
*/

extern crate capnpc;

use std::env;
use std::path::PathBuf;
use std::fs::File;
use std::io::{BufReader, BufWriter, BufRead, Write};

fn main() {
    let capnp_dir = "src";
    let out_dir = env::var("OUT_DIR").unwrap();

    let files = vec![
        "filenames_here.capnp",
    ];


    let filtered_file_paths = files
        .into_iter()
        .map(|filename| {
            let input_path = [&capnp_dir, filename].iter().collect::<PathBuf>();
            let output_path = [&out_dir, filename].iter().collect::<PathBuf>();

            let input_file = File::open(input_path.as_path()).map_err(|e| format!("{}: {}", filename, e.to_string()))?;
            let output_file = File::create(output_path.as_path()).map_err(|e| format!("{}: {}", filename, e.to_string()))?;

            let reader = BufReader::new(input_file);
            let mut writer = BufWriter::new(output_file);

            for line_result in reader.lines() {
                let line = line_result.map_err(|e| format!("{}: {}", filename, e.to_string()))?;
                if line.starts_with("using Go") || line.starts_with("$Go") {
                    continue
                }
                writer.write_all(line.as_bytes()).map_err(|e| format!("{}: {}", filename, e.to_string()))?;
                writer.write_all(b"\n").map_err(|e| format!("{}: {}", filename, e.to_string()))?;
            }

            Ok(output_path)
        })
        .collect::<Result<Vec<_>, String>>()
        .expect("Filtering capnp files");

    let mut capnp_cmd = capnpc::CompilerCommand::new();
    for filepath in filtered_file_paths {
        capnp_cmd.file(filepath);
    }
    capnp_cmd
        .src_prefix(out_dir)
        .run().expect("schema compiler command");
}