build.rs

//! build.rs is used to generate code at build time, which is then
//! imported elsewhere. This file is understood and executed by cargo.
use std::env;
use std::fs;
use std::path::Path;

/// Write version information to $OUT_DIR/build_rev.rs, which is included in src/lib.rs.
fn main() {
    println!("cargo:rerun-if-env-changed=RUN_TIME_CLOSURE");
    println!("cargo:rerun-if-changed=build.rs");

    fs::write(
        // cargo sets OUT_DIR: https://doc.rust-lang.org/cargo/reference/environment-variables.html
        Path::new(&env::var("OUT_DIR").unwrap()).join("build_rev.rs"),
        format!(
            r#"

/// Run-time closure parameters. This argument points to a file
/// generated by ./nix/runtime.nix in Lorri's source.
pub const RUN_TIME_CLOSURE: &str = "{runtime_closure}";
"#,
            runtime_closure = env::var("RUN_TIME_CLOSURE")
                .expect("RUN_TIME_CLOSURE not set, please reload nix-shell"),
        )
        .as_bytes(),
    )
    .unwrap();
}