Rust demo - Select custom mod by env

build.rs

fn main() {
    if std::env::var("TARGET_2").is_ok() {
        println!("cargo:rustc-cfg=target_2_enabled");
    }

    let manifest_dir =
        std::env::var("CARGO_MANIFEST_DIR").expect("Failed to get manifest directory");
    std::fs::write(
        std::path::Path::new(&manifest_dir)
            .join("src")
            .join("manifest_dir.rs"),
        format!("pub const MANIFEST_DIR: &str = {:?};", manifest_dir),
    )
    .unwrap();
}

main.rs

// Let all the components join the syntax check.
mod mod_target_1;
mod mod_target_2;

#[cfg(target_2_enabled)]
#[path = "mod_target_2.rs"]
mod mod_target;

#[cfg(not(target_2_enabled))]
#[path = "mod_target_1.rs"]
mod mod_target;

mod manifest_dir;

fn main() {
    mod_target::log();
    println!("MANIFEST_DIR: {}", manifest_dir::MANIFEST_DIR);
}

Makefile.toml

[config]
default_to_workspace = false
skip_core_tasks = true

[tasks.test-target-1]
script = '''
#!@duckscript

exec cargo run
'''

[tasks.test-target-2]
script = '''
#!@duckscript

set_env TARGET_2 true
exec cargo run
'''

mod_target_1.rs

#[allow(dead_code)]
pub fn log() {
    println!("Hello from {}", file!());
}

mod_target_2.rs

#[allow(dead_code)]
pub fn log() {
    println!("Hello from {}", file!());
}