How can I override a constant via a compiler option?
No, you can't define constants (read: const
bindings) with a compiler flag. But you can use the env!
macro for something similar. It reads some environment variable at compile time.
const MAX_DIMENSIONS_RAW: &'static str = env!("MAX_DIMENSIONS");
Sadly, this returns a string and not an integer. Furthermore we can't yet call arbitrary functions (like parse
) at compile time to calculate a constant. You could use lazy_static
to achieve something similar:
lazy_static! {
static ref MAX_DIMENSIONS: usize = MAX_DIMENSIONS_RAW.parse().unwrap();
}
Of course you should add proper error handling. If your user doesn't need to define the environment variable, you can use option_env!
.
With this approach, you can pass the setting at build time:
$ MAX_DIMENSIONS=1000 cargo build
Building on Lukas Kalbertodt's answer, you can get the environment variable as a constant number with some extra indirection, namely by using a build script.
build.rs
use std::{env, fs::File, io::Write, path::Path};
fn main() {
let out_dir = env::var("OUT_DIR").expect("No out dir");
let dest_path = Path::new(&out_dir).join("constants.rs");
let mut f = File::create(&dest_path).expect("Could not create file");
let max_dimensions = option_env!("MAX_DIMENSIONS");
let max_dimensions = max_dimensions
.map_or(Ok(10_000), str::parse)
.expect("Could not parse MAX_DIMENSIONS");
write!(&mut f, "const MAX_DIMENSIONS: usize = {};", max_dimensions)
.expect("Could not write file");
println!("cargo:rerun-if-env-changed=MAX_DIMENSIONS");
}
main.rs
include!(concat!(env!("OUT_DIR"), "/constants.rs"));
fn main() {
println!("The value is {} ({})", MAX_DIMENSIONS, MAX_DIMENSIONS + 1);
}
$ cargo run
The value is 10000 (10001)
$ MAX_DIMENSIONS=17 cargo run
The value is 17 (18)
$ MAX_DIMENSIONS=1 cargo run
The value is 1 (2)