I'm having a hard time figuring out how to use rerun-if-env-changed in build scripts.
I have a minimal project setup like this:
inner
- src
- lib.rs
- build.rs
- Cargo.toml
src
- main.rs
build.rs
Cargo.toml
In the outer Cargo.toml I define inner as a dependency:
[package]
name = "outer"
version = "0.1.0"
edition = "2018"
build = "build.rs"
[dependencies]
inner = { path = "./inner" }
The build script for outer defines an env var FOO, which I'd like inner to use during its build:
// outer/build.rs
fn main() {
println!("cargo:rustc-env=FOO=1");
}
And the inner build script:
// inner/build.rs
use std::env;
use std::fs::File;
use std::io::Write;
use std::path::Path;
fn main() {
println!("cargo:rerun-if-env-changed=FOO");
let out_dir = env::var("OUT_DIR").unwrap();
let dest = Path::new(&out_dir).join("build_constants.rs");
let mut file = File::create(&dest).unwrap();
let foo: i32 = option_env!("FOO")
.map_or(Ok(-1), str::parse)
.expect("failed to parse env variable");
write!(&mut file, "const BUILD_FOO: i32 = {};", foo).unwrap();
}
The inner crate publicly exports a the constant, which can be imported by outer:
// inner/src/lib.rs
include!(concat!(env!("OUT_DIR"), "/build_constants.rs"));
pub const FOO: i32 = BUILD_FOO;
I would expect this to work like this:
If outer is build first, then FOO is set to 1 and when inner is built, the constant is consequently set to 1.
If inner is build first, it is then built again (rerun), since FOO has changed during the build of outer.
However, this does not happen and the pub const FOO is always set to -1, the default value defined in the inner build script.
Am I doing something wrong here or is there a better way to pass around environment variables between crates?
