How can I make running a Cargo build script optional? - build

Question

I have a Rust project that generates a dynamic (cdylib) library. The project uses a cbindgen build script to create an according C header file, matching the exported functions of the library. Cargo.toml looks like this:
[package]
name = "example"
version = "0.1.0"
authors = ["Me <me#foo.bar>"]
build = "build.rs"
[lib]
name = "example"
crate-type = ["cdylib"]
[dependencies]
[build-dependencies]
cbindgen = "0.6.2"
Unfortunately RLS (Rust Language Server) doesn't work very well when the build script is active which makes editing in VS Code rather unpleasant. Is there a way to make running the build script optional, having it disabled by default and only manually enable it when requested on the command-line (i.e. something like cargo build --release --enable-build-scripts)?

Answer 1

You can't conditionally disable build scripts or pass variables to them via cargo build, but you can make use of environment variables instead.
Inside your build.rs:
use std::env;
fn main() {
let build_enabled = env::var("BUILD_ENABLED")
.map(|v| v == "1")
.unwrap_or(true); // run by default
if build_enabled {
// do your build
}
}
Build with your build script:
BUILD_ENABLED=1 cargo build
Build without your build script:
BUILD_ENABLED=0 cargo build

Answer 2

To extend the answer from #PeterHall one can use a Cargo "features" section to pass information on to the build script.
Insert the following lines into Cargo.toml:
[features]
headers = []
Then check for environment variable CARGO_FEATURE_HEADERS in build.rs:
use std::env;
fn write_headers() {
// call cbindgen ...
}
fn main() {
let headers_enabled = env::var_os("CARGO_FEATURE_HEADERS").is_some();
if headers_enabled {
write_headers();
}
}
To make a release build run cargo build --features=headers --release.
Now this solution still compiles the build script and all cbindgen dependencies when RLS updates its status or when manually running cargo test. But cbindgen run-time errors do not hamper RLS anymore.