Parser for CARGO_ENCODED_RUSTFLAGS

CARGO_ENCODED_RUSTFLAGS is one of the environment variables provided by Cargo to build scripts. It synthesizes several sources of flags affecting Cargo's rustc invocations that build scripts might care about:

• Flags passed via the RUSTFLAGS environment variable,
• Cargo config entries under target.<triple>.rustflags and target.<cfg>.rustflags and build.rustflags, including from the project-specific Cargo config file and the Cargo config in the user's CARGO_HOME.

If a build script needs to make some rustc invocations, or needs to characterize aspects of the upcoming rustc invocation, it likely needs these flags.

[build-dependencies]
rustflags = "0.1"

Example

This build script uses the cmake crate to compile some C code, and must configure it with a particular C preprocessor #define if the Rust build is being performed with sanitizers.

// build.rs

use rustflags::Flag;
use std::env;
use std::path::PathBuf;

fn main() {
    let manifest_dir = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
    let lib_source_dir = manifest_dir.join("lib");
    assert!(lib_source_dir.join("CMakeLists.txt").exists());

    let mut builder = cmake::Config::new(lib_source_dir);

    // Look for -Zsanitizer=address
    for flag in rustflags::from_env() {
        if matches!(flag, Flag::Z(z) if z == "sanitizer=address") {
            builder.define("ENABLE_SANITIZERS", "ON");
            builder.define("SANITIZERS", "address");
            break;
        }
    }

    builder.build();
}

License