use std::env::var; use std::io::Write; fn main() { use_feature_or_nothing("can_vector"); // https://github.com/rust-lang/rust/issues/69941 use_feature_or_nothing("write_all_vectored"); // https://github.com/rust-lang/rust/issues/70436 // Don't rerun this on changes other than build.rs, as we only depend on // the rustc version. println!("cargo:rerun-if-changed=build.rs"); } fn use_feature_or_nothing(feature: &str) { if has_feature(feature) { use_feature(feature); } } fn use_feature(feature: &str) { println!("cargo:rustc-cfg={}", feature); } /// Test whether the rustc at `var("RUSTC")` supports the given feature. fn has_feature(feature: &str) -> bool { can_compile(format!( "#![allow(stable_features)]\n#![feature({})]", feature )) } /// Test whether the rustc at `var("RUSTC")` can compile the given code. fn can_compile>(test: T) -> bool { use std::process::Stdio; let out_dir = var("OUT_DIR").unwrap(); let rustc = var("RUSTC").unwrap(); let target = var("TARGET").unwrap(); // Use `RUSTC_WRAPPER` if it's set, unless it's set to an empty string, as // documented [here]. // [here]: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads let wrapper = var("RUSTC_WRAPPER") .ok() .and_then(|w| if w.is_empty() { None } else { Some(w) }); let mut cmd = if let Some(wrapper) = wrapper { let mut cmd = std::process::Command::new(wrapper); // The wrapper's first argument is supposed to be the path to rustc. cmd.arg(rustc); cmd } else { std::process::Command::new(rustc) }; cmd.arg("--crate-type=rlib") // Don't require `main`. .arg("--emit=metadata") // Do as little as possible but still parse. .arg("--target") .arg(target) .arg("--out-dir") .arg(out_dir); // Put the output somewhere inconsequential. // If Cargo wants to set RUSTFLAGS, use that. if let Ok(rustflags) = var("CARGO_ENCODED_RUSTFLAGS") { if !rustflags.is_empty() { for arg in rustflags.split('\x1f') { cmd.arg(arg); } } } let mut child = cmd .arg("-") // Read from stdin. .stdin(Stdio::piped()) // Stdin is a pipe. .stderr(Stdio::null()) // Errors from feature detection aren't interesting and can be confusing. .spawn() .unwrap(); writeln!(child.stdin.take().unwrap(), "{}", test.as_ref()).unwrap(); child.wait().unwrap().success() }