1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
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
use_feature("io_lifetimes_use_std");
// 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<T: AsRef<str>>(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()
}