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use autocfg::AutoCfg;
const CONST_THREAD_LOCAL_PROBE: &str = r#"
{
thread_local! {
static MY_PROBE: usize = const { 10 };
}
MY_PROBE.with(|val| *val)
}
"#;
fn main() {
let mut enable_const_thread_local = false;
match AutoCfg::new() {
Ok(ac) => {
// These checks prefer to call only `probe_rustc_version` if that is
// enough to determine whether the feature is supported. This is
// because the `probe_expression` call involves a call to rustc,
// which the `probe_rustc_version` call avoids.
// Const-initialized thread locals were stabilized in 1.59.
if ac.probe_rustc_version(1, 60) {
enable_const_thread_local = true;
} else if ac.probe_rustc_version(1, 59) {
// This compiler claims to be 1.59, but there are some nightly
// compilers that claim to be 1.59 without supporting the
// feature. Explicitly probe to check if code using them
// compiles.
//
// The oldest nightly that supports the feature is 2021-12-06.
if ac.probe_expression(CONST_THREAD_LOCAL_PROBE) {
enable_const_thread_local = true;
}
}
}
Err(e) => {
// If we couldn't detect the compiler version and features, just
// print a warning. This isn't a fatal error: we can still build
// Tokio, we just can't enable cfgs automatically.
println!(
"cargo:warning=tokio: failed to detect compiler features: {}",
e
);
}
}
if !enable_const_thread_local {
// To disable this feature on compilers that support it, you can
// explicitly pass this flag with the following environment variable:
//
// RUSTFLAGS="--cfg tokio_no_const_thread_local"
autocfg::emit("tokio_no_const_thread_local")
}
}