use std::env;use std::process::Command;use std::str;// The rustc-cfg strings below are *not* public API. Please let us know by// opening a GitHub issue if your build environment requires some way to enable// these cfgs other than by executing our build script.fn main() {    let compiler = match rustc_version() {        Some(compiler) => compiler,        None => return,    };    if compiler.minor < 36 {        println!("cargo:rustc-cfg=syn_omit_await_from_token_macro");    }    if compiler.minor < 39 {        println!("cargo:rustc-cfg=syn_no_const_vec_new");    }    if compiler.minor < 40 {        println!("cargo:rustc-cfg=syn_no_non_exhaustive");    }    if compiler.minor < 56 {        println!("cargo:rustc-cfg=syn_no_negative_literal_parse");    }    if !compiler.nightly {        println!("cargo:rustc-cfg=syn_disable_nightly_tests");    }}struct Compiler {    minor: u32,    nightly: bool,}fn rustc_version() -> Option<Compiler> {    let rustc = env::var_os("RUSTC")?;    let output = Command::new(rustc).arg("--version").output().ok()?;    let version = str::from_utf8(&output.stdout).ok()?;    let mut pieces = version.split('.');    if pieces.next() != Some("rustc 1") {        return None;    }    let minor = pieces.next()?.parse().ok()?;    let nightly = version.contains("nightly") || version.ends_with("-dev");    Some(Compiler { minor, nightly })}