proc-macro2 0.4.25

A stable implementation of the upcoming new `proc_macro` API. Comes with an option, off by default, to also reimplement itself in terms of the upstream unstable API.
Documentation 
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// rustc-cfg emitted by the build script:
//
// "u128"
//     Include u128 and i128 constructors for proc_macro2::Literal. Enabled on
//     any compiler 1.26+.
//
// "use_proc_macro"
//     Link to extern crate proc_macro. Available on any compiler and any target
//     except wasm32. Requires "proc-macro" Cargo cfg to be enabled (default is
//     enabled). On wasm32 we never link to proc_macro even if "proc-macro" cfg
//     is enabled.
//
// "wrap_proc_macro"
//     Wrap types from libproc_macro rather than polyfilling the whole API.
//     Enabled on rustc 1.29+ as long as procmacro2_semver_exempt is not set,
//     because we can't emulate the unstable API without emulating everything
//     else. Also enabled unconditionally on nightly, in which case the
//     procmacro2_semver_exempt surface area is implemented by using the
//     nightly-only proc_macro API.
//
// "slow_extend"
//     Fallback when `impl Extend for TokenStream` is not available. These impls
//     were added one version later than the rest of the proc_macro token API.
//     Enabled on rustc 1.29 only.
//
// "nightly"
//     Enable the Span::unstable method. This is to support proc_macro_span and
//     proc_macro_diagnostic use on the nightly channel without requiring the
//     semver exemption opt-in. Enabled when building with nightly.
//
// "super_unstable"
//     Implement the semver exempt API in terms of the nightly-only proc_macro
//     API. Enabled when using procmacro2_semver_exempt on a nightly compiler.

use std::env;
use std::process::Command;
use std::str;

fn main() {
    println!("cargo:rerun-if-changed=build.rs");

    let target = env::var("TARGET").unwrap();

    let version = match rustc_version() {
        Some(version) => version,
        None => return,
    };

    if version.minor >= 26 {
        println!("cargo:rustc-cfg=u128");
    }

    if !enable_use_proc_macro(&target) {
        return;
    }
    println!("cargo:rustc-cfg=use_proc_macro");

    let semver_exempt = cfg!(procmacro2_semver_exempt);
    if semver_exempt {
        // https://github.com/alexcrichton/proc-macro2/issues/147
        println!("cargo:rustc-cfg=procmacro2_semver_exempt");
    }

    // Rust 1.29 stabilized the necessary APIs in the `proc_macro` crate
    if version.nightly || version.minor >= 29 && !semver_exempt {
        println!("cargo:rustc-cfg=wrap_proc_macro");
    }

    if version.minor == 29 {
        println!("cargo:rustc-cfg=slow_extend");
    }

    if version.nightly {
        println!("cargo:rustc-cfg=nightly");
    }

    if semver_exempt && version.nightly {
        println!("cargo:rustc-cfg=super_unstable");
    }
}

fn enable_use_proc_macro(target: &str) -> bool {
    // wasm targets don't have the `proc_macro` crate, disable this feature.
    if target.contains("wasm32") {
        return false;
    }

    // Otherwise, only enable it if our feature is actually enabled.
    cfg!(feature = "proc-macro")
}

struct RustcVersion {
    minor: u32,
    nightly: bool,
}

fn rustc_version() -> Option<RustcVersion> {
    macro_rules! otry {
        ($e:expr) => {
            match $e {
                Some(e) => e,
                None => return None,
            }
        };
    }

    let rustc = otry!(env::var_os("RUSTC"));
    let output = otry!(Command::new(rustc).arg("--version").output().ok());
    let version = otry!(str::from_utf8(&output.stdout).ok());
    let nightly = version.contains("nightly");
    let mut pieces = version.split('.');
    if pieces.next() != Some("rustc 1") {
        return None;
    }
    let minor = otry!(pieces.next());
    let minor = otry!(minor.parse().ok());

    Some(RustcVersion {
        minor: minor,
        nightly: nightly,
    })
}