2 files changed, 223 insertions, 0 deletions
diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..3347f87
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,202 @@
+// rustc-cfg emitted by the build script:
+//
+// "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.
+//
+// "hygiene"
+//    Enable Span::mixed_site() and non-dummy behavior of Span::resolved_at
+//    and Span::located_at. Enabled on Rust 1.45+.
+//
+// "proc_macro_span"
+//     Enable non-dummy behavior of Span::start and Span::end methods which
+//     requires an unstable compiler feature. Enabled when building with
+//     nightly, unless `-Z allow-feature` in RUSTFLAGS disallows unstable
+//     features.
+//
+// "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.
+//
+// "span_locations"
+//     Provide methods Span::start and Span::end which give the line/column
+//     location of a token. Enabled by procmacro2_semver_exempt or the
+//     "span-locations" Cargo cfg. This is behind a cfg because tracking
+//     location inside spans is a performance hit.
+//
+// "is_available"
+//     Use proc_macro::is_available() to detect if the proc macro API is
+//     available or needs to be polyfilled instead of trying to use the proc
+//     macro API and catching a panic if it isn't available. Enabled on Rust
+//     1.57+.
+
+use std::env;
+use std::ffi::OsString;
+use std::path::Path;
+use std::process::{self, Command, Stdio};
+use std::str;
+use std::u32;
+
+fn main() {
+    let rustc = rustc_minor_version().unwrap_or(u32::MAX);
+
+    let docs_rs = env::var_os("DOCS_RS").is_some();
+    let semver_exempt = cfg!(procmacro2_semver_exempt) || docs_rs;
+    if semver_exempt {
+        // https://github.com/dtolnay/proc-macro2/issues/147
+        println!("cargo:rustc-cfg=procmacro2_semver_exempt");
+    }
+
+    if semver_exempt || cfg!(feature = "span-locations") {
+        println!("cargo:rustc-cfg=span_locations");
+    }
+
+    if rustc < 57 {
+        println!("cargo:rustc-cfg=no_is_available");
+    }
+
+    if rustc < 66 {
+        println!("cargo:rustc-cfg=no_source_text");
+    }
+
+    if !cfg!(feature = "proc-macro") {
+        println!("cargo:rerun-if-changed=build.rs");
+        return;
+    }
+
+    println!("cargo:rerun-if-changed=build/probe.rs");
+
+    let proc_macro_span;
+    let consider_rustc_bootstrap;
+    if compile_probe(false) {
+        // This is a nightly or dev compiler, so it supports unstable features
+        // regardless of RUSTC_BOOTSTRAP. No need to rerun build script if
+        // RUSTC_BOOTSTRAP is changed.
+        proc_macro_span = true;
+        consider_rustc_bootstrap = false;
+    } else if let Some(rustc_bootstrap) = env::var_os("RUSTC_BOOTSTRAP") {
+        if compile_probe(true) {
+            // This is a stable or beta compiler for which the user has set
+            // RUSTC_BOOTSTRAP to turn on unstable features. Rerun build script
+            // if they change it.
+            proc_macro_span = true;
+            consider_rustc_bootstrap = true;
+        } else if rustc_bootstrap == "1" {
+            // This compiler does not support the proc macro Span API in the
+            // form that proc-macro2 expects. No need to pay attention to
+            // RUSTC_BOOTSTRAP.
+            proc_macro_span = false;
+            consider_rustc_bootstrap = false;
+        } else {
+            // This is a stable or beta compiler for which RUSTC_BOOTSTRAP is
+            // set to restrict the use of unstable features by this crate.
+            proc_macro_span = false;
+            consider_rustc_bootstrap = true;
+        }
+    } else {
+        // Without RUSTC_BOOTSTRAP, this compiler does not support the proc
+        // macro Span API in the form that proc-macro2 expects, but try again if
+        // the user turns on unstable features.
+        proc_macro_span = false;
+        consider_rustc_bootstrap = true;
+    }
+
+    if proc_macro_span || !semver_exempt {
+        println!("cargo:rustc-cfg=wrap_proc_macro");
+    }
+
+    if proc_macro_span {
+        println!("cargo:rustc-cfg=proc_macro_span");
+    }
+
+    if semver_exempt && proc_macro_span {
+        println!("cargo:rustc-cfg=super_unstable");
+    }
+
+    if consider_rustc_bootstrap {
+        println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
+    }
+}
+
+fn compile_probe(rustc_bootstrap: bool) -> bool {
+    if env::var_os("RUSTC_STAGE").is_some() {
+        // We are running inside rustc bootstrap. This is a highly non-standard
+        // environment with issues such as:
+        //
+        //     https://github.com/rust-lang/cargo/issues/11138
+        //     https://github.com/rust-lang/rust/issues/114839
+        //
+        // Let's just not use nightly features here.
+        return false;
+    }
+
+    let rustc = cargo_env_var("RUSTC");
+    let out_dir = cargo_env_var("OUT_DIR");
+    let probefile = Path::new("build").join("probe.rs");
+
+    // Make sure to pick up Cargo rustc configuration.
+    let mut cmd = if let Some(wrapper) = env::var_os("RUSTC_WRAPPER") {
+        let mut cmd = Command::new(wrapper);
+        // The wrapper's first argument is supposed to be the path to rustc.
+        cmd.arg(rustc);
+        cmd
+    } else {
+        Command::new(rustc)
+    };
+
+    if !rustc_bootstrap {
+        cmd.env_remove("RUSTC_BOOTSTRAP");
+    }
+
+    cmd.stderr(Stdio::null())
+        .arg("--edition=2021")
+        .arg("--crate-name=proc_macro2")
+        .arg("--crate-type=lib")
+        .arg("--emit=dep-info,metadata")
+        .arg("--out-dir")
+        .arg(out_dir)
+        .arg(probefile);
+
+    if let Some(target) = env::var_os("TARGET") {
+        cmd.arg("--target").arg(target);
+    }
+
+    // If Cargo wants to set RUSTFLAGS, use that.
+    if let Ok(rustflags) = env::var("CARGO_ENCODED_RUSTFLAGS") {
+        if !rustflags.is_empty() {
+            for arg in rustflags.split('\x1f') {
+                cmd.arg(arg);
+            }
+        }
+    }
+
+    match cmd.status() {
+        Ok(status) => status.success(),
+        Err(_) => false,
+    }
+}
+
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = cargo_env_var("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;
+    }
+    pieces.next()?.parse().ok()
+}
+
+fn cargo_env_var(key: &str) -> OsString {
+    env::var_os(key).unwrap_or_else(|| {
+        eprintln!(
+            "Environment variable ${} is not set during execution of build script",
+            key,
+        );
+        process::exit(1);
+    })
+}
diff --git a/build/probe.rs b/build/probe.rs
new file mode 100644
index 0000000..5afa13a
--- /dev/null
+++ b/build/probe.rs
@@ -0,0 +1,21 @@
+// This code exercises the surface area that we expect of Span's unstable API.
+// If the current toolchain is able to compile it, then proc-macro2 is able to
+// offer these APIs too.
+
+#![feature(proc_macro_span)]
+
+extern crate proc_macro;
+
+use core::ops::RangeBounds;
+use proc_macro::{Literal, Span};
+
+pub fn join(this: &Span, other: Span) -> Option<Span> {
+    this.join(other)
+}
+
+pub fn subspan<R: RangeBounds<usize>>(this: &Literal, range: R) -> Option<Span> {
+    this.subspan(range)
+}
+
+// Include in sccache cache key.
+const _: Option<&str> = option_env!("RUSTC_BOOTSTRAP");