Use autocfg instead of our own version parser

Cargo.toml

@@ -26,5 +26,8 @@ std = []
 # TODO
 # system_time = ["std"]
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 const_fn = "0.4"

build.rs

@@ -1,51 +1,25 @@
 #![forbid(unsafe_code)]
 #![warn(rust_2018_idioms, single_use_lifetimes)]
 
-use std::{env, process::Command, str};
+use autocfg::AutoCfg;
 
 // 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 minor = match rustc_minor_version() {
-        Some(minor) => minor,
-        None => {
-            println!("cargo:warning={}: unable to determine rustc version", env!("CARGO_PKG_NAME"));
+    let cfg = match AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
             return;
         }
     };
 
-    if minor >= 39 {
+    if cfg.probe_rustc_version(1, 39) {
         println!("cargo:rustc-cfg=stable_1_39");
     }
 }
-
-fn rustc_minor_version() -> Option<u32> {
-    let rustc = env::var_os("RUSTC")?;
-    let output = Command::new(rustc).args(&["--version", "--verbose"]).output().ok()?;
-    if !output.status.success() {
-        return None;
-    }
-    let output = str::from_utf8(&output.stdout).ok()?;
-
-    // Find the release line in the verbose version output.
-    let release = output
-        .lines()
-        .find(|line| line.starts_with("release: "))
-        .map(|line| &line["release: ".len()..])?;
-
-    // Split the version and channel info.
-    let mut version_channel = release.split('-');
-    let version = version_channel.next().unwrap();
-    let _channel = version_channel.next();
-
-    // Split the version into semver components.
-    let mut digits = version.splitn(3, '.');
-    let major = digits.next()?;
-    if major != "1" {
-        return None;
-    }
-    let minor = digits.next()?.parse().ok()?;
-    let _patch = digits.next()?;
-    Some(minor)
-}