build: refactor build.rs slightly

Hopefully the next MSRV bump will bring us beyond (or at) 1.27 and we will
no longer need this.

build.rs

@@ -3,36 +3,29 @@ use std::ffi::OsString;
 use std::process::Command;
 
 fn main() {
-    let rustc = env::var_os("RUSTC").unwrap_or(OsString::from("rustc"));
-    let output = Command::new(&rustc)
-        .arg("--version")
-        .output()
-        .unwrap()
-        .stdout;
-    let version = String::from_utf8(output).unwrap();
-
-    enable_simd_optimizations(&version);
+    let version = match Version::read() {
+        Ok(version) => version,
+        Err(err) => {
+            eprintln!("failed to parse `rustc --version`: {}", err);
+            return;
+        }
+    };
+    enable_simd_optimizations(version);
 }
 
-fn enable_simd_optimizations(version: &str) {
+fn enable_simd_optimizations(version: Version) {
     // We don't activate SIMD optimizations these if we've explicitly disabled
     // them. Disabling auto optimizations is intended for use in tests, so that
     // we can reliably test fallback implementations.
     if env::var_os("CARGO_CFG_REGEX_DISABLE_AUTO_OPTIMIZATIONS").is_some() {
         return;
     }
-    let parsed = match Version::parse(&version) {
-        Ok(parsed) => parsed,
-        Err(err) => {
-            eprintln!("failed to parse `rustc --version`: {}", err);
-            return;
-        }
-    };
-    let minimum = Version { major: 1, minor: 27, patch: 0 };
-    if version.contains("nightly") || parsed >= minimum {
-        println!("cargo:rustc-cfg=regex_runtime_teddy_ssse3");
-        println!("cargo:rustc-cfg=regex_runtime_teddy_avx2");
+    if version < (Version { major: 1, minor: 27, patch: 0 }) {
+        return;
     }
+
+    println!("cargo:rustc-cfg=regex_runtime_teddy_ssse3");
+    println!("cargo:rustc-cfg=regex_runtime_teddy_avx2");
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd, Ord)]
@@ -43,6 +36,16 @@ struct Version {
 }
 
 impl Version {
+    fn read() -> Result<Version, String> {
+        let rustc = env::var_os("RUSTC").unwrap_or(OsString::from("rustc"));
+        let output = Command::new(&rustc)
+            .arg("--version")
+            .output()
+            .unwrap()
+            .stdout;
+        Version::parse(&String::from_utf8(output).unwrap())
+    }
+
     fn parse(mut s: &str) -> Result<Version, String> {
         if !s.starts_with("rustc ") {
             return Err(format!("unrecognized version string: {}", s));