Get rustc version string at rustversion crate build time

Cargo.toml

@@ -8,6 +8,7 @@ description = "Conditional compilation according to rustc compiler version"
 repository = "https://github.com/dtolnay/rustversion"
 documentation = "https://docs.rs/rustversion"
 readme = "README.md"
+build = "build.rs"
 
 [lib]
 proc-macro = true

build.rs

@@ -0,0 +1,18 @@
+use std::env;
+use std::ffi::OsString;
+use std::fs;
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+ let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+ let output = Command::new(rustc)
+ .arg("--version")
+ .output()
+ .expect("Failed to exec rustc");
+
+ let string = String::from_utf8(output.stdout).expect("rustc output not utf8");
+ let dir = env::var_os("OUT_DIR").expect("OUT_DIR not set");
+
+ fs::write(&Path::new(&dir).join("version.txt"), &string).expect("failed to write version.txt")
+}

src/rustc.rs

@@ -1,15 +1,15 @@
 use std::env;
-use std::ffi::OsString;
 use std::fmt::{self, Display};
 use std::io;
-use std::process::Command;
 use std::str::FromStr;
 use std::string::FromUtf8Error;
 
 use crate::date::Date;
 use crate::version::{Channel::*, Version};
 use proc_macro2::Span;
 
+const RUSTC_VERSION: &str = include_str!(concat!(env!("OUT_DIR"), "/version.txt"));
+
 #[derive(Debug)]
 pub enum Error {
  Exec(io::Error),
@@ -48,16 +48,9 @@ impl From<Error> for syn::Error {
 }
 
 pub fn version() -> Result<Version> {
- let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
- let output = Command::new(rustc)
- .arg("--version")
- .output()
- .map_err(Error::Exec)?;
- let string = String::from_utf8(output.stdout)?;
-
- match parse(&string) {
+ match parse(RUSTC_VERSION) {
  Some(version) => Ok(version),
- None => Err(Error::Parse(string)),
+ None => Err(Error::Parse(RUSTC_VERSION.to_string())),
  }
 }