Determine rust channel by parsing rustc output if env vars do not exi…

…st (#163)

* Determine rust channel by parsing rustc output

The RUSTUP_TOOLCHAIN environment variable might not always be present.
This is the case for e.g. NixOS where rust is routinely not installed via
rustup, thus not setting this env var, causing build failures.
Instead, build.rs will now run `rustc -V` and check if the output contains the
word "nightly".

* Check env vars first, fall back to rustc in $PATH

* Try to check RUSTUP_TOOLCHAIN first

azalea/build.rs

@@ -1,8 +1,24 @@
 use std::env;
+use std::process::Command;
 
 fn main() {
-    let rust_toolchain = env::var("RUSTUP_TOOLCHAIN").unwrap();
-    if rust_toolchain.starts_with("stable") {
-        panic!("Azalea currently requires nightly Rust. You can use `rustup override set nightly` to set the toolchain for this directory.");
+    match env::var("RUSTUP_TOOLCHAIN") {
+        Ok(rust_toolchain) if !rust_toolchain.starts_with("nightly")  => { // stable & beta
+            panic!("Azalea currently requires nightly Rust. You can use `rustup override set nightly` to set the toolchain for this directory.");
+        }
+        Ok(_) => return, // nightly
+        Err(_) => { // probably not installed via rustup, run rustc and parse its output
+            let rustc_command = env::var("RUSTC")
+                .or_else(|_| env::var("CARGO_BUILD_RUSTC"))
+                .unwrap_or(String::from("rustc"));
+            let rustc_version_output = Command::new(rustc_command).arg("-V").output().unwrap();
+            if !rustc_version_output.status.success()
+                || !String::from_utf8(rustc_version_output.stdout)
+                    .unwrap()
+                    .contains("nightly")
+            {
+                panic!("Azalea currently requires nightly Rust. It seems that you did not install Rust via rustup. Please check the documentation for your installation method, to find out how to use nightly Rust.");
+            }
+        }
     }
 }