Move handling of RUSTC_PARALLEL_COMPILER to compile.rs

No longer needs to live in `rustc.rs`

Author: alexcrichton

src/bootstrap/bin/rustc.rs

@@ -170,10 +170,6 @@ fn main() {
         cmd.arg("-Z").arg("force-unstable-if-unmarked");
     }
 
-    if env::var_os("RUSTC_PARALLEL_COMPILER").is_some() {
-        cmd.arg("--cfg").arg("parallel_compiler");
-    }
-
     if verbose > 1 {
         eprintln!(
             "rustc command: {:?}={:?} {:?}",

src/bootstrap/compile.rs

@@ -480,7 +480,7 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo) {
         cargo.env("CFG_DEFAULT_LINKER", s);
     }
     if builder.config.rustc_parallel {
-        cargo.env("RUSTC_PARALLEL_COMPILER", "1");
+        cargo.rustflag("--cfg=parallel_compiler");
     }
     if builder.config.rust_verify_llvm_ir {
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");