bootstrap: Handle target-specific cargo env vars

This commit ensure that we handle target-specific env vars for RUSTFLAGS
through Cargo as well.

Author: alexcrichton

src/bootstrap/builder.rs

@@ -804,7 +804,7 @@ impl<'a> Builder<'a> {
             stage = compiler.stage;
         }
 
-        let mut rustflags = Rustflags::new();
+        let mut rustflags = Rustflags::new(&target, &mut cargo);
         if stage != 0 {
             rustflags.env("RUSTFLAGS_NOT_BOOTSTRAP");
         } else {
@@ -1258,9 +1258,19 @@ mod tests;
 struct Rustflags(String);
 
 impl Rustflags {
-    fn new() -> Rustflags {
+    fn new(target: &str, cmd: &mut Command) -> Rustflags {
         let mut ret = Rustflags(String::new());
+
+        // Inherit `RUSTFLAGS` by default
         ret.env("RUSTFLAGS");
+
+        // ... and also handle target-specific env RUSTFLAGS if they're
+        // configured. If this is configured we also remove it from the
+        // environment because Cargo will prefer it over RUSTFLAGS.
+        let target_specific = format!("CARGO_TARGET_{}_RUSTFLAGS", crate::envify(target));
+        ret.env(&target_specific);
+        cmd.env_remove(&target_specific);
+
         return ret;
     }