Make cargo miri run work within the rustc repo

Author: oli-obk

cargo-miri/bin.rs

@@ -871,6 +871,14 @@ fn phase_rustc(mut args: env::Args, phase: RustcPhase) {
             cmd.arg("-C").arg("panic=abort");
         }
     } else {
+        if !print {
+            // When we're running `cargo-miri` from `x.py` we need to pass the sysroot explicitly as rustc
+            // can't figure out the sysroot on its own unless it's from rustup.
+            if let Some(sysroot) = std::env::var_os("SYSROOT") {
+                cmd.arg("--sysroot").arg(sysroot);
+            }
+        }
+
         // For host crates or when we are printing, just forward everything.
         cmd.args(args);
     }