Auto merge of #870 - RalfJung:miri-rustc, r=oli-obk

Prepare Miri for rustc bootstrap building a separate libstd for it

src/bin/cargo-miri.rs

@@ -141,6 +141,13 @@ fn get_sysroot(mut cmd: Command) -> PathBuf {
             .unwrap_or_else(|_| panic!("Failed to canonicalize sysroot: {}", stdout))
     }
 
+    // We let the user skip this check if they really want to.
+    // (`bootstrap` needs this because Miri gets built by the stage1 compiler
+    // but run with the stage2 sysroot.)
+    if std::env::var("MIRI_SKIP_SYSROOT_CHECK").is_ok() {
+        return;
+    }
+
     let rustc_sysroot = get_sysroot(Command::new("rustc"));
     let miri_sysroot = get_sysroot(Command::new(find_miri()));
 

src/lib.rs

@@ -47,7 +47,5 @@
 /// Insert rustc arguments at the beginning of the argument list that Miri wants to be
 /// set per default, for maximal validation power.
 pub fn miri_default_args() -> &'static [&'static str] {
-    // The flags here should be kept in sync with what bootstrap adds when `test-miri` is
-    // set, which happens in `bootstrap/bin/rustc.rs` in the rustc sources.
     &["-Zalways-encode-mir", "-Zmir-emit-retag", "-Zmir-opt-level=0", "--cfg=miri"]
 }