compiletest/rmake: better explain why stage0 sysroot is needed if for…

…ced stage0

src/tools/compiletest/src/runtest.rs

@@ -3633,12 +3633,29 @@ impl<'test> TestCx<'test> {
  // annotated with `#[must_use]`.
  rustc.arg("-Dunused_must_use");
 
- // FIXME(jieyouxu): explain this!
+ // > `cg_clif` uses `COMPILETEST_FORCE_STAGE0=1 ./x.py test --stage 0` for running the rustc
+ // > test suite. With the introduction of rmake.rs this broke. `librun_make_support.rlib` is
+ // > compiled using the bootstrap rustc wrapper which sets `--sysroot
+ // > build/aarch64-unknown-linux-gnu/stage0-sysroot`, but then compiletest will compile
+ // > `rmake.rs` using the sysroot of the bootstrap compiler causing it to not find the
+ // > `libstd.rlib` against which `librun_make_support.rlib` is compiled.
+ //
+ // The gist here is that we have to pass the proper stage0 sysroot if we want
+ //
+ // ```
+ // $ COMPILETEST_FORCE_STAGE0=1 ./x test run-make --stage 0
+ // ```
+ //
+ // to work correctly.
+ //
+ // See <https://github.com/rust-lang/rust/pull/122248> for more background.
  if std::env::var_os("COMPILETEST_FORCE_STAGE0").is_some() {
- let mut stage0_sysroot = build_root.clone();
- stage0_sysroot.push("stage0-sysroot");
+ let stage0_sysroot = {
+ let mut p = build_root.clone();
+ p.push("stage0-sysroot");
+ p
+ };
  debug!(?stage0_sysroot);
- debug!(exists = stage0_sysroot.exists());
 
  rustc.arg("--sysroot").arg(&stage0_sysroot);
  }