Rollup merge of #144782 - jieyouxu:compiletest-selftests, r=Kobzol

Properly pass path to staged `rustc` to `compiletest` self-tests

Otherwise, this can do weird things like use a global rustc, or try to use stage 0 rustc. This must be properly configured, because `compiletest` is intended to only support one compiler target spec JSON format (of the in-tree compiler).

Historically, this was probably done so before `bootstrap` was really its own thing, and `compiletest` had to be runnable as a much more "self-standing" tool.

Follow-up to rust-lang/rust#144675, as I didn't realize this until Zalathar pointed it out in [#t-infra/bootstrap > Building vs testing `compiletest` @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Building.20vs.20testing.20.60compiletest.60/near/532040838).

r? ````@Kobzol````

1 files changed, 6 insertions, 0 deletions

diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 119fa4237bc..ffc5dfad459 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -749,6 +749,12 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             SourceType::InTree,
             &[],
         );
+
+        // Used for `compiletest` self-tests to have the path to the *staged* compiler. Getting this
+        // right is important, as `compiletest` is intended to only support one target spec JSON
+        // format, namely that of the staged compiler.
+        cargo.env("TEST_RUSTC", builder.rustc(compiler));
+
         cargo.allow_features(COMPILETEST_ALLOW_FEATURES);
         run_cargo_test(cargo, &[], &[], "compiletest self test", host, builder);
     }