Deny compiletest self-tests being run against stage 0 rustc unless explicitly allowed

Otherwise, `compiletest` would have to know e.g. how to parse two
different target spec, if target spec format was changed between beta
`rustc` and in-tree `rustc`.

Author: jieyouxu

src/bootstrap/src/core/build_steps/test.rs

@@ -8,6 +8,9 @@ use std::ffi::{OsStr, OsString};
 use std::path::{Path, PathBuf};
 use std::{env, fs, iter};
 
+#[cfg(feature = "tracing")]
+use tracing::instrument;
+
 use crate::core::build_steps::compile::{Std, run_cargo};
 use crate::core::build_steps::doc::DocumentationFormat;
 use crate::core::build_steps::gcc::{Gcc, add_cg_gcc_cargo_flags};
@@ -30,7 +33,7 @@ use crate::utils::helpers::{
     linker_flags, t, target_supports_cranelift_backend, up_to_date,
 };
 use crate::utils::render_tests::{add_flags_and_try_run_tests, try_run_tests};
-use crate::{CLang, DocTests, GitRepo, Mode, PathSet, envify};
+use crate::{CLang, DocTests, GitRepo, Mode, PathSet, debug, envify};
 
 const ADB_TEST_DIR: &str = "/data/local/tmp/work";
 
@@ -713,6 +716,10 @@ impl Step for CompiletestTest {
     }
 
     /// Runs `cargo test` for compiletest.
+    #[cfg_attr(
+        feature = "tracing",
+        instrument(level = "debug", name = "CompiletestTest::run", skip_all)
+    )]
     fn run(self, builder: &Builder<'_>) {
         let host = self.host;
 
@@ -725,6 +732,7 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         }
 
         let compiler = builder.compiler(builder.top_stage, host);
+        debug!(?compiler);
 
         // We need `ToolStd` for the locally-built sysroot because
         // compiletest uses unstable features of the `test` crate.