Split out a separate ./x test bootstrap-py step

When working on the Rust parts of bootstrap, it's unhelpful for
`./x test bootstrap` to also run Python unit tests.

Author: Zalathar

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

@@ -3337,33 +3337,57 @@ fn distcheck_rustc_dev(builder: &Builder<'_>, dir: &Path) {
     builder.remove_dir(dir);
 }
 
+/// Runs unit tests in `bootstrap_test.py`, which test the Python parts of bootstrap.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct Bootstrap;
+pub(crate) struct BootstrapPy;
 
-impl Step for Bootstrap {
+impl Step for BootstrapPy {
     type Output = ();
     const DEFAULT: bool = true;
     const IS_HOST: bool = true;
 
-    /// Tests the build system itself.
-    fn run(self, builder: &Builder<'_>) {
-        let host = builder.config.host_target;
-        let build_compiler = builder.compiler(0, host);
+    fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
+        // Bootstrap tests might not be perfectly self-contained and can depend
+        // on the environment, so only run them by default in CI, not locally.
+        // See `test::Bootstrap::should_run`.
+        let is_ci = run.builder.config.is_running_on_ci;
+        run.alias("bootstrap-py").default_condition(is_ci)
+    }
 
-        // Some tests require cargo submodule to be present.
-        builder.build.require_submodule("src/tools/cargo", None);
+    fn make_run(run: RunConfig<'_>) {
+        run.builder.ensure(BootstrapPy)
+    }
 
+    fn run(self, builder: &Builder<'_>) -> Self::Output {
         let mut check_bootstrap = command(builder.python());
         check_bootstrap
             .args(["-m", "unittest", "bootstrap_test.py"])
+            // Forward command-line args after `--` to unittest, for filtering etc.
+            .args(builder.config.test_args())
             .env("BUILD_DIR", &builder.out)
             .env("BUILD_PLATFORM", builder.build.host_target.triple)
             .env("BOOTSTRAP_TEST_RUSTC_BIN", &builder.initial_rustc)
             .env("BOOTSTRAP_TEST_CARGO_BIN", &builder.initial_cargo)
             .current_dir(builder.src.join("src/bootstrap/"));
-        // NOTE: we intentionally don't pass test_args here because the args for unittest and cargo test are mutually incompatible.
-        // Use `python -m unittest` manually if you want to pass arguments.
         check_bootstrap.delay_failure().run(builder);
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Bootstrap;
+
+impl Step for Bootstrap {
+    type Output = ();
+    const DEFAULT: bool = true;
+    const IS_HOST: bool = true;
+
+    /// Tests the build system itself.
+    fn run(self, builder: &Builder<'_>) {
+        let host = builder.config.host_target;
+        let build_compiler = builder.compiler(0, host);
+
+        // Some tests require cargo submodule to be present.
+        builder.build.require_submodule("src/tools/cargo", None);
 
         let mut cargo = tool::prepare_tool_cargo(
             builder,

src/bootstrap/src/core/builder/mod.rs

@@ -869,6 +869,7 @@ impl<'a> Builder<'a> {
             Kind::Test => describe!(
                 crate::core::build_steps::toolstate::ToolStateCheck,
                 test::Tidy,
+                test::BootstrapPy,
                 test::Bootstrap,
                 test::Ui,
                 test::Crashes,