Fix running bootstrap tests with a local Rust toolchain as the stage0

src/bootstrap/bootstrap_test.py

@@ -138,6 +138,25 @@ def build_args(self, configure_args=None, args=None, env=None):
         if env is None:
             env = {}
 
+        # This test ends up invoking build_bootstrap_cmd, which searches for
+        # the Cargo binary and errors out if it cannot be found. This is not a
+        # problem in most cases, but there is a scenario where it would cause
+        # the test to fail.
+        #
+        # When a custom local Cargo is configured in config.toml (with the
+        # build.cargo setting), no Cargo is downloaded to any location known by
+        # bootstrap, and bootstrap relies on that setting to find it.
+        #
+        # In this test though we are not using the config.toml of the caller:
+        # we are generating a blank one instead. If we don't set build.cargo in
+        # it, the test will have no way to find Cargo, failing the test.
+        cargo_bin = os.environ.get("BOOTSTRAP_TEST_CARGO_BIN")
+        if cargo_bin is not None:
+            configure_args += ["--set", "build.cargo=" + cargo_bin]
+        rustc_bin = os.environ.get("BOOTSTRAP_TEST_RUSTC_BIN")
+        if rustc_bin is not None:
+            configure_args += ["--set", "build.rustc=" + rustc_bin]
+
         env = env.copy()
         env["PATH"] = os.environ["PATH"]
 

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

@@ -3040,6 +3040,8 @@ impl Step for Bootstrap {
             .args(["-m", "unittest", "bootstrap_test.py"])
             .env("BUILD_DIR", &builder.out)
             .env("BUILD_PLATFORM", builder.build.build.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.

src/bootstrap/src/core/config/config.rs

@@ -1329,6 +1329,17 @@ impl Config {
             TomlConfig::default()
         };
 
+        if cfg!(test) {
+            // When configuring bootstrap for tests, make sure to set the rustc and Cargo to the
+            // same ones used to call the tests (if custom ones are not defined in the toml). If we
+            // don't do that, bootstrap will use its own detection logic to find a suitable rustc
+            // and Cargo, which doesn't work when the caller is specìfying a custom local rustc or
+            // Cargo in their config.toml.
+            let build = toml.build.get_or_insert_with(Default::default);
+            build.rustc = build.rustc.take().or(std::env::var_os("RUSTC").map(|p| p.into()));
+            build.cargo = build.cargo.take().or(std::env::var_os("CARGO").map(|p| p.into()));
+        }
+
         if let Some(include) = &toml.profile {
             // Allows creating alias for profile names, allowing
             // profiles to be renamed while maintaining back compatibility
@@ -1447,7 +1458,12 @@ impl Config {
             rustc
         } else {
             config.download_beta_toolchain();
-            config.out.join(config.build.triple).join("stage0/bin/rustc")
+            config
+                .out
+                .join(config.build.triple)
+                .join("stage0")
+                .join("bin")
+                .join(exe("rustc", config.build))
         };
 
         config.initial_cargo = if let Some(cargo) = cargo {
@@ -1457,7 +1473,12 @@ impl Config {
             cargo
         } else {
             config.download_beta_toolchain();
-            config.out.join(config.build.triple).join("stage0/bin/cargo")
+            config
+                .out
+                .join(config.build.triple)
+                .join("stage0")
+                .join("bin")
+                .join(exe("cargo", config.build))
         };
 
         // NOTE: it's important this comes *after* we set `initial_rustc` just above.