Build bootstrap with an explicit target

src/bootstrap/bootstrap.py

@@ -1001,17 +1001,20 @@ def bootstrap_out(self):
 
         >>> rb = RustBuild()
         >>> rb.build_dir = "build"
-        >>> rb.bootstrap_binary() == os.path.join("build", "bootstrap")
+        >>> rb.bootstrap_out() == os.path.join("build", "bootstrap", rb.build)
         True
         """
+        return os.path.join(self.bootstrap_cargo_target_dir(), self.build)
+
+    def bootstrap_cargo_target_dir(self):
         return os.path.join(self.build_dir, "bootstrap")
 
     def bootstrap_binary(self):
         """Return the path of the bootstrap binary
 
         >>> rb = RustBuild()
         >>> rb.build_dir = "build"
-        >>> rb.bootstrap_binary() == os.path.join("build", "bootstrap",
+        >>> rb.bootstrap_binary() == os.path.join("build", "bootstrap", rb.build,
         ... "debug", "bootstrap")
         True
         """
@@ -1033,8 +1036,7 @@ def build_bootstrap(self):
             print("::endgroup::")
 
     def build_bootstrap_cmd(self, env):
-        """For tests."""
-        build_dir = os.path.join(self.build_dir, "bootstrap")
+        build_dir = self.bootstrap_cargo_target_dir()
         if self.clean and os.path.exists(build_dir):
             shutil.rmtree(build_dir)
         # `CARGO_BUILD_TARGET` breaks bootstrap build.
@@ -1120,10 +1122,14 @@ def build_bootstrap_cmd(self, env):
             self.cargo(),
             "build",
             "--jobs=" + self.jobs,
+            "--target",
+            self.build,
             "--manifest-path",
             os.path.join(self.rust_root, "src/bootstrap/Cargo.toml"),
             "-Zroot-dir=" + self.rust_root,
+            "--bins",
         ]
+
         # verbose cargo output is very noisy, so only enable it with -vv
         args.extend("--verbose" for _ in range(self.verbose - 1))
         if self.verbose < 0:

src/bootstrap/bootstrap_test.py

@@ -238,6 +238,21 @@ def test_cargoflags(self):
         args, _ = self.build_args(env={"CARGOFLAGS": "--timings"})
         self.assertTrue("--timings" in args)
 
+    def test_build_bootstrap_uses_explicit_build_target(self):
+        args, _ = self.build_args(
+            configure_args=[
+                "--set",
+                "build.cargo=" + sys.executable,
+                "--set",
+                "build.rustc=" + sys.executable,
+            ],
+            args=["--build", "host-tuple"],
+        )
+        expected_build = os.environ.get("BUILD_PLATFORM", "host-tuple")
+        self.assertEqual(args[1], "build")
+        self.assertTrue("--bins" in args)
+        self.assertEqual(args[args.index("--target") + 1], expected_build)
+
     def test_warnings(self):
         for toml_warnings in ["false", "true", None]:
             configure_args = []