rust: Add test for using system install of rust

tests/languages/rust_test.py

@@ -76,3 +76,33 @@ def mocked_find_executable(exe: str) -> str | None:
 
     with rust.in_env(prefix, '1.64.0'):
         assert cmd_output('hello_world')[1] == 'Hello, world!\n'
+
+
+@pytest.mark.skipif(
+    cmd_output('cargo', '--version', retcode=None)[0] != 0,
+    reason="requires working system installation of rust/cargo",
+)
+def test_installs_with_system_rust(tmpdir):
+    tmpdir.join('src', 'main.rs').ensure().write(
+        'fn main() {'
+        '    println!("Hello, world!");'
+        '}',
+    )
+    tmpdir.join('Cargo.toml').ensure().write(
+        toml.dumps({
+            'package': {
+                'name': 'hello_world',
+                'version': '0.1.0',
+                'edition': '2021',
+            },
+            'dependencies': {},
+        }),
+    )
+    prefix = Prefix(str(tmpdir))
+    rust.install_environment(prefix, 'system', ())
+
+    # this directory shouldn't exist, make sure we succeed without it existing
+    shutil.rmtree(tmpdir.join('target'))
+
+    with rust.in_env(prefix, 'system'):
+        assert cmd_output('hello_world')[1] == 'Hello, world!\n'