rust: Fix get_default_version() on system install without toolchain

pre_commit/languages/rust.py

@@ -36,12 +36,16 @@
 
 @functools.lru_cache(maxsize=1)
 def get_default_version() -> str:
-    # if rust is already installed, we can save a bunch of setup time by
-    # using the installed version
-    if helpers.exe_exists('cargo'):
+    # If rust is already installed, we can save a bunch of setup time by
+    # using the installed version.
+    #
+    # Just detecting the executable does not suffice, because if rustup is
+    # installed but no toolchain is available, then `cargo` exists but
+    # cannot be used without installing a toolchain first.
+    retcode, _, _ = cmd_output_b('cargo', '--version', retcode=0)
+    if retcode == 0:
         return 'system'
-    else:
-        return C.DEFAULT
+    return C.DEFAULT
 
 
 def _envdir(prefix: Prefix, version: str) -> str:

tests/languages/rust_test.py

@@ -18,18 +18,18 @@
 
 
 @pytest.fixture
-def find_exe_mck():
-    with mock.patch.object(parse_shebang, 'find_executable') as mck:
+def cmd_output_b_mck():
+    with mock.patch.object(rust, 'cmd_output_b') as mck:
         yield mck
 
 
-def test_sets_system_when_rust_is_available(find_exe_mck):
-    find_exe_mck.return_value = '/path/to/exe'
+def test_sets_system_when_rust_is_available(cmd_output_b_mck):
+    cmd_output_b_mck.return_value = (0, b'', b'')
     assert ACTUAL_GET_DEFAULT_VERSION() == 'system'
 
 
-def test_uses_default_when_rust_is_not_available(find_exe_mck):
-    find_exe_mck.return_value = None
+def test_uses_default_when_rust_is_not_available(cmd_output_b_mck):
+    cmd_output_b_mck.return_value = (127, b'', b'error: not found')
     assert ACTUAL_GET_DEFAULT_VERSION() == C.DEFAULT