fix(providers/cargo): don't crash on workspace member with fixed version

When iterating workspace members in `CargoProvider.set_lock_version`, the
code subscripted `package["version"]["workspace"]` inside a try/except
block that only caught `NonExistentKey`. If a member's `Cargo.toml`
declared a hardcoded version (`version = "x.y.z"`) instead of
`version.workspace = true`, `package["version"]` was a tomlkit `String`
and the subscript raised `TypeError: string indices must be integers,
not 'str'`, which was not caught and propagated as a crash.

Replace the exception-driven check with an `isinstance` type guard so
the inheritance lookup only runs when `version` is an actual table.

Fixes #2001

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

commitizen/providers/cargo_provider.py

@@ -82,15 +82,15 @@ def set_lock_version(self, version: str) -> None:
                     ).get("package", {})
                     if TYPE_CHECKING:
                         assert isinstance(package_content, dict)
-                    try:
-                        version_workspace = package_content["version"]["workspace"]
-                        if version_workspace is True:
-                            package_name = package_content["name"]
-                            if TYPE_CHECKING:
-                                assert isinstance(package_name, str)
-                            members_inheriting.append(package_name)
-                    except NonExistentKey:
-                        pass
+                    version_field = package_content.get("version")
+                    if (
+                        isinstance(version_field, dict)
+                        and version_field.get("workspace") is True
+                    ):
+                        package_name = package_content["name"]
+                        if TYPE_CHECKING:
+                            assert isinstance(package_name, str)
+                        members_inheriting.append(package_name)
 
             for i, package in enumerate(packages):
                 if package["name"] in members_inheriting:

tests/providers/test_cargo_provider.py

@@ -451,3 +451,80 @@ def test_cargo_provider_workspace_member_without_workspace_key(
     assert file.read_text() == dedent(expected_workspace_toml)
     # The lock file should remain unchanged since the member doesn't inherit workspace version
     assert lock_file.read_text() == dedent(expected_lock_content)
+
+
+def test_cargo_provider_workspace_member_with_fixed_version(
+    config: BaseConfig,
+    chdir: Path,
+):
+    """Regression test for https://github.com/commitizen-tools/commitizen/issues/2001.
+
+    A workspace member with a hardcoded ``version = "x.y.z"`` string (rather
+    than ``version.workspace = true``) used to crash ``set_lock_version`` with
+    ``TypeError: string indices must be integers, not 'str'`` because the code
+    unconditionally subscripted ``package["version"]["workspace"]``.
+    """
+    workspace_toml = """\
+[workspace]
+members = ["member_with_fixed_version"]
+
+[workspace.package]
+version = "0.1.0"
+"""
+
+    # Real Cargo syntax for a workspace member that opts out of the
+    # workspace-inherited version by pinning its own.
+    member_content = """\
+[package]
+name = "member_with_fixed_version"
+version = "0.9.0"
+"""
+
+    lock_content = """\
+[[package]]
+name = "member_with_fixed_version"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "123abc"
+"""
+
+    expected_workspace_toml = """\
+[workspace]
+members = ["member_with_fixed_version"]
+
+[workspace.package]
+version = "42.1"
+"""
+
+    # The pinned member must not be bumped because it does not inherit the
+    # workspace version.
+    expected_lock_content = """\
+[[package]]
+name = "member_with_fixed_version"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "123abc"
+"""
+
+    filename = CargoProvider.filename
+    file = chdir / filename
+    file.write_text(dedent(workspace_toml))
+
+    os.mkdir(chdir / "member_with_fixed_version")
+    member_file = chdir / "member_with_fixed_version" / "Cargo.toml"
+    member_file.write_text(dedent(member_content))
+
+    lock_filename = CargoProvider.lock_filename
+    lock_file = chdir / lock_filename
+    lock_file.write_text(dedent(lock_content))
+
+    config.settings["version_provider"] = "cargo"
+
+    provider = get_provider(config)
+    assert isinstance(provider, CargoProvider)
+    assert provider.get_version() == "0.1.0"
+
+    # Must not raise TypeError: string indices must be integers, not 'str'.
+    provider.set_version("42.1")
+    assert file.read_text() == dedent(expected_workspace_toml)
+    assert lock_file.read_text() == dedent(expected_lock_content)