Change Rust to install environment with cargo add over toml

pre_commit/languages/rust.py

@@ -11,8 +11,6 @@
 from typing import Generator
 from typing import Sequence
 
-import toml
-
 import pre_commit.constants as C
 from pre_commit import parse_shebang
 from pre_commit.envcontext import envcontext
@@ -82,18 +80,13 @@ def in_env(
 
 
 def _add_dependencies(
-        cargo_toml_path: str,
+        prefix: Prefix,
         additional_dependencies: set[str],
 ) -> None:
-    with open(cargo_toml_path, 'r+') as f:
-        cargo_toml = toml.load(f)
-        cargo_toml.setdefault('dependencies', {})
-        for dep in additional_dependencies:
-            name, _, spec = dep.partition(':')
-            cargo_toml['dependencies'][name] = spec or '*'
-        f.seek(0)
-        toml.dump(cargo_toml, f)
-        f.truncate()
+    for dep in additional_dependencies:
+        name, _, spec = dep.partition(':')
+        crate = name + (f'@{spec}' if spec else '')
+        helpers.run_setup_cmd(prefix, ('cargo', 'add', crate))
 
 
 def install_rust_with_toolchain(toolchain: str) -> None:
@@ -152,7 +145,7 @@ def install_environment(
     lib_deps = set(additional_dependencies) - cli_deps
 
     if len(lib_deps) > 0:
-        _add_dependencies(prefix.path('Cargo.toml'), lib_deps)
+        _add_dependencies(prefix, lib_deps)
 
     with clean_path_on_failure(directory):
         packages_to_install: set[tuple[str, ...]] = {('--path', '.')}

tests/repository_test.py

@@ -485,7 +485,7 @@ def test_additional_rust_lib_dependencies_installed(
     path = make_repo(tempdir_factory, 'rust_hooks_repo')
     config = make_config_from_repo(path)
     # A small rust package with no dependencies.
-    deps = ['shellharden:3.1.0']
+    deps = ['shellharden:3.1.0', 'git-version']
     config['hooks'][0]['additional_dependencies'] = deps
     hook = _get_hook(config, store, 'rust-hook')
     binaries = os.listdir(