rust: Remove TOOLCHAIN_FROM_VERSION mapping

pre_commit/languages/rust.py

@@ -26,10 +26,6 @@
 from pre_commit.util import win_exe
 
 ENVIRONMENT_DIR = 'rustenv'
-TOOLCHAIN_FROM_VERSION = {
-    'system': None,
-    C.DEFAULT: 'stable',
-}
 
 health_check = helpers.basic_health_check
 
@@ -48,19 +44,35 @@ def get_default_version() -> str:
     return C.DEFAULT
 
 
+def _rust_toolchain(language_version: str) -> str:
+    '''Transform the language version into a rust toolchain version.'''
+    if language_version == 'system':
+        raise AssertionError(
+            'system rust install was requested, but pre-commit to set '
+            'a toolchain version -- this is probably a bug',
+        )
+
+    if language_version == C.DEFAULT:
+        return 'stable'
+
+    return language_version
+
+
 def _envdir(prefix: Prefix, version: str) -> str:
     directory = helpers.environment_dir(ENVIRONMENT_DIR, version)
     return prefix.path(directory)
 
 
 def get_env_patch(target_dir: str, version: str) -> PatchesT:
-    toolchain = TOOLCHAIN_FROM_VERSION.get(version, version)
     return (
         ('CARGO_HOME', target_dir),
         ('PATH', (os.path.join(target_dir, 'bin'), os.pathsep, Var('PATH'))),
         # Only set RUSTUP_TOOLCHAIN if we don't want use the system's default
         # toolchain
-        *((('RUSTUP_TOOLCHAIN', toolchain),) if toolchain else ()),
+        *(
+            (('RUSTUP_TOOLCHAIN', _rust_toolchain(version)),)
+            if version != 'system' else ()
+        ),
     )
 
 
@@ -171,9 +183,8 @@ def install_environment(
                 packages_to_install.add((package,))
 
         with in_env(prefix, version):
-            toolchain = TOOLCHAIN_FROM_VERSION.get(version, version)
-            if toolchain is not None:
-                install_rust_with_toolchain(toolchain)
+            if version != 'system':
+                install_rust_with_toolchain(_rust_toolchain(version))
 
             for args in packages_to_install:
                 cmd_output_b(