use an immutable list for an lru_cached functions

When mutable items are stored in an lru cache, changing the returned
items changes the cached items as well. Therefore we want to ensure that
we're not mutating them. Using the ImmutableListProtocol allows mypy to
find mutations and reject them. This doesn't solve the problem of
mutable values inside the values, so you could have to do things like:

```python
ImmutableListProtocol[ImmutableListProtocol[str]]
```

or equally hacky. It can also be used for input types and acts a bit
like C's const:

```python
def foo(arg: ImmutableListProtocol[str]) -> T.List[str]:
    arg[1] = 'foo'  # works while running, but mypy errors
```

1 files changed, 2 insertions, 1 deletions

diff --git a/mesonbuild/mesonlib/universal.py b/mesonbuild/mesonlib/universal.py
index 66149f9..b8fc763 100644
--- a/mesonbuild/mesonlib/universal.py
+++ b/mesonbuild/mesonlib/universal.py
@@ -30,6 +30,7 @@ import textwrap
 from mesonbuild import mlog
 
 if T.TYPE_CHECKING:
+    from .._typing import ImmutableListProtocol
     from ..build import ConfigurationData
     from ..coredata import KeyedOptionDictType, UserOption
     from ..compilers.compilers import CompilerType
@@ -659,7 +660,7 @@ def exe_exists(arglist: T.List[str]) -> bool:
 
 
 @lru_cache(maxsize=None)
-def darwin_get_object_archs(objpath: str) -> T.List[str]:
+def darwin_get_object_archs(objpath: str) -> 'ImmutableListProtocol[str]':
     '''
     For a specific object (executable, static library, dylib, etc), run `lipo`
     to fetch the list of archs supported by it. Supports both thin objects and