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, 6 insertions, 6 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index 1e5a13e..484e4cc 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -36,6 +36,7 @@ from ..mesonlib import (
 )
 
 if T.TYPE_CHECKING:
+    from .._typing import ImmutableListProtocol
     from ..arglist import CompilerArgs
     from ..compilers import Compiler
     from ..interpreter import Interpreter, Test
@@ -1103,21 +1104,20 @@ class Backend:
         return result
 
     @lru_cache(maxsize=None)
-    def get_custom_target_provided_by_generated_source(self, generated_source):
-        libs = []
+    def get_custom_target_provided_by_generated_source(self, generated_source: build.CustomTarget) -> 'ImmutableListProtocol[str]':
+        libs: T.List[str] = []
         for f in generated_source.get_outputs():
             if self.environment.is_library(f):
                 libs.append(os.path.join(self.get_target_dir(generated_source), f))
         return libs
 
     @lru_cache(maxsize=None)
-    def get_custom_target_provided_libraries(self, target):
-        libs = []
+    def get_custom_target_provided_libraries(self, target: T.Union[build.BuildTarget, build.CustomTarget]) -> 'ImmutableListProtocol[str]':
+        libs: T.List[str] = []
         for t in target.get_generated_sources():
             if not isinstance(t, build.CustomTarget):
                 continue
-            l = self.get_custom_target_provided_by_generated_source(t)
-            libs = libs + l
+            libs.extend(self.get_custom_target_provided_by_generated_source(t))
         return libs
 
     def is_unity(self, target):