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
```

3 files changed, 7 insertions, 4 deletions

diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 4eeaec7..361f5d6 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -1023,6 +1023,7 @@ class Compiler(metaclass=abc.ABCMeta):
         raise EnvironmentException('This compiler does not have a preprocessor')
 
     def get_default_include_dirs(self) -> T.List[str]:
+        # TODO: This is a candidate for returning an immutable list
         return []
 
     def get_largefile_args(self) -> T.List[str]:
diff --git a/mesonbuild/compilers/mixins/clike.py b/mesonbuild/compilers/mixins/clike.py
index 0637439..0c0ade3 100644
--- a/mesonbuild/compilers/mixins/clike.py
+++ b/mesonbuild/compilers/mixins/clike.py
@@ -42,6 +42,7 @@ from .visualstudio import VisualStudioLikeCompiler
 
 if T.TYPE_CHECKING:
     from ...dependencies import Dependency
+    from ..._typing import ImmutableListProtocol
     from ...environment import Environment
     from ...compilers.compilers import Compiler
     from ...programs import ExternalProgram
@@ -207,7 +208,7 @@ class CLikeCompiler(Compiler):
 
     @functools.lru_cache()
     def _get_library_dirs(self, env: 'Environment',
-                          elf_class: T.Optional[int] = None) -> T.List[str]:
+                          elf_class: T.Optional[int] = None) -> 'ImmutableListProtocol[str]':
         # TODO: replace elf_class with enum
         dirs = self.get_compiler_dirs(env, 'libraries')
         if elf_class is None or elf_class == 0:
@@ -253,7 +254,7 @@ class CLikeCompiler(Compiler):
         return self._get_library_dirs(env, elf_class).copy()
 
     @functools.lru_cache()
-    def _get_program_dirs(self, env: 'Environment') -> T.List[str]:
+    def _get_program_dirs(self, env: 'Environment') -> 'ImmutableListProtocol[str]':
         '''
         Programs used by the compiler. Also where toolchain DLLs such as
         libstdc++-6.dll are found with MinGW.
diff --git a/mesonbuild/compilers/mixins/gnu.py b/mesonbuild/compilers/mixins/gnu.py
index b007ff0..9a94703 100644
--- a/mesonbuild/compilers/mixins/gnu.py
+++ b/mesonbuild/compilers/mixins/gnu.py
@@ -28,6 +28,7 @@ from ... import mlog
 from ...mesonlib import OptionKey
 
 if T.TYPE_CHECKING:
+    from ..._typing import ImmutableListProtocol
     from ...environment import Environment
     from ..compilers import Compiler
 else:
@@ -92,7 +93,7 @@ gnu_color_args = {
 
 
 @functools.lru_cache(maxsize=None)
-def gnulike_default_include_dirs(compiler: T.Tuple[str], lang: str) -> T.List[str]:
+def gnulike_default_include_dirs(compiler: T.Tuple[str, ...], lang: str) -> 'ImmutableListProtocol[str]':
     lang_map = {
         'c': 'c',
         'cpp': 'c++',
@@ -189,7 +190,7 @@ class GnuLikeCompiler(Compiler, metaclass=abc.ABCMeta):
         return gnulike_instruction_set_args.get(instruction_set, None)
 
     def get_default_include_dirs(self) -> T.List[str]:
-        return gnulike_default_include_dirs(tuple(self.exelist), self.language)
+        return gnulike_default_include_dirs(tuple(self.exelist), self.language).copy()
 
     @abc.abstractmethod
     def openmp_flags(self) -> T.List[str]: