refactor: Refactor BothLibraries logic

This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.

5 files changed, 26 insertions, 4 deletions

diff --git a/mesonbuild/interpreterbase/__init__.py b/mesonbuild/interpreterbase/__init__.py
index d776ae1..298a034 100644
--- a/mesonbuild/interpreterbase/__init__.py
+++ b/mesonbuild/interpreterbase/__init__.py
@@ -33,12 +33,14 @@ __all__ = [
     'check_stringlist',
     'default_resolve_key',
     'flatten',
+    'resolve_second_level_holders',
 
     'noPosargs',
     'builtinMethodNoKwargs',
     'noKwargs',
     'stringArgs',
     'noArgsFlattening',
+    'noSecondLevelHolderResolving',
     'unholder_return',
     'disablerIfNotFound',
     'permittedKwargs',
@@ -91,6 +93,7 @@ from .decorators import (
     noKwargs,
     stringArgs,
     noArgsFlattening,
+    noSecondLevelHolderResolving,
     unholder_return,
     disablerIfNotFound,
     permittedKwargs,
@@ -115,5 +118,5 @@ from .exceptions import (
 )
 
 from .disabler import Disabler, is_disabled
-from .helpers import check_stringlist, default_resolve_key, flatten
+from .helpers import check_stringlist, default_resolve_key, flatten, resolve_second_level_holders
 from .interpreterbase import MesonVersionString, InterpreterBase
diff --git a/mesonbuild/interpreterbase/baseobjects.py b/mesonbuild/interpreterbase/baseobjects.py
index d82aad2..e65e4cb 100644
--- a/mesonbuild/interpreterbase/baseobjects.py
+++ b/mesonbuild/interpreterbase/baseobjects.py
@@ -14,7 +14,7 @@
 
 from .. import mparser
 from .exceptions import InvalidCode
-from .helpers import flatten
+from .helpers import flatten, resolve_second_level_holders
 from ..mesonlib import HoldableObject
 
 import typing as T
@@ -57,6 +57,8 @@ class InterpreterObject:
             method = self.methods[method_name]
             if not getattr(method, 'no-args-flattening', False):
                 args = flatten(args)
+            if not getattr(method, 'no-second-level-holder-flattening', False):
+                args, kwargs = resolve_second_level_holders(args, kwargs)
             return method(args, kwargs)
         raise InvalidCode(f'Unknown method "{method_name}" in object {self} of type {type(self).__name__}.')
 
diff --git a/mesonbuild/interpreterbase/decorators.py b/mesonbuild/interpreterbase/decorators.py
index b05df5b..b91e9d5 100644
--- a/mesonbuild/interpreterbase/decorators.py
+++ b/mesonbuild/interpreterbase/decorators.py
@@ -68,6 +68,10 @@ def noArgsFlattening(f: TV_func) -> TV_func:
     setattr(f, 'no-args-flattening', True)  # noqa: B010
     return f
 
+def noSecondLevelHolderResolving(f: TV_func) -> TV_func:
+    setattr(f, 'no-second-level-holder-flattening', True)  # noqa: B010
+    return f
+
 def unholder_return(f: TV_func) -> T.Callable[..., TYPE_var]:
     @wraps(f)
     def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
diff --git a/mesonbuild/interpreterbase/helpers.py b/mesonbuild/interpreterbase/helpers.py
index 2602e80..2352577 100644
--- a/mesonbuild/interpreterbase/helpers.py
+++ b/mesonbuild/interpreterbase/helpers.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from .. import mparser, mlog
+from .. import mesonlib, mparser, mlog
 from .exceptions import InvalidArguments, InterpreterException
 
 import collections.abc
@@ -38,6 +38,17 @@ def flatten(args: T.Union['TYPE_var', T.List['TYPE_var']]) -> T.List['TYPE_var']
             result.append(a)
     return result
 
+def resolve_second_level_holders(args: T.List['TYPE_var'], kwargs: 'TYPE_kwargs') -> T.Tuple[T.List['TYPE_var'], 'TYPE_kwargs']:
+    def resolver(arg: 'TYPE_var') -> 'TYPE_var':
+        if isinstance(arg, list):
+            return [resolver(x) for x in arg]
+        if isinstance(arg, dict):
+            return {k: resolver(v) for k, v in arg.items()}
+        if isinstance(arg, mesonlib.SecondLevelHolder):
+            return arg.get_default_object()
+        return arg
+    return [resolver(x) for x in args], {k: resolver(v) for k, v in kwargs.items()}
+
 def check_stringlist(a: T.Any, msg: str = 'Arguments must be strings.') -> None:
     if not isinstance(a, list):
         mlog.debug('Not a list:', str(a))
diff --git a/mesonbuild/interpreterbase/interpreterbase.py b/mesonbuild/interpreterbase/interpreterbase.py
index 3ea10f4..3f5726b 100644
--- a/mesonbuild/interpreterbase/interpreterbase.py
+++ b/mesonbuild/interpreterbase/interpreterbase.py
@@ -42,7 +42,7 @@ from .exceptions import (
 
 from .decorators import FeatureNew, builtinMethodNoKwargs
 from .disabler import Disabler, is_disabled
-from .helpers import check_stringlist, default_resolve_key, flatten
+from .helpers import check_stringlist, default_resolve_key, flatten, resolve_second_level_holders
 from ._unholder import _unholder
 
 import os, copy, re
@@ -546,6 +546,8 @@ The result of this is undefined and will become a hard error in a future Meson r
             func_args = posargs
             if not getattr(func, 'no-args-flattening', False):
                 func_args = flatten(posargs)
+            if not getattr(func, 'no-second-level-holder-flattening', False):
+                func_args, kwargs = resolve_second_level_holders(func_args, kwargs)
             res = func(node, func_args, kwargs)
             return self._holderify(res)
         else: