Add submodule dependency filesHEADmaster

Change-Id: Iaf8d18082d3991dec7c0ebbea540f092188eb4ec

8 files changed, 2059 insertions, 0 deletions

diff --git a/meson/mesonbuild/interpreterbase/__init__.py b/meson/mesonbuild/interpreterbase/__init__.py
new file mode 100644
index 000000000..8e45cdb8f
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/__init__.py
@@ -0,0 +1,122 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+__all__ = [
+    'InterpreterObject',
+    'MesonInterpreterObject',
+    'ObjectHolder',
+    'RangeHolder',
+    'MesonVersionString',
+    'MutableInterpreterObject',
+
+    'Disabler',
+    'is_disabled',
+
+    'InterpreterException',
+    'InvalidCode',
+    'InvalidArguments',
+    'SubdirDoneRequest',
+    'ContinueRequest',
+    'BreakRequest',
+
+    'check_stringlist',
+    'default_resolve_key',
+    'flatten',
+    'resolve_second_level_holders',
+
+    'noPosargs',
+    'builtinMethodNoKwargs',
+    'noKwargs',
+    'stringArgs',
+    'noArgsFlattening',
+    'noSecondLevelHolderResolving',
+    'permissive_unholder_return',
+    'disablerIfNotFound',
+    'permittedKwargs',
+    'typed_pos_args',
+    'ContainerTypeInfo',
+    'KwargInfo',
+    'typed_kwargs',
+    'FeatureCheckBase',
+    'FeatureNew',
+    'FeatureDeprecated',
+    'FeatureNewKwargs',
+    'FeatureDeprecatedKwargs',
+
+    'InterpreterBase',
+
+    'TV_fw_var',
+    'TV_fw_args',
+    'TV_fw_kwargs',
+    'TV_func',
+    'TYPE_elementary',
+    'TYPE_var',
+    'TYPE_nvar',
+    'TYPE_kwargs',
+    'TYPE_nkwargs',
+    'TYPE_key_resolver',
+]
+
+from .baseobjects import (
+    InterpreterObject,
+    MesonInterpreterObject,
+    ObjectHolder,
+    RangeHolder,
+    MutableInterpreterObject,
+
+    TV_fw_var,
+    TV_fw_args,
+    TV_fw_kwargs,
+    TV_func,
+    TYPE_elementary,
+    TYPE_var,
+    TYPE_nvar,
+    TYPE_kwargs,
+    TYPE_nkwargs,
+    TYPE_key_resolver,
+)
+
+from .decorators import (
+    noPosargs,
+    builtinMethodNoKwargs,
+    noKwargs,
+    stringArgs,
+    noArgsFlattening,
+    noSecondLevelHolderResolving,
+    permissive_unholder_return,
+    disablerIfNotFound,
+    permittedKwargs,
+    typed_pos_args,
+    ContainerTypeInfo,
+    KwargInfo,
+    typed_kwargs,
+    FeatureCheckBase,
+    FeatureNew,
+    FeatureDeprecated,
+    FeatureNewKwargs,
+    FeatureDeprecatedKwargs,
+)
+
+from .exceptions import (
+    InterpreterException,
+    InvalidCode,
+    InvalidArguments,
+    SubdirDoneRequest,
+    ContinueRequest,
+    BreakRequest,
+)
+
+from .disabler import Disabler, is_disabled
+from .helpers import check_stringlist, default_resolve_key, flatten, resolve_second_level_holders
+from .interpreterbase import MesonVersionString, InterpreterBase
diff --git a/meson/mesonbuild/interpreterbase/_unholder.py b/meson/mesonbuild/interpreterbase/_unholder.py
new file mode 100644
index 000000000..10c7cfc8f
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/_unholder.py
@@ -0,0 +1,39 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .baseobjects import InterpreterObject, MesonInterpreterObject, ObjectHolder, TYPE_var
+from .exceptions import InvalidArguments
+from ..mesonlib import HoldableObject, MesonBugException
+
+import typing as T
+
+def _unholder(obj: T.Union[TYPE_var, InterpreterObject], *, permissive: bool = False) -> TYPE_var:
+    if isinstance(obj, (int, bool, str)):
+        return obj
+    elif isinstance(obj, list):
+        return [_unholder(x, permissive=permissive) for x in obj]
+    elif isinstance(obj, dict):
+        return {k: _unholder(v, permissive=permissive) for k, v in obj.items()}
+    elif isinstance(obj, ObjectHolder):
+        assert isinstance(obj.held_object, HoldableObject)
+        return obj.held_object
+    elif isinstance(obj, MesonInterpreterObject):
+        return obj
+    elif isinstance(obj, HoldableObject) and permissive:
+        return obj
+    elif isinstance(obj, HoldableObject):
+        raise MesonBugException(f'Argument {obj} of type {type(obj).__name__} is not held by an ObjectHolder.')
+    elif isinstance(obj, InterpreterObject):
+        raise InvalidArguments(f'Argument {obj} of type {type(obj).__name__} cannot be passed to a method or function')
+    raise MesonBugException(f'Unknown object {obj} of type {type(obj).__name__} in the parameters.')
diff --git a/meson/mesonbuild/interpreterbase/baseobjects.py b/meson/mesonbuild/interpreterbase/baseobjects.py
new file mode 100644
index 000000000..8b1293ca2
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/baseobjects.py
@@ -0,0 +1,96 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .. import mparser
+from .exceptions import InvalidCode
+from .helpers import flatten, resolve_second_level_holders
+from ..mesonlib import HoldableObject
+
+import typing as T
+
+if T.TYPE_CHECKING:
+    # Object holders need the actual interpreter
+    from ..interpreter import Interpreter
+
+TV_fw_var = T.Union[str, int, bool, list, dict, 'InterpreterObject']
+TV_fw_args = T.List[T.Union[mparser.BaseNode, TV_fw_var]]
+TV_fw_kwargs = T.Dict[str, T.Union[mparser.BaseNode, TV_fw_var]]
+
+TV_func = T.TypeVar('TV_func', bound=T.Callable[..., T.Any])
+
+TYPE_elementary = T.Union[str, int, bool, T.List[T.Any], T.Dict[str, T.Any]]
+TYPE_var = T.Union[TYPE_elementary, HoldableObject, 'MesonInterpreterObject']
+TYPE_nvar = T.Union[TYPE_var, mparser.BaseNode]
+TYPE_kwargs = T.Dict[str, TYPE_var]
+TYPE_nkwargs = T.Dict[str, TYPE_nvar]
+TYPE_key_resolver = T.Callable[[mparser.BaseNode], str]
+
+class InterpreterObject:
+    def __init__(self, *, subproject: T.Optional[str] = None) -> None:
+        self.methods: T.Dict[
+            str,
+            T.Callable[[T.List[TYPE_var], TYPE_kwargs], TYPE_var]
+        ] = {}
+        # Current node set during a method call. This can be used as location
+        # when printing a warning message during a method call.
+        self.current_node:  mparser.BaseNode = None
+        self.subproject: str = subproject or ''
+
+    def method_call(
+                self,
+                method_name: str,
+                args: T.List[TYPE_var],
+                kwargs: TYPE_kwargs
+            ) -> TYPE_var:
+        if method_name in self.methods:
+            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__}.')
+
+class MesonInterpreterObject(InterpreterObject):
+    ''' All non-elementary objects and non-object-holders should be derived from this '''
+
+class MutableInterpreterObject:
+    ''' Dummy class to mark the object type as mutable '''
+
+InterpreterObjectTypeVar = T.TypeVar('InterpreterObjectTypeVar', bound=HoldableObject)
+
+class ObjectHolder(InterpreterObject, T.Generic[InterpreterObjectTypeVar]):
+    def __init__(self, obj: InterpreterObjectTypeVar, interpreter: 'Interpreter') -> None:
+        super().__init__(subproject=interpreter.subproject)
+        assert isinstance(obj, HoldableObject), f'This is a bug: Trying to hold object of type `{type(obj).__name__}` that is not an `HoldableObject`'
+        self.held_object = obj
+        self.interpreter = interpreter
+        self.env = self.interpreter.environment
+
+    def __repr__(self) -> str:
+        return f'<[{type(self).__name__}] holds [{type(self.held_object).__name__}]: {self.held_object!r}>'
+
+class RangeHolder(MesonInterpreterObject):
+    def __init__(self, start: int, stop: int, step: int, *, subproject: str) -> None:
+        super().__init__(subproject=subproject)
+        self.range = range(start, stop, step)
+
+    def __iter__(self) -> T.Iterator[int]:
+        return iter(self.range)
+
+    def __getitem__(self, key: int) -> int:
+        return self.range[key]
+
+    def __len__(self) -> int:
+        return len(self.range)
diff --git a/meson/mesonbuild/interpreterbase/decorators.py b/meson/mesonbuild/interpreterbase/decorators.py
new file mode 100644
index 000000000..eabc6d864
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/decorators.py
@@ -0,0 +1,650 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .. import mesonlib, mlog
+from .baseobjects import TV_func, TYPE_var
+from .disabler import Disabler
+from .exceptions import InterpreterException, InvalidArguments
+from .helpers import check_stringlist, get_callee_args
+from ._unholder import _unholder
+
+from functools import wraps
+import abc
+import itertools
+import typing as T
+
+def noPosargs(f: TV_func) -> TV_func:
+    @wraps(f)
+    def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+        args = get_callee_args(wrapped_args)[2]
+        if args:
+            raise InvalidArguments('Function does not take positional arguments.')
+        return f(*wrapped_args, **wrapped_kwargs)
+    return T.cast(TV_func, wrapped)
+
+def builtinMethodNoKwargs(f: TV_func) -> TV_func:
+    @wraps(f)
+    def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+        node = wrapped_args[0].current_node
+        method_name = wrapped_args[2]
+        kwargs = wrapped_args[4]
+        if kwargs:
+            mlog.warning(f'Method {method_name!r} does not take keyword arguments.',
+                         'This will become a hard error in the future',
+                         location=node)
+        return f(*wrapped_args, **wrapped_kwargs)
+    return T.cast(TV_func, wrapped)
+
+def noKwargs(f: TV_func) -> TV_func:
+    @wraps(f)
+    def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+        kwargs = get_callee_args(wrapped_args)[3]
+        if kwargs:
+            raise InvalidArguments('Function does not take keyword arguments.')
+        return f(*wrapped_args, **wrapped_kwargs)
+    return T.cast(TV_func, wrapped)
+
+def stringArgs(f: TV_func) -> TV_func:
+    @wraps(f)
+    def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+        args = get_callee_args(wrapped_args)[2]
+        assert(isinstance(args, list))
+        check_stringlist(args)
+        return f(*wrapped_args, **wrapped_kwargs)
+    return T.cast(TV_func, wrapped)
+
+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 permissive_unholder_return(f: TV_func) -> T.Callable[..., TYPE_var]:
+    @wraps(f)
+    def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+        res = f(*wrapped_args, **wrapped_kwargs)
+        return _unholder(res, permissive=True)
+    return T.cast(T.Callable[..., TYPE_var], wrapped)
+
+def disablerIfNotFound(f: TV_func) -> TV_func:
+    @wraps(f)
+    def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+        kwargs = get_callee_args(wrapped_args)[3]
+        disabler = kwargs.pop('disabler', False)
+        ret = f(*wrapped_args, **wrapped_kwargs)
+        if disabler and not ret.found():
+            return Disabler()
+        return ret
+    return T.cast(TV_func, wrapped)
+
+class permittedKwargs:
+
+    def __init__(self, permitted: T.Set[str]):
+        self.permitted = permitted  # type: T.Set[str]
+
+    def __call__(self, f: TV_func) -> TV_func:
+        @wraps(f)
+        def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+            s, node, args, kwargs, _ = get_callee_args(wrapped_args)
+            for k in kwargs:
+                if k not in self.permitted:
+                    mlog.warning(f'''Passed invalid keyword argument "{k}".''', location=node)
+                    mlog.warning('This will become a hard error in the future.')
+            return f(*wrapped_args, **wrapped_kwargs)
+        return T.cast(TV_func, wrapped)
+
+
+def typed_pos_args(name: str, *types: T.Union[T.Type, T.Tuple[T.Type, ...]],
+                   varargs: T.Optional[T.Union[T.Type, T.Tuple[T.Type, ...]]] = None,
+                   optargs: T.Optional[T.List[T.Union[T.Type, T.Tuple[T.Type, ...]]]] = None,
+                   min_varargs: int = 0, max_varargs: int = 0) -> T.Callable[..., T.Any]:
+    """Decorator that types type checking of positional arguments.
+
+    This supports two different models of optional aguments, the first is the
+    variadic argument model. Variadic arguments are a possibly bounded,
+    possibly unbounded number of arguments of the same type (unions are
+    supported). The second is the standard default value model, in this case
+    a number of optional arguments may be provided, but they are still
+    ordered, and they may have different types.
+
+    This function does not support mixing variadic and default arguments.
+
+    :name: The name of the decorated function (as displayed in error messages)
+    :varargs: They type(s) of any variadic arguments the function takes. If
+        None the function takes no variadic args
+    :min_varargs: the minimum number of variadic arguments taken
+    :max_varargs: the maximum number of variadic arguments taken. 0 means unlimited
+    :optargs: The types of any optional arguments parameters taken. If None
+        then no optional paramters are taken.
+
+    Some examples of usage blow:
+    >>> @typed_pos_args('mod.func', str, (str, int))
+    ... def func(self, state: ModuleState, args: T.Tuple[str, T.Union[str, int]], kwargs: T.Dict[str, T.Any]) -> T.Any:
+    ...     pass
+
+    >>> @typed_pos_args('method', str, varargs=str)
+    ... def method(self, node: BaseNode, args: T.Tuple[str, T.List[str]], kwargs: T.Dict[str, T.Any]) -> T.Any:
+    ...     pass
+
+    >>> @typed_pos_args('method', varargs=str, min_varargs=1)
+    ... def method(self, node: BaseNode, args: T.Tuple[T.List[str]], kwargs: T.Dict[str, T.Any]) -> T.Any:
+    ...     pass
+
+    >>> @typed_pos_args('method', str, optargs=[(str, int), str])
+    ... def method(self, node: BaseNode, args: T.Tuple[str, T.Optional[T.Union[str, int]], T.Optional[str]], kwargs: T.Dict[str, T.Any]) -> T.Any:
+    ...     pass
+
+    When should you chose `typed_pos_args('name', varargs=str,
+    min_varargs=1)` vs `typed_pos_args('name', str, varargs=str)`?
+
+    The answer has to do with the semantics of the function, if all of the
+    inputs are the same type (such as with `files()`) then the former is
+    correct, all of the arguments are string names of files. If the first
+    argument is something else the it should be separated.
+    """
+    def inner(f: TV_func) -> TV_func:
+
+        @wraps(f)
+        def wrapper(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+            args = get_callee_args(wrapped_args)[2]
+
+            # These are implementation programming errors, end users should never see them.
+            assert isinstance(args, list), args
+            assert max_varargs >= 0, 'max_varags cannot be negative'
+            assert min_varargs >= 0, 'min_varags cannot be negative'
+            assert optargs is None or varargs is None, \
+                'varargs and optargs not supported together as this would be ambiguous'
+
+            num_args = len(args)
+            num_types = len(types)
+            a_types = types
+
+            if varargs:
+                min_args = num_types + min_varargs
+                max_args = num_types + max_varargs
+                if max_varargs == 0 and num_args < min_args:
+                    raise InvalidArguments(f'{name} takes at least {min_args} arguments, but got {num_args}.')
+                elif max_varargs != 0 and (num_args < min_args or num_args > max_args):
+                    raise InvalidArguments(f'{name} takes between {min_args} and {max_args} arguments, but got {num_args}.')
+            elif optargs:
+                if num_args < num_types:
+                    raise InvalidArguments(f'{name} takes at least {num_types} arguments, but got {num_args}.')
+                elif num_args > num_types + len(optargs):
+                    raise InvalidArguments(f'{name} takes at most {num_types + len(optargs)} arguments, but got {num_args}.')
+                # Add the number of positional arguments required
+                if num_args > num_types:
+                    diff = num_args - num_types
+                    a_types = tuple(list(types) + list(optargs[:diff]))
+            elif num_args != num_types:
+                raise InvalidArguments(f'{name} takes exactly {num_types} arguments, but got {num_args}.')
+
+            for i, (arg, type_) in enumerate(itertools.zip_longest(args, a_types, fillvalue=varargs), start=1):
+                if not isinstance(arg, type_):
+                    if isinstance(type_, tuple):
+                        shouldbe = 'one of: {}'.format(", ".join(f'"{t.__name__}"' for t in type_))
+                    else:
+                        shouldbe = f'"{type_.__name__}"'
+                    raise InvalidArguments(f'{name} argument {i} was of type "{type(arg).__name__}" but should have been {shouldbe}')
+
+            # Ensure that we're actually passing a tuple.
+            # Depending on what kind of function we're calling the length of
+            # wrapped_args can vary.
+            nargs = list(wrapped_args)
+            i = nargs.index(args)
+            if varargs:
+                # if we have varargs we need to split them into a separate
+                # tuple, as python's typing doesn't understand tuples with
+                # fixed elements and variadic elements, only one or the other.
+                # so in that case we need T.Tuple[int, str, float, T.Tuple[str, ...]]
+                pos = args[:len(types)]
+                var = list(args[len(types):])
+                pos.append(var)
+                nargs[i] = tuple(pos)
+            elif optargs:
+                if num_args < num_types + len(optargs):
+                    diff =  num_types + len(optargs) - num_args
+                    nargs[i] = tuple(list(args) + [None] * diff)
+                else:
+                    nargs[i] = args
+            else:
+                nargs[i] = tuple(args)
+            return f(*nargs, **wrapped_kwargs)
+
+        return T.cast(TV_func, wrapper)
+    return inner
+
+
+class ContainerTypeInfo:
+
+    """Container information for keyword arguments.
+
+    For keyword arguments that are containers (list or dict), this class encodes
+    that information.
+
+    :param container: the type of container
+    :param contains: the types the container holds
+    :param pairs: if the container is supposed to be of even length.
+        This is mainly used for interfaces that predate the addition of dictionaries, and use
+        `[key, value, key2, value2]` format.
+    :param allow_empty: Whether this container is allowed to be empty
+        There are some cases where containers not only must be passed, but must
+        not be empty, and other cases where an empty container is allowed.
+    """
+
+    def __init__(self, container: T.Type, contains: T.Union[T.Type, T.Tuple[T.Type, ...]], *,
+                 pairs: bool = False, allow_empty: bool = True) :
+        self.container = container
+        self.contains = contains
+        self.pairs = pairs
+        self.allow_empty = allow_empty
+
+    def check(self, value: T.Any) -> T.Optional[str]:
+        """Check that a value is valid.
+
+        :param value: A value to check
+        :return: If there is an error then a string message, otherwise None
+        """
+        if not isinstance(value, self.container):
+            return f'container type was "{type(value).__name__}", but should have been "{self.container.__name__}"'
+        iter_ = iter(value.values()) if isinstance(value, dict) else iter(value)
+        for each in iter_:
+            if not isinstance(each, self.contains):
+                if isinstance(self.contains, tuple):
+                    shouldbe = 'one of: {}'.format(", ".join(f'"{t.__name__}"' for t in self.contains))
+                else:
+                    shouldbe = f'"{self.contains.__name__}"'
+                return f'contained a value of type "{type(each).__name__}" but should have been {shouldbe}'
+        if self.pairs and len(value) % 2 != 0:
+            return 'container should be of even length, but is not'
+        if not value and not self.allow_empty:
+            return 'container is empty, but not allowed to be'
+        return None
+
+
+_T = T.TypeVar('_T')
+
+class _NULL_T:
+    """Special null type for evolution, this is an implementation detail."""
+
+
+_NULL = _NULL_T()
+
+class KwargInfo(T.Generic[_T]):
+
+    """A description of a keyword argument to a meson function
+
+    This is used to describe a value to the :func:typed_kwargs function.
+
+    :param name: the name of the parameter
+    :param types: A type or tuple of types that are allowed, or a :class:ContainerType
+    :param required: Whether this is a required keyword argument. defaults to False
+    :param listify: If true, then the argument will be listified before being
+        checked. This is useful for cases where the Meson DSL allows a scalar or
+        a container, but internally we only want to work with containers
+    :param default: A default value to use if this isn't set. defaults to None,
+        this may be safely set to a mutable type, as long as that type does not
+        itself contain mutable types, typed_kwargs will copy the default
+    :param since: Meson version in which this argument has been added. defaults to None
+    :param deprecated: Meson version in which this argument has been deprecated. defaults to None
+    :param validator: A callable that does additional validation. This is mainly
+        intended for cases where a string is expected, but only a few specific
+        values are accepted. Must return None if the input is valid, or a
+        message if the input is invalid
+    :param convertor: A callable that converts the raw input value into a
+        different type. This is intended for cases such as the meson DSL using a
+        string, but the implementation using an Enum. This should not do
+        validation, just converstion.
+    :param deprecated_values: a dictionary mapping a value to the version of
+        meson it was deprecated in.
+    :param since_values: a dictionary mapping a value to the version of meson it was
+        added in.
+    :param not_set_warning: A warning messsage that is logged if the kwarg is not
+        set by the user.
+    """
+
+    def __init__(self, name: str, types: T.Union[T.Type[_T], T.Tuple[T.Type[_T], ...], ContainerTypeInfo],
+                 *, required: bool = False, listify: bool = False,
+                 default: T.Optional[_T] = None,
+                 since: T.Optional[str] = None,
+                 since_values: T.Optional[T.Dict[str, str]] = None,
+                 deprecated: T.Optional[str] = None,
+                 deprecated_values: T.Optional[T.Dict[str, str]] = None,
+                 validator: T.Optional[T.Callable[[_T], T.Optional[str]]] = None,
+                 convertor: T.Optional[T.Callable[[_T], TYPE_var]] = None,
+                 not_set_warning: T.Optional[str] = None):
+        self.name = name
+        self.types = types
+        self.required = required
+        self.listify = listify
+        self.default = default
+        self.since_values = since_values
+        self.since = since
+        self.deprecated = deprecated
+        self.deprecated_values = deprecated_values
+        self.validator = validator
+        self.convertor = convertor
+        self.not_set_warning = not_set_warning
+
+    def evolve(self, *,
+               required: T.Union[bool, _NULL_T] = _NULL,
+               listify: T.Union[bool, _NULL_T] = _NULL,
+               default: T.Union[_T, None, _NULL_T] = _NULL,
+               since: T.Union[str, None, _NULL_T] = _NULL,
+               since_values: T.Union[T.Dict[str, str], None, _NULL_T] = _NULL,
+               deprecated: T.Union[str, None, _NULL_T] = _NULL,
+               deprecated_values: T.Union[T.Dict[str, str], None, _NULL_T] = _NULL,
+               validator: T.Union[T.Callable[[_T], T.Optional[str]], None, _NULL_T] = _NULL,
+               convertor: T.Union[T.Callable[[_T], TYPE_var], None, _NULL_T] = _NULL) -> 'KwargInfo':
+        """Create a shallow copy of this KwargInfo, with modifications.
+
+        This allows us to create a new copy of a KwargInfo with modifications.
+        This allows us to use a shared kwarg that implements complex logic, but
+        has slight differences in usage, such as being added to different
+        functions in different versions of Meson.
+
+        The use the _NULL special value here allows us to pass None, which has
+        meaning in many of these cases. _NULL itself is never stored, always
+        being replaced by either the copy in self, or the provided new version.
+        """
+        return type(self)(
+            self.name,
+            self.types,
+            listify=listify if not isinstance(listify, _NULL_T) else self.listify,
+            required=required if not isinstance(required, _NULL_T) else self.required,
+            default=default if not isinstance(default, _NULL_T) else self.default,
+            since=since if not isinstance(since, _NULL_T) else self.since,
+            since_values=since_values if not isinstance(since_values, _NULL_T) else self.since_values,
+            deprecated=deprecated if not isinstance(deprecated, _NULL_T) else self.deprecated,
+            deprecated_values=deprecated_values if not isinstance(deprecated_values, _NULL_T) else self.deprecated_values,
+            validator=validator if not isinstance(validator, _NULL_T) else self.validator,
+            convertor=convertor if not isinstance(convertor, _NULL_T) else self.convertor,
+        )
+
+
+
+def typed_kwargs(name: str, *types: KwargInfo) -> T.Callable[..., T.Any]:
+    """Decorator for type checking keyword arguments.
+
+    Used to wrap a meson DSL implementation function, where it checks various
+    things about keyword arguments, including the type, and various other
+    information. For non-required values it sets the value to a default, which
+    means the value will always be provided.
+
+    If type tyhpe is a :class:ContainerTypeInfo, then the default value will be
+    passed as an argument to the container initializer, making a shallow copy
+
+    :param name: the name of the function, including the object it's attached ot
+        (if applicable)
+    :param *types: KwargInfo entries for each keyword argument.
+    """
+    def inner(f: TV_func) -> TV_func:
+
+        @wraps(f)
+        def wrapper(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+            kwargs, subproject = get_callee_args(wrapped_args, want_subproject=True)[3:5]
+
+            all_names = {t.name for t in types}
+            unknowns = set(kwargs).difference(all_names)
+            if unknowns:
+                # Warn about unknown argumnts, delete them and continue. This
+                # keeps current behavior
+                ustr = ', '.join([f'"{u}"' for u in sorted(unknowns)])
+                mlog.warning(f'{name} got unknown keyword arguments {ustr}')
+                for u in unknowns:
+                    del kwargs[u]
+
+            for info in types:
+                value = kwargs.get(info.name)
+                if value is not None:
+                    if info.since:
+                        feature_name = info.name + ' arg in ' + name
+                        FeatureNew.single_use(feature_name, info.since, subproject)
+                    if info.deprecated:
+                        feature_name = info.name + ' arg in ' + name
+                        FeatureDeprecated.single_use(feature_name, info.deprecated, subproject)
+                    if info.listify:
+                        kwargs[info.name] = value = mesonlib.listify(value)
+                    if isinstance(info.types, ContainerTypeInfo):
+                        msg = info.types.check(value)
+                        if msg is not None:
+                            raise InvalidArguments(f'{name} keyword argument "{info.name}" {msg}')
+                    else:
+                        if not isinstance(value, info.types):
+                            if isinstance(info.types, tuple):
+                                shouldbe = 'one of: {}'.format(", ".join(f'"{t.__name__}"' for t in info.types))
+                            else:
+                                shouldbe = f'"{info.types.__name__}"'
+                            raise InvalidArguments(f'{name} keyword argument "{info.name}"" was of type "{type(value).__name__}" but should have been {shouldbe}')
+
+                    if info.validator is not None:
+                        msg = info.validator(value)
+                        if msg is not None:
+                            raise InvalidArguments(f'{name} keyword argument "{info.name}" {msg}')
+
+                    warn: bool
+                    if info.deprecated_values is not None:
+                        for n, version in info.deprecated_values.items():
+                            if isinstance(value, (dict, list)):
+                                warn = n in value
+                            else:
+                                warn = n == value
+
+                            if warn:
+                                FeatureDeprecated.single_use(f'"{name}" keyword argument "{info.name}" value "{n}"', version, subproject)
+
+                    if info.since_values is not None:
+                        for n, version in info.since_values.items():
+                            if isinstance(value, (dict, list)):
+                                warn = n in value
+                            else:
+                                warn = n == value
+
+                            if warn:
+                                FeatureNew.single_use(f'"{name}" keyword argument "{info.name}" value "{n}"', version, subproject)
+
+                elif info.required:
+                    raise InvalidArguments(f'{name} is missing required keyword argument "{info.name}"')
+                else:
+                    # set the value to the default, this ensuring all kwargs are present
+                    # This both simplifies the typing checking and the usage
+                    # Create a shallow copy of the container (and do a type
+                    # conversion if necessary). This allows mutable types to
+                    # be used safely as default values
+                    if isinstance(info.types, ContainerTypeInfo):
+                        kwargs[info.name] = info.types.container(info.default)
+                    else:
+                        kwargs[info.name] = info.default
+                    if info.not_set_warning:
+                        mlog.warning(info.not_set_warning)
+
+                if info.convertor:
+                    kwargs[info.name] = info.convertor(kwargs[info.name])
+
+            return f(*wrapped_args, **wrapped_kwargs)
+        return T.cast(TV_func, wrapper)
+    return inner
+
+
+class FeatureCheckBase(metaclass=abc.ABCMeta):
+    "Base class for feature version checks"
+
+    # In python 3.6 we can just forward declare this, but in 3.5 we can't
+    # This will be overwritten by the subclasses by necessity
+    feature_registry = {}  # type: T.ClassVar[T.Dict[str, T.Dict[str, T.Set[str]]]]
+
+    def __init__(self, feature_name: str, version: str, extra_message: T.Optional[str] = None):
+        self.feature_name = feature_name  # type: str
+        self.feature_version = version    # type: str
+        self.extra_message = extra_message or ''  # type: str
+
+    @staticmethod
+    def get_target_version(subproject: str) -> str:
+        # Don't do any checks if project() has not been parsed yet
+        if subproject not in mesonlib.project_meson_versions:
+            return ''
+        return mesonlib.project_meson_versions[subproject]
+
+    @staticmethod
+    @abc.abstractmethod
+    def check_version(target_version: str, feature_Version: str) -> bool:
+        pass
+
+    def use(self, subproject: str) -> None:
+        tv = self.get_target_version(subproject)
+        # No target version
+        if tv == '':
+            return
+        # Target version is new enough
+        if self.check_version(tv, self.feature_version):
+            return
+        # Feature is too new for target version, register it
+        if subproject not in self.feature_registry:
+            self.feature_registry[subproject] = {self.feature_version: set()}
+        register = self.feature_registry[subproject]
+        if self.feature_version not in register:
+            register[self.feature_version] = set()
+        if self.feature_name in register[self.feature_version]:
+            # Don't warn about the same feature multiple times
+            # FIXME: This is needed to prevent duplicate warnings, but also
+            # means we won't warn about a feature used in multiple places.
+            return
+        register[self.feature_version].add(self.feature_name)
+        self.log_usage_warning(tv)
+
+    @classmethod
+    def report(cls, subproject: str) -> None:
+        if subproject not in cls.feature_registry:
+            return
+        warning_str = cls.get_warning_str_prefix(cls.get_target_version(subproject))
+        fv = cls.feature_registry[subproject]
+        for version in sorted(fv.keys()):
+            warning_str += '\n * {}: {}'.format(version, fv[version])
+        mlog.warning(warning_str)
+
+    def log_usage_warning(self, tv: str) -> None:
+        raise InterpreterException('log_usage_warning not implemented')
+
+    @staticmethod
+    def get_warning_str_prefix(tv: str) -> str:
+        raise InterpreterException('get_warning_str_prefix not implemented')
+
+    def __call__(self, f: TV_func) -> TV_func:
+        @wraps(f)
+        def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+            subproject = get_callee_args(wrapped_args, want_subproject=True)[4]
+            if subproject is None:
+                raise AssertionError(f'{wrapped_args!r}')
+            self.use(subproject)
+            return f(*wrapped_args, **wrapped_kwargs)
+        return T.cast(TV_func, wrapped)
+
+    @classmethod
+    def single_use(cls, feature_name: str, version: str, subproject: str,
+                   extra_message: T.Optional[str] = None) -> None:
+        """Oneline version that instantiates and calls use()."""
+        cls(feature_name, version, extra_message).use(subproject)
+
+
+class FeatureNew(FeatureCheckBase):
+    """Checks for new features"""
+
+    # Class variable, shared across all instances
+    #
+    # Format: {subproject: {feature_version: set(feature_names)}}
+    feature_registry = {}  # type: T.ClassVar[T.Dict[str, T.Dict[str, T.Set[str]]]]
+
+    @staticmethod
+    def check_version(target_version: str, feature_version: str) -> bool:
+        return mesonlib.version_compare_condition_with_min(target_version, feature_version)
+
+    @staticmethod
+    def get_warning_str_prefix(tv: str) -> str:
+        return f'Project specifies a minimum meson_version \'{tv}\' but uses features which were added in newer versions:'
+
+    def log_usage_warning(self, tv: str) -> None:
+        args = [
+            'Project targeting', f"'{tv}'",
+            'but tried to use feature introduced in',
+            f"'{self.feature_version}':",
+            f'{self.feature_name}.',
+        ]
+        if self.extra_message:
+            args.append(self.extra_message)
+        mlog.warning(*args)
+
+class FeatureDeprecated(FeatureCheckBase):
+    """Checks for deprecated features"""
+
+    # Class variable, shared across all instances
+    #
+    # Format: {subproject: {feature_version: set(feature_names)}}
+    feature_registry = {}  # type: T.ClassVar[T.Dict[str, T.Dict[str, T.Set[str]]]]
+
+    @staticmethod
+    def check_version(target_version: str, feature_version: str) -> bool:
+        # For deprecation checks we need to return the inverse of FeatureNew checks
+        return not mesonlib.version_compare_condition_with_min(target_version, feature_version)
+
+    @staticmethod
+    def get_warning_str_prefix(tv: str) -> str:
+        return 'Deprecated features used:'
+
+    def log_usage_warning(self, tv: str) -> None:
+        args = [
+            'Project targeting', f"'{tv}'",
+            'but tried to use feature deprecated since',
+            f"'{self.feature_version}':",
+            f'{self.feature_name}.',
+        ]
+        if self.extra_message:
+            args.append(self.extra_message)
+        mlog.warning(*args)
+
+
+class FeatureCheckKwargsBase(metaclass=abc.ABCMeta):
+
+    @property
+    @abc.abstractmethod
+    def feature_check_class(self) -> T.Type[FeatureCheckBase]:
+        pass
+
+    def __init__(self, feature_name: str, feature_version: str,
+                 kwargs: T.List[str], extra_message: T.Optional[str] = None):
+        self.feature_name = feature_name
+        self.feature_version = feature_version
+        self.kwargs = kwargs
+        self.extra_message = extra_message
+
+    def __call__(self, f: TV_func) -> TV_func:
+        @wraps(f)
+        def wrapped(*wrapped_args: T.Any, **wrapped_kwargs: T.Any) -> T.Any:
+            kwargs, subproject = get_callee_args(wrapped_args, want_subproject=True)[3:5]
+            if subproject is None:
+                raise AssertionError(f'{wrapped_args!r}')
+            for arg in self.kwargs:
+                if arg not in kwargs:
+                    continue
+                name = arg + ' arg in ' + self.feature_name
+                self.feature_check_class.single_use(
+                        name, self.feature_version, subproject, self.extra_message)
+            return f(*wrapped_args, **wrapped_kwargs)
+        return T.cast(TV_func, wrapped)
+
+class FeatureNewKwargs(FeatureCheckKwargsBase):
+    feature_check_class = FeatureNew
+
+class FeatureDeprecatedKwargs(FeatureCheckKwargsBase):
+    feature_check_class = FeatureDeprecated
diff --git a/meson/mesonbuild/interpreterbase/disabler.py b/meson/mesonbuild/interpreterbase/disabler.py
new file mode 100644
index 000000000..81f526466
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/disabler.py
@@ -0,0 +1,42 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .baseobjects import MesonInterpreterObject
+import typing as T
+
+class Disabler(MesonInterpreterObject):
+    def __init__(self) -> None:
+        super().__init__()
+        self.methods.update({'found': self.found_method})
+
+    def found_method(self, args: T.Sequence[T.Any], kwargs: T.Dict[str, T.Any]) -> bool:
+        return False
+
+def _is_arg_disabled(arg: T.Any) -> bool:
+    if isinstance(arg, Disabler):
+        return True
+    if isinstance(arg, list):
+        for i in arg:
+            if _is_arg_disabled(i):
+                return True
+    return False
+
+def is_disabled(args: T.Sequence[T.Any], kwargs: T.Dict[str, T.Any]) -> bool:
+    for i in args:
+        if _is_arg_disabled(i):
+            return True
+    for i in kwargs.values():
+        if _is_arg_disabled(i):
+            return True
+    return False
diff --git a/meson/mesonbuild/interpreterbase/exceptions.py b/meson/mesonbuild/interpreterbase/exceptions.py
new file mode 100644
index 000000000..cdbe0fb3b
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/exceptions.py
@@ -0,0 +1,33 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from ..mesonlib import MesonException
+
+class InterpreterException(MesonException):
+    pass
+
+class InvalidCode(InterpreterException):
+    pass
+
+class InvalidArguments(InterpreterException):
+    pass
+
+class SubdirDoneRequest(BaseException):
+    pass
+
+class ContinueRequest(BaseException):
+    pass
+
+class BreakRequest(BaseException):
+    pass
diff --git a/meson/mesonbuild/interpreterbase/helpers.py b/meson/mesonbuild/interpreterbase/helpers.py
new file mode 100644
index 000000000..235257795
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/helpers.py
@@ -0,0 +1,118 @@
+# Copyright 2013-2021 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from .. import mesonlib, mparser, mlog
+from .exceptions import InvalidArguments, InterpreterException
+
+import collections.abc
+import typing as T
+
+if T.TYPE_CHECKING:
+    from .baseobjects import TYPE_var, TYPE_kwargs
+
+def flatten(args: T.Union['TYPE_var', T.List['TYPE_var']]) -> T.List['TYPE_var']:
+    if isinstance(args, mparser.StringNode):
+        assert isinstance(args.value, str)
+        return [args.value]
+    if not isinstance(args, collections.abc.Sequence):
+        return [args]
+    result: T.List['TYPE_var'] = []
+    for a in args:
+        if isinstance(a, list):
+            rest = flatten(a)
+            result = result + rest
+        elif isinstance(a, mparser.StringNode):
+            result.append(a.value)
+        else:
+            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))
+        raise InvalidArguments('Argument not a list.')
+    if not all(isinstance(s, str) for s in a):
+        mlog.debug('Element not a string:', str(a))
+        raise InvalidArguments(msg)
+
+def default_resolve_key(key: mparser.BaseNode) -> str:
+    if not isinstance(key, mparser.IdNode):
+        raise InterpreterException('Invalid kwargs format.')
+    return key.value
+
+def get_callee_args(wrapped_args: T.Sequence[T.Any], want_subproject: bool = False) -> T.Tuple[T.Any, mparser.BaseNode, T.List['TYPE_var'], 'TYPE_kwargs', T.Optional[str]]:
+    s = wrapped_args[0]
+    n = len(wrapped_args)
+    # Raise an error if the codepaths are not there
+    subproject = None  # type: T.Optional[str]
+    if want_subproject and n == 2:
+        if hasattr(s, 'subproject'):
+            # Interpreter base types have 2 args: self, node
+            node = wrapped_args[1]
+            # args and kwargs are inside the node
+            args = None
+            kwargs = None
+            subproject = s.subproject
+        elif hasattr(wrapped_args[1], 'subproject'):
+            # Module objects have 2 args: self, interpreter
+            node = wrapped_args[1].current_node
+            # args and kwargs are inside the node
+            args = None
+            kwargs = None
+            subproject = wrapped_args[1].subproject
+        else:
+            raise AssertionError(f'Unknown args: {wrapped_args!r}')
+    elif n == 3:
+        # Methods on objects (*Holder, MesonMain, etc) have 3 args: self, args, kwargs
+        node = s.current_node
+        args = wrapped_args[1]
+        kwargs = wrapped_args[2]
+        if want_subproject:
+            if hasattr(s, 'subproject'):
+                subproject = s.subproject
+            elif hasattr(s, 'interpreter'):
+                subproject = s.interpreter.subproject
+    elif n == 4:
+        # Meson functions have 4 args: self, node, args, kwargs
+        # Module functions have 4 args: self, state, args, kwargs
+        from .interpreterbase import InterpreterBase  # TODO: refactor to avoid this import
+        if isinstance(s, InterpreterBase):
+            node = wrapped_args[1]
+        else:
+            node = wrapped_args[1].current_node
+        args = wrapped_args[2]
+        kwargs = wrapped_args[3]
+        if want_subproject:
+            if isinstance(s, InterpreterBase):
+                subproject = s.subproject
+            else:
+                subproject = wrapped_args[1].subproject
+    else:
+        raise AssertionError(f'Unknown args: {wrapped_args!r}')
+    # Sometimes interpreter methods are called internally with None instead of
+    # empty list/dict
+    args = args if args is not None else []
+    kwargs = kwargs if kwargs is not None else {}
+    return s, node, args, kwargs, subproject
diff --git a/meson/mesonbuild/interpreterbase/interpreterbase.py b/meson/mesonbuild/interpreterbase/interpreterbase.py
new file mode 100644
index 000000000..115e24be0
--- /dev/null
+++ b/meson/mesonbuild/interpreterbase/interpreterbase.py
@@ -0,0 +1,959 @@
+# Copyright 2016-2017 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This class contains the basic functionality needed to run any interpreter
+# or an interpreter-based tool.
+
+from .. import mparser, mesonlib, mlog
+from .. import environment
+
+from .baseobjects import (
+    InterpreterObject,
+    MesonInterpreterObject,
+    MutableInterpreterObject,
+    InterpreterObjectTypeVar,
+    ObjectHolder,
+    RangeHolder,
+
+    TYPE_elementary,
+    TYPE_var,
+    TYPE_kwargs,
+)
+
+from .exceptions import (
+    InterpreterException,
+    InvalidCode,
+    InvalidArguments,
+    SubdirDoneRequest,
+    ContinueRequest,
+    BreakRequest
+)
+
+from .decorators import FeatureNew, builtinMethodNoKwargs
+from .disabler import Disabler, is_disabled
+from .helpers import check_stringlist, default_resolve_key, flatten, resolve_second_level_holders
+from ._unholder import _unholder
+
+import os, copy, re
+import typing as T
+
+if T.TYPE_CHECKING:
+    from ..interpreter import Interpreter
+
+HolderMapType = T.Dict[
+    T.Type[mesonlib.HoldableObject],
+    # For some reason, this has to be a callable and can't just be ObjectHolder[InterpreterObjectTypeVar]
+    T.Callable[[InterpreterObjectTypeVar, 'Interpreter'], ObjectHolder[InterpreterObjectTypeVar]]
+]
+
+FunctionType = T.Dict[
+    str,
+    T.Callable[[mparser.BaseNode, T.List[TYPE_var], T.Dict[str, TYPE_var]], TYPE_var]
+]
+
+class MesonVersionString(str):
+    pass
+
+class InterpreterBase:
+    elementary_types = (int, str, bool, list)
+
+    def __init__(self, source_root: str, subdir: str, subproject: str):
+        self.source_root = source_root
+        self.funcs: FunctionType = {}
+        self.builtin: T.Dict[str, InterpreterObject] = {}
+        # Holder maps store a mapping from an HoldableObject to a class ObjectHolder
+        self.holder_map: HolderMapType = {}
+        self.bound_holder_map: HolderMapType = {}
+        self.subdir = subdir
+        self.root_subdir = subdir
+        self.subproject = subproject
+        # TODO: This should actually be more strict: T.Union[TYPE_elementary, InterpreterObject]
+        self.variables: T.Dict[str, T.Union[TYPE_var, InterpreterObject]] = {}
+        self.argument_depth = 0
+        self.current_lineno = -1
+        # Current node set during a function call. This can be used as location
+        # when printing a warning message during a method call.
+        self.current_node = None  # type: mparser.BaseNode
+        # This is set to `version_string` when this statement is evaluated:
+        # meson.version().compare_version(version_string)
+        # If it was part of a if-clause, it is used to temporally override the
+        # current meson version target within that if-block.
+        self.tmp_meson_version = None # type: T.Optional[str]
+
+    def load_root_meson_file(self) -> None:
+        mesonfile = os.path.join(self.source_root, self.subdir, environment.build_filename)
+        if not os.path.isfile(mesonfile):
+            raise InvalidArguments('Missing Meson file in %s' % mesonfile)
+        with open(mesonfile, encoding='utf-8') as mf:
+            code = mf.read()
+        if code.isspace():
+            raise InvalidCode('Builder file is empty.')
+        assert(isinstance(code, str))
+        try:
+            self.ast = mparser.Parser(code, mesonfile).parse()
+        except mesonlib.MesonException as me:
+            me.file = mesonfile
+            raise me
+
+    def join_path_strings(self, args: T.Sequence[str]) -> str:
+        return os.path.join(*args).replace('\\', '/')
+
+    def parse_project(self) -> None:
+        """
+        Parses project() and initializes languages, compilers etc. Do this
+        early because we need this before we parse the rest of the AST.
+        """
+        self.evaluate_codeblock(self.ast, end=1)
+
+    def sanity_check_ast(self) -> None:
+        if not isinstance(self.ast, mparser.CodeBlockNode):
+            raise InvalidCode('AST is of invalid type. Possibly a bug in the parser.')
+        if not self.ast.lines:
+            raise InvalidCode('No statements in code.')
+        first = self.ast.lines[0]
+        if not isinstance(first, mparser.FunctionNode) or first.func_name != 'project':
+            raise InvalidCode('First statement must be a call to project')
+
+    def run(self) -> None:
+        # Evaluate everything after the first line, which is project() because
+        # we already parsed that in self.parse_project()
+        try:
+            self.evaluate_codeblock(self.ast, start=1)
+        except SubdirDoneRequest:
+            pass
+
+    def evaluate_codeblock(self, node: mparser.CodeBlockNode, start: int = 0, end: T.Optional[int] = None) -> None:
+        if node is None:
+            return
+        if not isinstance(node, mparser.CodeBlockNode):
+            e = InvalidCode('Tried to execute a non-codeblock. Possibly a bug in the parser.')
+            e.lineno = node.lineno
+            e.colno = node.colno
+            raise e
+        statements = node.lines[start:end]
+        i = 0
+        while i < len(statements):
+            cur = statements[i]
+            try:
+                self.current_lineno = cur.lineno
+                self.evaluate_statement(cur)
+            except Exception as e:
+                if getattr(e, 'lineno', None) is None:
+                    # We are doing the equivalent to setattr here and mypy does not like it
+                    e.lineno = cur.lineno                                                             # type: ignore
+                    e.colno = cur.colno                                                               # type: ignore
+                    e.file = os.path.join(self.source_root, self.subdir, environment.build_filename)  # type: ignore
+                raise e
+            i += 1 # In THE FUTURE jump over blocks and stuff.
+
+    def evaluate_statement(self, cur: mparser.BaseNode) -> T.Optional[T.Union[TYPE_var, InterpreterObject]]:
+        self.current_node = cur
+        if isinstance(cur, mparser.FunctionNode):
+            return self.function_call(cur)
+        elif isinstance(cur, mparser.AssignmentNode):
+            self.assignment(cur)
+        elif isinstance(cur, mparser.MethodNode):
+            return self.method_call(cur)
+        elif isinstance(cur, mparser.StringNode):
+            return cur.value
+        elif isinstance(cur, mparser.BooleanNode):
+            return cur.value
+        elif isinstance(cur, mparser.IfClauseNode):
+            return self.evaluate_if(cur)
+        elif isinstance(cur, mparser.IdNode):
+            return self.get_variable(cur.value)
+        elif isinstance(cur, mparser.ComparisonNode):
+            return self.evaluate_comparison(cur)
+        elif isinstance(cur, mparser.ArrayNode):
+            return self.evaluate_arraystatement(cur)
+        elif isinstance(cur, mparser.DictNode):
+            return self.evaluate_dictstatement(cur)
+        elif isinstance(cur, mparser.NumberNode):
+            return cur.value
+        elif isinstance(cur, mparser.AndNode):
+            return self.evaluate_andstatement(cur)
+        elif isinstance(cur, mparser.OrNode):
+            return self.evaluate_orstatement(cur)
+        elif isinstance(cur, mparser.NotNode):
+            return self.evaluate_notstatement(cur)
+        elif isinstance(cur, mparser.UMinusNode):
+            return self.evaluate_uminusstatement(cur)
+        elif isinstance(cur, mparser.ArithmeticNode):
+            return self.evaluate_arithmeticstatement(cur)
+        elif isinstance(cur, mparser.ForeachClauseNode):
+            self.evaluate_foreach(cur)
+        elif isinstance(cur, mparser.PlusAssignmentNode):
+            self.evaluate_plusassign(cur)
+        elif isinstance(cur, mparser.IndexNode):
+            return self.evaluate_indexing(cur)
+        elif isinstance(cur, mparser.TernaryNode):
+            return self.evaluate_ternary(cur)
+        elif isinstance(cur, mparser.FormatStringNode):
+            return self.evaluate_fstring(cur)
+        elif isinstance(cur, mparser.ContinueNode):
+            raise ContinueRequest()
+        elif isinstance(cur, mparser.BreakNode):
+            raise BreakRequest()
+        elif isinstance(cur, self.elementary_types):
+            return cur
+        else:
+            raise InvalidCode("Unknown statement.")
+        return None
+
+    def evaluate_arraystatement(self, cur: mparser.ArrayNode) -> T.List[T.Union[TYPE_var, InterpreterObject]]:
+        (arguments, kwargs) = self.reduce_arguments(cur.args)
+        if len(kwargs) > 0:
+            raise InvalidCode('Keyword arguments are invalid in array construction.')
+        return arguments
+
+    @FeatureNew('dict', '0.47.0')
+    def evaluate_dictstatement(self, cur: mparser.DictNode) -> T.Union[TYPE_var, InterpreterObject]:
+        def resolve_key(key: mparser.BaseNode) -> str:
+            if not isinstance(key, mparser.StringNode):
+                FeatureNew.single_use('Dictionary entry using non literal key', '0.53.0', self.subproject)
+            str_key = self.evaluate_statement(key)
+            if not isinstance(str_key, str):
+                raise InvalidArguments('Key must be a string')
+            return str_key
+        arguments, kwargs = self.reduce_arguments(cur.args, key_resolver=resolve_key, duplicate_key_error='Duplicate dictionary key: {}')
+        assert not arguments
+        return kwargs
+
+    def evaluate_notstatement(self, cur: mparser.NotNode) -> T.Union[bool, Disabler]:
+        v = self.evaluate_statement(cur.value)
+        if isinstance(v, Disabler):
+            return v
+        if not isinstance(v, bool):
+            raise InterpreterException('Argument to "not" is not a boolean.')
+        return not v
+
+    def evaluate_if(self, node: mparser.IfClauseNode) -> T.Optional[Disabler]:
+        assert(isinstance(node, mparser.IfClauseNode))
+        for i in node.ifs:
+            # Reset self.tmp_meson_version to know if it gets set during this
+            # statement evaluation.
+            self.tmp_meson_version = None
+            result = self.evaluate_statement(i.condition)
+            if isinstance(result, Disabler):
+                return result
+            if not(isinstance(result, bool)):
+                raise InvalidCode(f'If clause {result!r} does not evaluate to true or false.')
+            if result:
+                prev_meson_version = mesonlib.project_meson_versions[self.subproject]
+                if self.tmp_meson_version:
+                    mesonlib.project_meson_versions[self.subproject] = self.tmp_meson_version
+                try:
+                    self.evaluate_codeblock(i.block)
+                finally:
+                    mesonlib.project_meson_versions[self.subproject] = prev_meson_version
+                return None
+        if not isinstance(node.elseblock, mparser.EmptyNode):
+            self.evaluate_codeblock(node.elseblock)
+        return None
+
+    def validate_comparison_types(self, val1: T.Any, val2: T.Any) -> bool:
+        if type(val1) != type(val2):
+            return False
+        return True
+
+    def evaluate_in(self, val1: T.Any, val2: T.Any) -> bool:
+        if not isinstance(val1, (str, int, float, mesonlib.HoldableObject)):
+            raise InvalidArguments('lvalue of "in" operator must be a string, integer, float, or object')
+        if not isinstance(val2, (list, dict)):
+            raise InvalidArguments('rvalue of "in" operator must be an array or a dict')
+        return val1 in val2
+
+    def evaluate_comparison(self, node: mparser.ComparisonNode) -> T.Union[bool, Disabler]:
+        val1 = self.evaluate_statement(node.left)
+        if isinstance(val1, Disabler):
+            return val1
+        val2 = self.evaluate_statement(node.right)
+        if isinstance(val2, Disabler):
+            return val2
+        # Do not compare the ObjectHolders but the actual held objects
+        val1 = _unholder(val1)
+        val2 = _unholder(val2)
+        if node.ctype == 'in':
+            return self.evaluate_in(val1, val2)
+        elif node.ctype == 'notin':
+            return not self.evaluate_in(val1, val2)
+        valid = self.validate_comparison_types(val1, val2)
+        # Ordering comparisons of different types isn't allowed since PR #1810
+        # (0.41.0).  Since PR #2884 we also warn about equality comparisons of
+        # different types, which will one day become an error.
+        if not valid and (node.ctype == '==' or node.ctype == '!='):
+            mlog.warning('''Trying to compare values of different types ({}, {}) using {}.
+The result of this is undefined and will become a hard error in a future Meson release.'''
+                         .format(type(val1).__name__, type(val2).__name__, node.ctype), location=node)
+        if node.ctype == '==':
+            return val1 == val2
+        elif node.ctype == '!=':
+            return val1 != val2
+        elif not valid:
+            raise InterpreterException(
+                'Values of different types ({}, {}) cannot be compared using {}.'.format(type(val1).__name__,
+                                                                                         type(val2).__name__,
+                                                                                         node.ctype))
+        elif not isinstance(val1, self.elementary_types):
+            raise InterpreterException('{} can only be compared for equality.'.format(getattr(node.left, 'value', '<ERROR>')))
+        elif not isinstance(val2, self.elementary_types):
+            raise InterpreterException('{} can only be compared for equality.'.format(getattr(node.right, 'value', '<ERROR>')))
+        # Use type: ignore because mypy will complain that we are comparing two Unions,
+        # but we actually guarantee earlier that both types are the same
+        elif node.ctype == '<':
+            return val1 < val2   # type: ignore
+        elif node.ctype == '<=':
+            return val1 <= val2  # type: ignore
+        elif node.ctype == '>':
+            return val1 > val2   # type: ignore
+        elif node.ctype == '>=':
+            return val1 >= val2  # type: ignore
+        else:
+            raise InvalidCode('You broke my compare eval.')
+
+    def evaluate_andstatement(self, cur: mparser.AndNode) -> T.Union[bool, Disabler]:
+        l = self.evaluate_statement(cur.left)
+        if isinstance(l, Disabler):
+            return l
+        if not isinstance(l, bool):
+            raise InterpreterException('First argument to "and" is not a boolean.')
+        if not l:
+            return False
+        r = self.evaluate_statement(cur.right)
+        if isinstance(r, Disabler):
+            return r
+        if not isinstance(r, bool):
+            raise InterpreterException('Second argument to "and" is not a boolean.')
+        return r
+
+    def evaluate_orstatement(self, cur: mparser.OrNode) -> T.Union[bool, Disabler]:
+        l = self.evaluate_statement(cur.left)
+        if isinstance(l, Disabler):
+            return l
+        if not isinstance(l, bool):
+            raise InterpreterException('First argument to "or" is not a boolean.')
+        if l:
+            return True
+        r = self.evaluate_statement(cur.right)
+        if isinstance(r, Disabler):
+            return r
+        if not isinstance(r, bool):
+            raise InterpreterException('Second argument to "or" is not a boolean.')
+        return r
+
+    def evaluate_uminusstatement(self, cur: mparser.UMinusNode) -> T.Union[int, Disabler]:
+        v = self.evaluate_statement(cur.value)
+        if isinstance(v, Disabler):
+            return v
+        if not isinstance(v, int):
+            raise InterpreterException('Argument to negation is not an integer.')
+        return -v
+
+    @FeatureNew('/ with string arguments', '0.49.0')
+    def evaluate_path_join(self, l: str, r: str) -> str:
+        if not isinstance(l, str):
+            raise InvalidCode('The division operator can only append to a string.')
+        if not isinstance(r, str):
+            raise InvalidCode('The division operator can only append a string.')
+        return self.join_path_strings((l, r))
+
+    def evaluate_division(self, l: T.Any, r: T.Any) -> T.Union[int, str]:
+        if isinstance(l, str) or isinstance(r, str):
+            return self.evaluate_path_join(l, r)
+        if isinstance(l, int) and isinstance(r, int):
+            if r == 0:
+                raise InvalidCode('Division by zero.')
+            return l // r
+        raise InvalidCode('Division works only with strings or integers.')
+
+    def evaluate_arithmeticstatement(self, cur: mparser.ArithmeticNode) -> T.Union[int, str, dict, list, Disabler]:
+        l = self.evaluate_statement(cur.left)
+        if isinstance(l, Disabler):
+            return l
+        r = self.evaluate_statement(cur.right)
+        if isinstance(r, Disabler):
+            return r
+
+        if cur.operation == 'add':
+            if isinstance(l, dict) and isinstance(r, dict):
+                return {**l, **r}
+            try:
+                # MyPy error due to handling two Unions (we are catching all exceptions anyway)
+                return l + r  # type: ignore
+            except Exception as e:
+                raise InvalidCode('Invalid use of addition: ' + str(e))
+        elif cur.operation == 'sub':
+            if not isinstance(l, int) or not isinstance(r, int):
+                raise InvalidCode('Subtraction works only with integers.')
+            return l - r
+        elif cur.operation == 'mul':
+            if not isinstance(l, int) or not isinstance(r, int):
+                raise InvalidCode('Multiplication works only with integers.')
+            return l * r
+        elif cur.operation == 'div':
+            return self.evaluate_division(l, r)
+        elif cur.operation == 'mod':
+            if not isinstance(l, int) or not isinstance(r, int):
+                raise InvalidCode('Modulo works only with integers.')
+            return l % r
+        else:
+            raise InvalidCode('You broke me.')
+
+    def evaluate_ternary(self, node: mparser.TernaryNode) -> T.Union[TYPE_var, InterpreterObject]:
+        assert(isinstance(node, mparser.TernaryNode))
+        result = self.evaluate_statement(node.condition)
+        if isinstance(result, Disabler):
+            return result
+        if not isinstance(result, bool):
+            raise InterpreterException('Ternary condition is not boolean.')
+        if result:
+            return self.evaluate_statement(node.trueblock)
+        else:
+            return self.evaluate_statement(node.falseblock)
+
+    @FeatureNew('format strings', '0.58.0')
+    def evaluate_fstring(self, node: mparser.FormatStringNode) -> TYPE_var:
+        assert(isinstance(node, mparser.FormatStringNode))
+
+        def replace(match: T.Match[str]) -> str:
+            var = str(match.group(1))
+            try:
+                val = self.variables[var]
+                if not isinstance(val, (str, int, float, bool)):
+                    raise InvalidCode(f'Identifier "{var}" does not name a formattable variable ' +
+                        '(has to be an integer, a string, a floating point number or a boolean).')
+
+                return str(val)
+            except KeyError:
+                raise InvalidCode(f'Identifier "{var}" does not name a variable.')
+
+        return re.sub(r'@([_a-zA-Z][_0-9a-zA-Z]*)@', replace, node.value)
+
+    def evaluate_foreach(self, node: mparser.ForeachClauseNode) -> None:
+        assert(isinstance(node, mparser.ForeachClauseNode))
+        items = self.evaluate_statement(node.items)
+
+        if isinstance(items, (list, RangeHolder)):
+            if len(node.varnames) != 1:
+                raise InvalidArguments('Foreach on array does not unpack')
+            varname = node.varnames[0]
+            for item in items:
+                self.set_variable(varname, item)
+                try:
+                    self.evaluate_codeblock(node.block)
+                except ContinueRequest:
+                    continue
+                except BreakRequest:
+                    break
+        elif isinstance(items, dict):
+            if len(node.varnames) != 2:
+                raise InvalidArguments('Foreach on dict unpacks key and value')
+            for key, value in sorted(items.items()):
+                self.set_variable(node.varnames[0], key)
+                self.set_variable(node.varnames[1], value)
+                try:
+                    self.evaluate_codeblock(node.block)
+                except ContinueRequest:
+                    continue
+                except BreakRequest:
+                    break
+        else:
+            raise InvalidArguments('Items of foreach loop must be an array or a dict')
+
+    def evaluate_plusassign(self, node: mparser.PlusAssignmentNode) -> None:
+        assert(isinstance(node, mparser.PlusAssignmentNode))
+        varname = node.var_name
+        addition = self.evaluate_statement(node.value)
+
+        # Remember that all variables are immutable. We must always create a
+        # full new variable and then assign it.
+        old_variable = self.get_variable(varname)
+        new_value = None  # type: T.Union[str, int, float, bool, dict, list]
+        if isinstance(old_variable, str):
+            if not isinstance(addition, str):
+                raise InvalidArguments('The += operator requires a string on the right hand side if the variable on the left is a string')
+            new_value = old_variable + addition
+        elif isinstance(old_variable, int):
+            if not isinstance(addition, int):
+                raise InvalidArguments('The += operator requires an int on the right hand side if the variable on the left is an int')
+            new_value = old_variable + addition
+        elif isinstance(old_variable, list):
+            if isinstance(addition, list):
+                new_value = old_variable + addition
+            else:
+                new_value = old_variable + [addition]
+        elif isinstance(old_variable, dict):
+            if not isinstance(addition, dict):
+                raise InvalidArguments('The += operator requires a dict on the right hand side if the variable on the left is a dict')
+            new_value = {**old_variable, **addition}
+        # Add other data types here.
+        else:
+            raise InvalidArguments('The += operator currently only works with arrays, dicts, strings or ints')
+        self.set_variable(varname, new_value)
+
+    def evaluate_indexing(self, node: mparser.IndexNode) -> T.Union[TYPE_elementary, InterpreterObject]:
+        assert(isinstance(node, mparser.IndexNode))
+        iobject = self.evaluate_statement(node.iobject)
+        if isinstance(iobject, Disabler):
+            return iobject
+        if not hasattr(iobject, '__getitem__'):
+            raise InterpreterException(
+                'Tried to index an object that doesn\'t support indexing.')
+        index = self.evaluate_statement(node.index)
+
+        if isinstance(iobject, dict):
+            if not isinstance(index, str):
+                raise InterpreterException('Key is not a string')
+            try:
+                # The cast is required because we don't have recursive types...
+                return T.cast(T.Union[TYPE_elementary, InterpreterObject], iobject[index])
+            except KeyError:
+                raise InterpreterException('Key %s is not in dict' % index)
+        else:
+            if not isinstance(index, int):
+                raise InterpreterException('Index value is not an integer.')
+            try:
+                # Ignore the MyPy error, since we don't know all indexable types here
+                # and we handle non indexable types with an exception
+                # TODO maybe find a better solution
+                res = iobject[index]  # type: ignore
+                # Only holderify if we are dealing with `InterpreterObject`, since raw
+                # lists already store ObjectHolders
+                if isinstance(iobject, InterpreterObject):
+                    return self._holderify(res)
+                else:
+                    return res
+            except IndexError:
+                # We are already checking for the existence of __getitem__, so this should be save
+                raise InterpreterException('Index %d out of bounds of array of size %d.' % (index, len(iobject)))  # type: ignore
+
+    def function_call(self, node: mparser.FunctionNode) -> T.Optional[T.Union[TYPE_elementary, InterpreterObject]]:
+        func_name = node.func_name
+        (h_posargs, h_kwargs) = self.reduce_arguments(node.args)
+        (posargs, kwargs) = self._unholder_args(h_posargs, h_kwargs)
+        if is_disabled(posargs, kwargs) and func_name not in {'get_variable', 'set_variable', 'is_disabler'}:
+            return Disabler()
+        if func_name in self.funcs:
+            func = self.funcs[func_name]
+            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:
+            self.unknown_function_called(func_name)
+            return None
+
+    def method_call(self, node: mparser.MethodNode) -> T.Optional[T.Union[TYPE_var, InterpreterObject]]:
+        invokable = node.source_object
+        obj: T.Union[TYPE_var, InterpreterObject]
+        if isinstance(invokable, mparser.IdNode):
+            object_name = invokable.value
+            obj = self.get_variable(object_name)
+        else:
+            obj = self.evaluate_statement(invokable)
+        method_name = node.name
+        (h_args, h_kwargs) = self.reduce_arguments(node.args)
+        (args, kwargs) = self._unholder_args(h_args, h_kwargs)
+        if is_disabled(args, kwargs):
+            return Disabler()
+        if isinstance(obj, str):
+            return self.string_method_call(obj, method_name, args, kwargs)
+        if isinstance(obj, bool):
+            return self.bool_method_call(obj, method_name, args, kwargs)
+        if isinstance(obj, int):
+            return self.int_method_call(obj, method_name, args, kwargs)
+        if isinstance(obj, list):
+            return self.array_method_call(obj, method_name, args, kwargs)
+        if isinstance(obj, dict):
+            return self.dict_method_call(obj, method_name, args, kwargs)
+        if not isinstance(obj, InterpreterObject):
+            raise InvalidArguments('Variable "%s" is not callable.' % object_name)
+        # Special case. This is the only thing you can do with a disabler
+        # object. Every other use immediately returns the disabler object.
+        if isinstance(obj, Disabler):
+            if method_name == 'found':
+                return False
+            else:
+                return Disabler()
+        # TODO: InterpreterBase **really** shouldn't be in charge of checking this
+        if method_name == 'extract_objects':
+            if not isinstance(obj, ObjectHolder):
+                raise InvalidArguments(f'Invalid operation "extract_objects" on variable "{object_name}" of type {type(obj).__name__}')
+            self.validate_extraction(obj.held_object)
+        obj.current_node = node
+        return self._holderify(obj.method_call(method_name, args, kwargs))
+
+    def _holderify(self, res: T.Union[TYPE_var, InterpreterObject, None]) -> T.Union[TYPE_elementary, InterpreterObject]:
+        if res is None:
+            return None
+        if isinstance(res, (int, bool, str)):
+            return res
+        elif isinstance(res, list):
+            return [self._holderify(x) for x in res]
+        elif isinstance(res, dict):
+            return {k: self._holderify(v) for k, v in res.items()}
+        elif isinstance(res, mesonlib.HoldableObject):
+            # Always check for an exact match first.
+            cls = self.holder_map.get(type(res), None)
+            if cls is not None:
+                # Casts to Interpreter are required here since an assertion would
+                # not work for the `ast` module.
+                return cls(res, T.cast('Interpreter', self))
+            # Try the boundary types next.
+            for typ, cls in self.bound_holder_map.items():
+                if isinstance(res, typ):
+                    return cls(res, T.cast('Interpreter', self))
+            raise mesonlib.MesonBugException(f'Object {res} of type {type(res).__name__} is neither in self.holder_map nor self.bound_holder_map.')
+        elif isinstance(res, ObjectHolder):
+            raise mesonlib.MesonBugException(f'Returned object {res} of type {type(res).__name__} is an object holder.')
+        elif isinstance(res, MesonInterpreterObject):
+            return res
+        raise mesonlib.MesonBugException(f'Unknown returned object {res} of type {type(res).__name__} in the parameters.')
+
+    def _unholder_args(self,
+                       args: T.List[T.Union[TYPE_var, InterpreterObject]],
+                       kwargs: T.Dict[str, T.Union[TYPE_var, InterpreterObject]]) -> T.Tuple[T.List[TYPE_var], TYPE_kwargs]:
+        return [_unholder(x) for x in args], {k: _unholder(v) for k, v in kwargs.items()}
+
+    @builtinMethodNoKwargs
+    def bool_method_call(self, obj: bool, method_name: str, posargs: T.List[TYPE_var], kwargs: TYPE_kwargs) -> T.Union[str, int]:
+        if method_name == 'to_string':
+            if not posargs:
+                if obj:
+                    return 'true'
+                else:
+                    return 'false'
+            elif len(posargs) == 2 and isinstance(posargs[0], str) and isinstance(posargs[1], str):
+                if obj:
+                    return posargs[0]
+                else:
+                    return posargs[1]
+            else:
+                raise InterpreterException('bool.to_string() must have either no arguments or exactly two string arguments that signify what values to return for true and false.')
+        elif method_name == 'to_int':
+            if obj:
+                return 1
+            else:
+                return 0
+        else:
+            raise InterpreterException('Unknown method "%s" for a boolean.' % method_name)
+
+    @builtinMethodNoKwargs
+    def int_method_call(self, obj: int, method_name: str, posargs: T.List[TYPE_var], kwargs: TYPE_kwargs) -> T.Union[str, bool]:
+        if method_name == 'is_even':
+            if not posargs:
+                return obj % 2 == 0
+            else:
+                raise InterpreterException('int.is_even() must have no arguments.')
+        elif method_name == 'is_odd':
+            if not posargs:
+                return obj % 2 != 0
+            else:
+                raise InterpreterException('int.is_odd() must have no arguments.')
+        elif method_name == 'to_string':
+            if not posargs:
+                return str(obj)
+            else:
+                raise InterpreterException('int.to_string() must have no arguments.')
+        else:
+            raise InterpreterException('Unknown method "%s" for an integer.' % method_name)
+
+    @staticmethod
+    def _get_one_string_posarg(posargs: T.List[TYPE_var], method_name: str) -> str:
+        if len(posargs) > 1:
+            raise InterpreterException(f'{method_name}() must have zero or one arguments')
+        elif len(posargs) == 1:
+            s = posargs[0]
+            if not isinstance(s, str):
+                raise InterpreterException(f'{method_name}() argument must be a string')
+            return s
+        return None
+
+    @builtinMethodNoKwargs
+    def string_method_call(self, obj: str, method_name: str, posargs: T.List[TYPE_var], kwargs: TYPE_kwargs) -> T.Union[str, int, bool, T.List[str]]:
+        if method_name == 'strip':
+            s1 = self._get_one_string_posarg(posargs, 'strip')
+            if s1 is not None:
+                return obj.strip(s1)
+            return obj.strip()
+        elif method_name == 'format':
+            return self.format_string(obj, posargs)
+        elif method_name == 'to_upper':
+            return obj.upper()
+        elif method_name == 'to_lower':
+            return obj.lower()
+        elif method_name == 'underscorify':
+            return re.sub(r'[^a-zA-Z0-9]', '_', obj)
+        elif method_name == 'split':
+            s2 = self._get_one_string_posarg(posargs, 'split')
+            if s2 is not None:
+                return obj.split(s2)
+            return obj.split()
+        elif method_name == 'startswith' or method_name == 'contains' or method_name == 'endswith':
+            s3 = posargs[0]
+            if not isinstance(s3, str):
+                raise InterpreterException('Argument must be a string.')
+            if method_name == 'startswith':
+                return obj.startswith(s3)
+            elif method_name == 'contains':
+                return obj.find(s3) >= 0
+            return obj.endswith(s3)
+        elif method_name == 'to_int':
+            try:
+                return int(obj)
+            except Exception:
+                raise InterpreterException(f'String {obj!r} cannot be converted to int')
+        elif method_name == 'join':
+            if len(posargs) != 1:
+                raise InterpreterException('Join() takes exactly one argument.')
+            strlist = posargs[0]
+            check_stringlist(strlist)
+            assert isinstance(strlist, list)  # Required for mypy
+            return obj.join(strlist)
+        elif method_name == 'version_compare':
+            if len(posargs) != 1:
+                raise InterpreterException('Version_compare() takes exactly one argument.')
+            cmpr = posargs[0]
+            if not isinstance(cmpr, str):
+                raise InterpreterException('Version_compare() argument must be a string.')
+            if isinstance(obj, MesonVersionString):
+                self.tmp_meson_version = cmpr
+            return mesonlib.version_compare(obj, cmpr)
+        elif method_name == 'substring':
+            if len(posargs) > 2:
+                raise InterpreterException('substring() takes maximum two arguments.')
+            start = 0
+            end = len(obj)
+            if len (posargs) > 0:
+                if not isinstance(posargs[0], int):
+                    raise InterpreterException('substring() argument must be an int')
+                start = posargs[0]
+            if len (posargs) > 1:
+                if not isinstance(posargs[1], int):
+                    raise InterpreterException('substring() argument must be an int')
+                end = posargs[1]
+            return obj[start:end]
+        elif method_name == 'replace':
+            FeatureNew.single_use('str.replace', '0.58.0', self.subproject)
+            if len(posargs) != 2:
+                raise InterpreterException('replace() takes exactly two arguments.')
+            if not isinstance(posargs[0], str) or not isinstance(posargs[1], str):
+                raise InterpreterException('replace() requires that both arguments be strings')
+            return obj.replace(posargs[0], posargs[1])
+        raise InterpreterException('Unknown method "%s" for a string.' % method_name)
+
+    def format_string(self, templ: str, args: T.List[TYPE_var]) -> str:
+        arg_strings = []
+        for arg in args:
+            if isinstance(arg, mparser.BaseNode):
+                arg = self.evaluate_statement(arg)
+            if isinstance(arg, bool): # Python boolean is upper case.
+                arg = str(arg).lower()
+            arg_strings.append(str(arg))
+
+        def arg_replace(match: T.Match[str]) -> str:
+            idx = int(match.group(1))
+            if idx >= len(arg_strings):
+                raise InterpreterException(f'Format placeholder @{idx}@ out of range.')
+            return arg_strings[idx]
+
+        return re.sub(r'@(\d+)@', arg_replace, templ)
+
+    def unknown_function_called(self, func_name: str) -> None:
+        raise InvalidCode('Unknown function "%s".' % func_name)
+
+    @builtinMethodNoKwargs
+    def array_method_call(self,
+                          obj: T.List[T.Union[TYPE_elementary, InterpreterObject]],
+                          method_name: str,
+                          posargs: T.List[TYPE_var],
+                          kwargs: TYPE_kwargs) -> T.Union[TYPE_var, InterpreterObject]:
+        if method_name == 'contains':
+            def check_contains(el: T.List[TYPE_var]) -> bool:
+                if len(posargs) != 1:
+                    raise InterpreterException('Contains method takes exactly one argument.')
+                item = posargs[0]
+                for element in el:
+                    if isinstance(element, list):
+                        found = check_contains(element)
+                        if found:
+                            return True
+                    if element == item:
+                        return True
+                return False
+            return check_contains([_unholder(x) for x in obj])
+        elif method_name == 'length':
+            return len(obj)
+        elif method_name == 'get':
+            index = posargs[0]
+            fallback = None
+            if len(posargs) == 2:
+                fallback = self._holderify(posargs[1])
+            elif len(posargs) > 2:
+                m = 'Array method \'get()\' only takes two arguments: the ' \
+                    'index and an optional fallback value if the index is ' \
+                    'out of range.'
+                raise InvalidArguments(m)
+            if not isinstance(index, int):
+                raise InvalidArguments('Array index must be a number.')
+            if index < -len(obj) or index >= len(obj):
+                if fallback is None:
+                    m = 'Array index {!r} is out of bounds for array of size {!r}.'
+                    raise InvalidArguments(m.format(index, len(obj)))
+                if isinstance(fallback, mparser.BaseNode):
+                    return self.evaluate_statement(fallback)
+                return fallback
+            return obj[index]
+        raise InterpreterException(f'Arrays do not have a method called {method_name!r}.')
+
+    @builtinMethodNoKwargs
+    def dict_method_call(self,
+                         obj: T.Dict[str, T.Union[TYPE_elementary, InterpreterObject]],
+                         method_name: str,
+                         posargs: T.List[TYPE_var],
+                         kwargs: TYPE_kwargs) -> T.Union[TYPE_var, InterpreterObject]:
+        if method_name in ('has_key', 'get'):
+            if method_name == 'has_key':
+                if len(posargs) != 1:
+                    raise InterpreterException('has_key() takes exactly one argument.')
+            else:
+                if len(posargs) not in (1, 2):
+                    raise InterpreterException('get() takes one or two arguments.')
+
+            key = posargs[0]
+            if not isinstance(key, (str)):
+                raise InvalidArguments('Dictionary key must be a string.')
+
+            has_key = key in obj
+
+            if method_name == 'has_key':
+                return has_key
+
+            if has_key:
+                return obj[key]
+
+            if len(posargs) == 2:
+                fallback = self._holderify(posargs[1])
+                if isinstance(fallback, mparser.BaseNode):
+                    return self.evaluate_statement(fallback)
+                return fallback
+
+            raise InterpreterException(f'Key {key!r} is not in the dictionary.')
+
+        if method_name == 'keys':
+            if len(posargs) != 0:
+                raise InterpreterException('keys() takes no arguments.')
+            return sorted(obj.keys())
+
+        raise InterpreterException('Dictionaries do not have a method called "%s".' % method_name)
+
+    def reduce_arguments(
+                self,
+                args: mparser.ArgumentNode,
+                key_resolver: T.Callable[[mparser.BaseNode], str] = default_resolve_key,
+                duplicate_key_error: T.Optional[str] = None,
+            ) -> T.Tuple[
+                T.List[T.Union[TYPE_var, InterpreterObject]],
+                T.Dict[str, T.Union[TYPE_var, InterpreterObject]]
+            ]:
+        assert(isinstance(args, mparser.ArgumentNode))
+        if args.incorrect_order():
+            raise InvalidArguments('All keyword arguments must be after positional arguments.')
+        self.argument_depth += 1
+        reduced_pos: T.List[T.Union[TYPE_var, InterpreterObject]] = [self.evaluate_statement(arg) for arg in args.arguments]
+        reduced_kw: T.Dict[str, T.Union[TYPE_var, InterpreterObject]] = {}
+        for key, val in args.kwargs.items():
+            reduced_key = key_resolver(key)
+            assert isinstance(val, mparser.BaseNode)
+            reduced_val = self.evaluate_statement(val)
+            if duplicate_key_error and reduced_key in reduced_kw:
+                raise InvalidArguments(duplicate_key_error.format(reduced_key))
+            reduced_kw[reduced_key] = reduced_val
+        self.argument_depth -= 1
+        final_kw = self.expand_default_kwargs(reduced_kw)
+        return reduced_pos, final_kw
+
+    def expand_default_kwargs(self, kwargs: T.Dict[str, T.Union[TYPE_var, InterpreterObject]]) -> T.Dict[str, T.Union[TYPE_var, InterpreterObject]]:
+        if 'kwargs' not in kwargs:
+            return kwargs
+        to_expand = kwargs.pop('kwargs')
+        if not isinstance(to_expand, dict):
+            raise InterpreterException('Value of "kwargs" must be dictionary.')
+        if 'kwargs' in to_expand:
+            raise InterpreterException('Kwargs argument must not contain a "kwargs" entry. Points for thinking meta, though. :P')
+        for k, v in to_expand.items():
+            if k in kwargs:
+                raise InterpreterException(f'Entry "{k}" defined both as a keyword argument and in a "kwarg" entry.')
+            kwargs[k] = v
+        return kwargs
+
+    def assignment(self, node: mparser.AssignmentNode) -> None:
+        assert(isinstance(node, mparser.AssignmentNode))
+        if self.argument_depth != 0:
+            raise InvalidArguments('''Tried to assign values inside an argument list.
+To specify a keyword argument, use : instead of =.''')
+        var_name = node.var_name
+        if not isinstance(var_name, str):
+            raise InvalidArguments('Tried to assign value to a non-variable.')
+        value = self.evaluate_statement(node.value)
+        if not self.is_assignable(value):
+            raise InvalidCode(f'Tried to assign the invalid value "{value}" of type {type(value).__name__} to variable.')
+        # For mutable objects we need to make a copy on assignment
+        if isinstance(value, MutableInterpreterObject):
+            value = copy.deepcopy(value)
+        self.set_variable(var_name, value)
+        return None
+
+    def set_variable(self, varname: str, variable: T.Union[TYPE_var, InterpreterObject], *, holderify: bool = False) -> None:
+        if variable is None:
+            raise InvalidCode('Can not assign None to variable.')
+        if holderify:
+            variable = self._holderify(variable)
+        else:
+            # Ensure that we are never storing a HoldableObject
+            def check(x: T.Union[TYPE_var, InterpreterObject]) -> None:
+                if isinstance(x, mesonlib.HoldableObject):
+                    raise mesonlib.MesonBugException(f'set_variable in InterpreterBase called with a HoldableObject {x} of type {type(x).__name__}')
+                elif isinstance(x, list):
+                    for y in x:
+                        check(y)
+                elif isinstance(x, dict):
+                    for v in x.values():
+                        check(v)
+            check(variable)
+        if not isinstance(varname, str):
+            raise InvalidCode('First argument to set_variable must be a string.')
+        if not self.is_assignable(variable):
+            raise InvalidCode(f'Assigned value "{variable}" of type {type(variable).__name__} is not an assignable type.')
+        if re.match('[_a-zA-Z][_0-9a-zA-Z]*$', varname) is None:
+            raise InvalidCode('Invalid variable name: ' + varname)
+        if varname in self.builtin:
+            raise InvalidCode('Tried to overwrite internal variable "%s"' % varname)
+        self.variables[varname] = variable
+
+    def get_variable(self, varname: str) -> T.Union[TYPE_var, InterpreterObject]:
+        if varname in self.builtin:
+            return self.builtin[varname]
+        if varname in self.variables:
+            return self.variables[varname]
+        raise InvalidCode('Unknown variable "%s".' % varname)
+
+    def is_assignable(self, value: T.Any) -> bool:
+        return isinstance(value, (InterpreterObject, str, int, list, dict))
+
+    def validate_extraction(self, buildtarget: mesonlib.HoldableObject) -> None:
+        raise InterpreterException('validate_extraction is not implemented in this context (please file a bug)')