typing: more fixes

19 files changed, 215 insertions, 161 deletions

diff --git a/mesonbuild/ast/introspection.py b/mesonbuild/ast/introspection.py
index 73cd422..19fedbf 100644
--- a/mesonbuild/ast/introspection.py
+++ b/mesonbuild/ast/introspection.py
@@ -25,10 +25,11 @@ from ..build import BuildTarget, Executable, Jar, SharedLibrary, SharedModule, S
 from ..mparser import BaseNode, ArithmeticNode, ArrayNode, ElementaryNode, IdNode, FunctionNode, StringNode
 import typing as T
 import os
+import argparse
 
 build_target_functions = ['executable', 'jar', 'library', 'shared_library', 'shared_module', 'static_library', 'both_libraries']
 
-class IntrospectionHelper:
+class IntrospectionHelper(argparse.Namespace):
     # mimic an argparse namespace
     def __init__(self, cross_file: str):
         self.cross_file = cross_file  # type: str
diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index e0bab7c..b12932a 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -36,7 +36,7 @@ from ..mesonlib import (
 )
 
 if T.TYPE_CHECKING:
-    from ..interpreter import Interpreter
+    from ..interpreter import Interpreter, Test
 
 
 class TestProtocol(enum.Enum):
@@ -104,7 +104,7 @@ class TargetInstallData:
 
 class ExecutableSerialisation:
     def __init__(self, cmd_args, env=None, exe_wrapper=None,
-                 workdir=None, extra_paths=None, capture=None):
+                 workdir=None, extra_paths=None, capture=None) -> None:
         self.cmd_args = cmd_args
         self.env = env or {}
         if exe_wrapper is not None:
@@ -182,11 +182,15 @@ class Backend:
         self.interpreter = interpreter
         self.environment = build.environment
         self.processed_targets = {}
+        self.name = '<UNKNOWN>'
         self.build_dir = self.environment.get_build_dir()
         self.source_dir = self.environment.get_source_dir()
         self.build_to_src = mesonlib.relpath(self.environment.get_source_dir(),
                                              self.environment.get_build_dir())
 
+    def generate(self) -> None:
+        raise RuntimeError('generate is not implemented in {}'.format(type(self).__name__))
+
     def get_target_filename(self, t, *, warn_multi_output: bool = True):
         if isinstance(t, build.CustomTarget):
             if warn_multi_output and len(t.get_outputs()) != 1:
@@ -794,7 +798,7 @@ class Backend:
     def write_test_file(self, datafile):
         self.write_test_serialisation(self.build.get_tests(), datafile)
 
-    def create_test_serialisation(self, tests):
+    def create_test_serialisation(self, tests: T.List['Test']) -> T.List[TestSerialisation]:
         arr = []
         for t in sorted(tests, key=lambda tst: -1 * tst.priority):
             exe = t.get_exe()
@@ -864,7 +868,7 @@ class Backend:
             arr.append(ts)
         return arr
 
-    def write_test_serialisation(self, tests, datafile):
+    def write_test_serialisation(self, tests: T.List['Test'], datafile: str):
         pickle.dump(self.create_test_serialisation(tests), datafile)
 
     def construct_target_rel_path(self, a, workdir):
@@ -1128,7 +1132,7 @@ class Backend:
         cmd = [i.replace('\\', '/') for i in cmd]
         return inputs, outputs, cmd
 
-    def run_postconf_scripts(self):
+    def run_postconf_scripts(self) -> None:
         env = {'MESON_SOURCE_ROOT': self.environment.get_source_dir(),
                'MESON_BUILD_ROOT': self.environment.get_build_dir(),
                'MESONINTROSPECT': ' '.join([shlex.quote(x) for x in self.environment.get_build_command() + ['introspect']]),
@@ -1140,7 +1144,7 @@ class Backend:
             cmd = s['exe'] + s['args']
             subprocess.check_call(cmd, env=child_env)
 
-    def create_install_data(self):
+    def create_install_data(self) -> InstallData:
         strip_bin = self.environment.lookup_binary_entry(MachineChoice.HOST, 'strip')
         if strip_bin is None:
             if self.environment.is_cross_build():
@@ -1338,7 +1342,7 @@ class Backend:
             d.install_subdirs.append([src_dir, dst_dir, sd.install_mode,
                                       sd.exclude])
 
-    def get_introspection_data(self, target_id, target):
+    def get_introspection_data(self, target_id: str, target: build.Target) -> T.List[T.Dict[str, T.Union[bool, str, T.List[T.Union[str, T.Dict[str, T.Union[str, T.List[str], bool]]]]]]]:
         '''
         Returns a list of source dicts with the following format for a given target:
         [
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 9004165..0cf1d54 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -2997,7 +2997,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         elem = NinjaBuildElement(self.all_outputs, deps, 'phony', '')
         self.add_build(elem)
 
-    def get_introspection_data(self, target_id, target):
+    def get_introspection_data(self, target_id: str, target: build.Target) -> T.List[T.Dict[str, T.Union[bool, str, T.List[T.Union[str, T.Dict[str, T.Union[str, T.List[str], bool]]]]]]]:
         if target_id not in self.introspection_data or len(self.introspection_data[target_id]) == 0:
             return super().get_introspection_data(target_id, target)
 
diff --git a/mesonbuild/backend/vs2010backend.py b/mesonbuild/backend/vs2010backend.py
index f282d02..4fc3d5c 100644
--- a/mesonbuild/backend/vs2010backend.py
+++ b/mesonbuild/backend/vs2010backend.py
@@ -192,11 +192,11 @@ class Vs2010Backend(backends.Backend):
         Vs2010Backend.touch_regen_timestamp(self.environment.get_build_dir())
 
     @staticmethod
-    def get_regen_stampfile(build_dir):
+    def get_regen_stampfile(build_dir: str) -> None:
         return os.path.join(os.path.join(build_dir, Environment.private_dir), 'regen.stamp')
 
     @staticmethod
-    def touch_regen_timestamp(build_dir):
+    def touch_regen_timestamp(build_dir: str) -> None:
         with open(Vs2010Backend.get_regen_stampfile(build_dir), 'w'):
             pass
 
diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index edd1506..bf325b0 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -37,6 +37,9 @@ from .compilers import (
 from .linkers import StaticLinker
 from .interpreterbase import FeatureNew
 
+if T.TYPE_CHECKING:
+    from .interpreter import Test
+
 pch_kwargs = set(['c_pch', 'cpp_pch'])
 
 lang_arg_kwargs = set([
@@ -128,14 +131,14 @@ class Build:
         self.project_version = None
         self.environment = environment
         self.projects = {}
-        self.targets = OrderedDict()
+        self.targets = OrderedDict()                  # type: T.Dict[str, 'Target']
         self.run_target_names = set() # type: T.Set[T.Tuple[str, str]]
         self.global_args = PerMachine({}, {})         # type: PerMachine[T.Dict[str, T.List[str]]]
         self.projects_args = PerMachine({}, {})       # type: PerMachine[T.Dict[str, T.List[str]]]
         self.global_link_args = PerMachine({}, {})    # type: PerMachine[T.Dict[str, T.List[str]]]
         self.projects_link_args = PerMachine({}, {})  # type: PerMachine[T.Dict[str, T.List[str]]]
-        self.tests = []
-        self.benchmarks = []
+        self.tests = []                               # type: T.List['Test']
+        self.benchmarks = []                          # type: T.List['Test']
         self.headers = []
         self.man = []
         self.data = []
@@ -178,13 +181,13 @@ class Build:
     def get_subproject_dir(self):
         return self.subproject_dir
 
-    def get_targets(self):
+    def get_targets(self) -> T.Dict[str, 'Target']:
         return self.targets
 
-    def get_tests(self):
+    def get_tests(self) -> T.List['Test']:
         return self.tests
 
-    def get_benchmarks(self):
+    def get_benchmarks(self) -> T.List['Test']:
         return self.benchmarks
 
     def get_headers(self):
@@ -403,13 +406,13 @@ a hard error in the future.'''.format(name))
             outdirs[0] = default_install_dir
         return outdirs, custom_install_dir
 
-    def get_basename(self):
+    def get_basename(self) -> str:
         return self.name
 
-    def get_subdir(self):
+    def get_subdir(self) -> str:
         return self.subdir
 
-    def get_typename(self):
+    def get_typename(self) -> str:
         return self.typename
 
     @staticmethod
@@ -423,7 +426,7 @@ a hard error in the future.'''.format(name))
         return h.hexdigest()[:7]
 
     @staticmethod
-    def construct_id_from_path(subdir, name, type_suffix):
+    def construct_id_from_path(subdir: str, name: str, type_suffix: str) -> str:
         """Construct target ID from subdir, name and type suffix.
 
         This helper function is made public mostly for tests."""
@@ -441,7 +444,7 @@ a hard error in the future.'''.format(name))
             return subdir_part + '@@' + my_id
         return my_id
 
-    def get_id(self):
+    def get_id(self) -> str:
         return self.construct_id_from_path(
             self.subdir, self.name, self.type_suffix())
 
@@ -480,6 +483,12 @@ a hard error in the future.'''.format(name))
     def is_linkable_target(self) -> bool:
         return False
 
+    def get_outputs(self) -> T.List[str]:
+        return []
+
+    def should_install(self) -> bool:
+        return False
+
 class BuildTarget(Target):
     known_kwargs = known_build_target_kwargs
 
@@ -1006,7 +1015,7 @@ This will become a hard error in a future Meson release.''')
     def get_filename(self):
         return self.filename
 
-    def get_outputs(self):
+    def get_outputs(self) -> T.List[str]:
         return self.outputs
 
     def get_extra_args(self, language):
@@ -1036,7 +1045,7 @@ This will become a hard error in a future Meson release.''')
     def get_generated_sources(self):
         return self.generated
 
-    def should_install(self):
+    def should_install(self) -> bool:
         return self.need_install
 
     def has_pch(self):
@@ -1474,7 +1483,7 @@ class GeneratedList:
     def get_inputs(self):
         return self.infilelist
 
-    def get_outputs(self):
+    def get_outputs(self) -> T.List[str]:
         return self.outfilelist
 
     def get_outputs_for(self, filename):
@@ -2192,7 +2201,7 @@ class CustomTarget(Target):
     def get_dependencies(self):
         return self.dependencies
 
-    def should_install(self):
+    def should_install(self) -> bool:
         return self.install
 
     def get_custom_install_dir(self):
@@ -2201,7 +2210,7 @@ class CustomTarget(Target):
     def get_custom_install_mode(self):
         return self.install_mode
 
-    def get_outputs(self):
+    def get_outputs(self) -> T.List[str]:
         return self.outputs
 
     def get_filename(self):
@@ -2289,13 +2298,13 @@ class RunTarget(Target):
     def get_sources(self):
         return []
 
-    def should_install(self):
+    def should_install(self) -> bool:
         return False
 
-    def get_filename(self):
+    def get_filename(self) -> str:
         return self.name
 
-    def get_outputs(self):
+    def get_outputs(self) -> T.List[str]:
         if isinstance(self.name, str):
             return [self.name]
         elif isinstance(self.name, list):
@@ -2367,7 +2376,7 @@ class CustomTargetIndex:
         return '<CustomTargetIndex: {!r}[{}]>'.format(
             self.target, self.target.get_outputs().index(self.output))
 
-    def get_outputs(self):
+    def get_outputs(self) -> T.List[str]:
         return [self.output]
 
     def get_subdir(self):
@@ -2509,6 +2518,6 @@ def load(build_dir: str) -> Build:
         raise MesonException(load_fail_msg)
     return obj
 
-def save(obj, filename):
+def save(obj: Build, filename: str) -> None:
     with open(filename, 'wb') as f:
         pickle.dump(obj, f)
diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index ce03fbc..4fc4999 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -47,7 +47,7 @@ _T = T.TypeVar('_T')
 
 class MesonVersionMismatchException(MesonException):
     '''Build directory generated with Meson version incompatible with current version'''
-    def __init__(self, old_version, current_version):
+    def __init__(self, old_version: str, current_version: str) -> None:
         super().__init__('Build directory has been generated with Meson version {}, '
                          'which is incompatible with current version {}.'
                          .format(old_version, current_version))
@@ -56,7 +56,7 @@ class MesonVersionMismatchException(MesonException):
 
 
 class UserOption(T.Generic[_T]):
-    def __init__(self, description, choices, yielding):
+    def __init__(self, description: str, choices: T.Optional[T.Union[str, T.List[_T]]], yielding: T.Optional[bool]):
         super().__init__()
         self.choices = choices
         self.description = description
@@ -66,7 +66,8 @@ class UserOption(T.Generic[_T]):
             raise MesonException('Value of "yielding" must be a boolean.')
         self.yielding = yielding
 
-    def printable_value(self):
+    def printable_value(self) -> T.Union[str, int, bool, T.List[T.Union[str, int, bool]]]:
+        assert isinstance(self.value, (str, int, bool, list))
         return self.value
 
     # Check that the input is a valid value and return the
@@ -75,30 +76,32 @@ class UserOption(T.Generic[_T]):
     def validate_value(self, value: T.Any) -> _T:
         raise RuntimeError('Derived option class did not override validate_value.')
 
-    def set_value(self, newvalue):
+    def set_value(self, newvalue: T.Any) -> None:
         self.value = self.validate_value(newvalue)
 
 class UserStringOption(UserOption[str]):
-    def __init__(self, description, value, choices=None, yielding=None):
-        super().__init__(description, choices, yielding)
+    def __init__(self, description: str, value: T.Any, yielding: T.Optional[bool] = None):
+        super().__init__(description, None, yielding)
         self.set_value(value)
 
-    def validate_value(self, value):
+    def validate_value(self, value: T.Any) -> str:
         if not isinstance(value, str):
             raise MesonException('Value "%s" for string option is not a string.' % str(value))
         return value
 
 class UserBooleanOption(UserOption[bool]):
-    def __init__(self, description, value, yielding=None):
+    def __init__(self, description: str, value, yielding: T.Optional[bool] = None) -> None:
         super().__init__(description, [True, False], yielding)
         self.set_value(value)
 
     def __bool__(self) -> bool:
         return self.value
 
-    def validate_value(self, value) -> bool:
+    def validate_value(self, value: T.Any) -> bool:
         if isinstance(value, bool):
             return value
+        if not isinstance(value, str):
+            raise MesonException('Value {} cannot be converted to a boolean'.format(value))
         if value.lower() == 'true':
             return True
         if value.lower() == 'false':
@@ -106,7 +109,7 @@ class UserBooleanOption(UserOption[bool]):
         raise MesonException('Value %s is not boolean (true or false).' % value)
 
 class UserIntegerOption(UserOption[int]):
-    def __init__(self, description, value, yielding=None):
+    def __init__(self, description: str, value: T.Any, yielding: T.Optional[bool] = None):
         min_value, max_value, default_value = value
         self.min_value = min_value
         self.max_value = max_value
@@ -119,7 +122,7 @@ class UserIntegerOption(UserOption[int]):
         super().__init__(description, choices, yielding)
         self.set_value(default_value)
 
-    def validate_value(self, value) -> int:
+    def validate_value(self, value: T.Any) -> int:
         if isinstance(value, str):
             value = self.toint(value)
         if not isinstance(value, int):
@@ -130,35 +133,35 @@ class UserIntegerOption(UserOption[int]):
             raise MesonException('New value %d is more than maximum value %d.' % (value, self.max_value))
         return value
 
-    def toint(self, valuestring) -> int:
+    def toint(self, valuestring: str) -> int:
         try:
             return int(valuestring)
         except ValueError:
             raise MesonException('Value string "%s" is not convertible to an integer.' % valuestring)
 
 class UserUmaskOption(UserIntegerOption, UserOption[T.Union[str, int]]):
-    def __init__(self, description, value, yielding=None):
+    def __init__(self, description: str, value: T.Any, yielding: T.Optional[bool] = None):
         super().__init__(description, (0, 0o777, value), yielding)
         self.choices = ['preserve', '0000-0777']
 
-    def printable_value(self):
+    def printable_value(self) -> str:
         if self.value == 'preserve':
             return self.value
         return format(self.value, '04o')
 
-    def validate_value(self, value):
+    def validate_value(self, value: T.Any) -> T.Union[str, int]:
         if value is None or value == 'preserve':
             return 'preserve'
         return super().validate_value(value)
 
-    def toint(self, valuestring):
+    def toint(self, valuestring: T.Union[str, int]) -> int:
         try:
             return int(valuestring, 8)
         except ValueError as e:
             raise MesonException('Invalid mode: {}'.format(e))
 
 class UserComboOption(UserOption[str]):
-    def __init__(self, description, choices: T.List[str], value, yielding=None):
+    def __init__(self, description: str, choices: T.List[str], value: T.Any, yielding: T.Optional[bool] = None):
         super().__init__(description, choices, yielding)
         if not isinstance(self.choices, list):
             raise MesonException('Combo choices must be an array.')
@@ -167,7 +170,7 @@ class UserComboOption(UserOption[str]):
                 raise MesonException('Combo choice elements must be strings.')
         self.set_value(value)
 
-    def validate_value(self, value):
+    def validate_value(self, value: T.Any) -> str:
         if value not in self.choices:
             if isinstance(value, bool):
                 _type = 'boolean'
@@ -182,13 +185,13 @@ class UserComboOption(UserOption[str]):
         return value
 
 class UserArrayOption(UserOption[T.List[str]]):
-    def __init__(self, description, value, split_args=False, user_input=False, allow_dups=False, **kwargs):
+    def __init__(self, description: str, value: T.Union[str, T.List[str]], split_args: bool = False, user_input: bool = False, allow_dups: bool = False, **kwargs: T.Any) -> None:
         super().__init__(description, kwargs.get('choices', []), yielding=kwargs.get('yielding', None))
         self.split_args = split_args
         self.allow_dups = allow_dups
         self.value = self.validate_value(value, user_input=user_input)
 
-    def validate_value(self, value, user_input: bool = True) -> T.List[str]:
+    def validate_value(self, value: T.Union[str, T.List[str]], user_input: bool = True) -> T.List[str]:
         # User input is for options defined on the command line (via -D
         # options). Users can put their input in as a comma separated
         # string, but for defining options in meson_options.txt the format
@@ -232,16 +235,16 @@ class UserArrayOption(UserOption[T.List[str]]):
 class UserFeatureOption(UserComboOption):
     static_choices = ['enabled', 'disabled', 'auto']
 
-    def __init__(self, description, value, yielding=None):
+    def __init__(self, description: str, value: T.Any, yielding: T.Optional[bool] = None):
         super().__init__(description, self.static_choices, value, yielding)
 
-    def is_enabled(self):
+    def is_enabled(self) -> bool:
         return self.value == 'enabled'
 
-    def is_disabled(self):
+    def is_disabled(self) -> bool:
         return self.value == 'disabled'
 
-    def is_auto(self):
+    def is_auto(self) -> bool:
         return self.value == 'auto'
 
 if T.TYPE_CHECKING:
@@ -534,7 +537,7 @@ class CoreData:
             value = None
         opts_map[optname] = opt.init_option(key, value, default_prefix())
 
-    def init_backend_options(self, backend_name):
+    def init_backend_options(self, backend_name: str) -> None:
         if backend_name == 'ninja':
             self.backend_options['backend_max_links'] = \
                 UserIntegerOption(
@@ -547,7 +550,7 @@ class CoreData:
                     'Default project to execute in Visual Studio',
                     '')
 
-    def get_builtin_option(self, optname, subproject=''):
+    def get_builtin_option(self, optname: str, subproject: str = '') -> T.Union[str, int, bool]:
         raw_optname = optname
         if subproject:
             optname = subproject + ':' + optname
@@ -683,7 +686,7 @@ class CoreData:
     def get_external_link_args(self, for_machine: MachineChoice, lang):
         return self.compiler_options[for_machine][lang]['link_args'].value
 
-    def merge_user_options(self, options):
+    def merge_user_options(self, options: T.Dict[str, T.Union[str, bool, int]]) -> None:
         for (name, value) in options.items():
             if name not in self.user_options:
                 self.user_options[name] = value
@@ -715,7 +718,7 @@ class CoreData:
                 if k in build_opts:
                     build_opts[k].set_value(o.value)
 
-    def set_options(self, options, *, subproject='', warn_unknown=True):
+    def set_options(self, options: T.Dict[str, T.Any], subproject: str = '', warn_unknown: bool = True) -> None:
         if not self.is_cross_build():
             options = self.strip_build_option_names(options)
         # Set prefix first because it's needed to sanitize other options
@@ -912,10 +915,10 @@ def parse_machine_files(filenames):
     parser = MachineFileParser(filenames)
     return parser.sections
 
-def get_cmd_line_file(build_dir):
+def get_cmd_line_file(build_dir: str) -> str:
     return os.path.join(build_dir, 'meson-private', 'cmd_line.txt')
 
-def read_cmd_line_file(build_dir, options):
+def read_cmd_line_file(build_dir: str, options: argparse.Namespace) -> None:
     filename = get_cmd_line_file(build_dir)
     if not os.path.isfile(filename):
         return
@@ -937,10 +940,10 @@ def read_cmd_line_file(build_dir, options):
         # literal_eval to get it into the list of strings.
         options.native_file = ast.literal_eval(properties.get('native_file', '[]'))
 
-def cmd_line_options_to_string(options):
+def cmd_line_options_to_string(options: argparse.Namespace) -> T.Dict[str, str]:
     return {k: str(v) for k, v in options.cmd_line_options.items()}
 
-def write_cmd_line_file(build_dir, options):
+def write_cmd_line_file(build_dir: str, options: argparse.Namespace) -> None:
     filename = get_cmd_line_file(build_dir)
     config = CmdLineFileParser()
 
@@ -955,7 +958,7 @@ def write_cmd_line_file(build_dir, options):
     with open(filename, 'w') as f:
         config.write(f)
 
-def update_cmd_line_file(build_dir, options):
+def update_cmd_line_file(build_dir: str, options: argparse.Namespace):
     filename = get_cmd_line_file(build_dir)
     config = CmdLineFileParser()
     config.read(filename)
@@ -963,7 +966,7 @@ def update_cmd_line_file(build_dir, options):
     with open(filename, 'w') as f:
         config.write(f)
 
-def get_cmd_line_options(build_dir, options):
+def get_cmd_line_options(build_dir: str, options: argparse.Namespace) -> str:
     copy = argparse.Namespace(**vars(options))
     read_cmd_line_file(build_dir, copy)
     cmdline = ['-D{}={}'.format(k, v) for k, v in copy.cmd_line_options.items()]
@@ -976,7 +979,7 @@ def get_cmd_line_options(build_dir, options):
 def major_versions_differ(v1, v2):
     return v1.split('.')[0:2] != v2.split('.')[0:2]
 
-def load(build_dir):
+def load(build_dir: str) -> CoreData:
     filename = os.path.join(build_dir, 'meson-private', 'coredata.dat')
     load_fail_msg = 'Coredata file {!r} is corrupted. Try with a fresh build tree.'.format(filename)
     try:
@@ -995,7 +998,7 @@ def load(build_dir):
         raise MesonVersionMismatchException(obj.version, version)
     return obj
 
-def save(obj, build_dir):
+def save(obj: CoreData, build_dir: str) -> str:
     filename = os.path.join(build_dir, 'meson-private', 'coredata.dat')
     prev_filename = filename + '.prev'
     tempfilename = filename + '~'
@@ -1012,7 +1015,7 @@ def save(obj, build_dir):
     return filename
 
 
-def register_builtin_arguments(parser):
+def register_builtin_arguments(parser: argparse.ArgumentParser) -> None:
     for n, b in BUILTIN_OPTIONS.items():
         b.add_to_argparse(n, parser, '', '')
     for n, b in BUILTIN_OPTIONS_PER_MACHINE.items():
@@ -1021,7 +1024,7 @@ def register_builtin_arguments(parser):
     parser.add_argument('-D', action='append', dest='projectoptions', default=[], metavar="option",
                         help='Set the value of an option, can be used several times to set multiple options.')
 
-def create_options_dict(options):
+def create_options_dict(options: T.List[str]) -> T.Dict[str, str]:
     result = OrderedDict()
     for o in options:
         try:
@@ -1031,7 +1034,7 @@ def create_options_dict(options):
         result[key] = value
     return result
 
-def parse_cmd_line_options(args):
+def parse_cmd_line_options(args: argparse.Namespace) -> None:
     args.cmd_line_options = create_options_dict(args.projectoptions)
 
     # Merge builtin options set with --option into the dict.
diff --git a/mesonbuild/dependencies/boost.py b/mesonbuild/dependencies/boost.py
index 8497d2c..6e95860 100644
--- a/mesonbuild/dependencies/boost.py
+++ b/mesonbuild/dependencies/boost.py
@@ -341,7 +341,9 @@ class BoostLibraryFile():
 class BoostDependency(ExternalDependency):
     def __init__(self, environment: Environment, kwargs: T.Dict[str, T.Any]) -> None:
         super().__init__('boost', environment, kwargs, language='cpp')
-        self.debug = environment.coredata.get_builtin_option('buildtype').startswith('debug')
+        buildtype = environment.coredata.get_builtin_option('buildtype')
+        assert isinstance(buildtype, str)
+        self.debug = buildtype.startswith('debug')
         self.multithreading = kwargs.get('threading', 'multi') == 'multi'
 
         self.boost_root = None  # type: Path
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index f8282c0..5cd6069 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -132,6 +132,9 @@ build_filename = 'meson.build'
 
 CompilersDict = T.Dict[str, Compiler]
 
+if T.TYPE_CHECKING:
+    import argparse
+
 def detect_gcovr(min_version='3.3', new_rootdir_version='4.2', log=False):
     gcovr_exe = 'gcovr'
     try:
@@ -153,7 +156,7 @@ def detect_llvm_cov():
             return tool
     return None
 
-def find_coverage_tools():
+def find_coverage_tools() -> T.Tuple[T.Optional[str], T.Optional[str], T.Optional[str], T.Optional[str], T.Optional[str]]:
     gcovr_exe, gcovr_new_rootdir = detect_gcovr()
 
     llvm_cov_exe = detect_llvm_cov()
@@ -522,7 +525,7 @@ class Environment:
     log_dir = 'meson-logs'
     info_dir = 'meson-info'
 
-    def __init__(self, source_dir, build_dir, options):
+    def __init__(self, source_dir: T.Optional[str], build_dir: T.Optional[str], options: 'argparse.Namespace') -> None:
         self.source_dir = source_dir
         self.build_dir = build_dir
         # Do not try to create build directories when build_dir is none.
@@ -535,7 +538,7 @@ class Environment:
             os.makedirs(self.log_dir, exist_ok=True)
             os.makedirs(self.info_dir, exist_ok=True)
             try:
-                self.coredata = coredata.load(self.get_build_dir())
+                self.coredata = coredata.load(self.get_build_dir())  # type: coredata.CoreData
                 self.first_invocation = False
             except FileNotFoundError:
                 self.create_new_coredata(options)
@@ -807,7 +810,7 @@ class Environment:
         self.default_pkgconfig = ['pkg-config']
         self.wrap_resolver = None
 
-    def create_new_coredata(self, options):
+    def create_new_coredata(self, options: 'argparse.Namespace') -> None:
         # WARNING: Don't use any values from coredata in __init__. It gets
         # re-initialized with project options by the interpreter during
         # build file parsing.
@@ -819,17 +822,17 @@ class Environment:
     def is_cross_build(self, when_building_for: MachineChoice = MachineChoice.HOST) -> bool:
         return self.coredata.is_cross_build(when_building_for)
 
-    def dump_coredata(self):
+    def dump_coredata(self) -> str:
         return coredata.save(self.coredata, self.get_build_dir())
 
-    def get_script_dir(self):
+    def get_script_dir(self) -> str:
         import mesonbuild.scripts
         return os.path.dirname(mesonbuild.scripts.__file__)
 
-    def get_log_dir(self):
+    def get_log_dir(self) -> str:
         return self.log_dir
 
-    def get_coredata(self):
+    def get_coredata(self) -> coredata.CoreData:
         return self.coredata
 
     def get_build_command(self, unbuffered=False):
@@ -1535,7 +1538,7 @@ class Environment:
 
         self._handle_exceptions(popen_exceptions, compilers)
 
-    def get_scratch_dir(self):
+    def get_scratch_dir(self) -> str:
         return self.scratch_dir
 
     def detect_objc_compiler(self, for_machine: MachineInfo) -> 'Compiler':
@@ -1974,10 +1977,10 @@ class Environment:
         self._handle_exceptions(popen_exceptions, linkers, 'linker')
         raise EnvironmentException('Unknown static linker "{}"'.format(' '.join(linkers)))
 
-    def get_source_dir(self):
+    def get_source_dir(self) -> str:
         return self.source_dir
 
-    def get_build_dir(self):
+    def get_build_dir(self) -> str:
         return self.build_dir
 
     def get_import_lib_dir(self) -> str:
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 322cc26..031bed0 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -31,9 +31,10 @@ from .interpreterbase import InterpreterException, InvalidArguments, InvalidCode
 from .interpreterbase import InterpreterObject, MutableInterpreterObject, Disabler, disablerIfNotFound
 from .interpreterbase import FeatureNew, FeatureDeprecated, FeatureNewKwargs, FeatureDeprecatedKwargs
 from .interpreterbase import ObjectHolder, MesonVersionString
-from .modules import ModuleReturnValue
+from .interpreterbase import TYPE_var, TYPE_nkwargs
+from .modules import ModuleReturnValue, ExtensionModule
 from .cmake import CMakeInterpreter
-from .backend.backends import TestProtocol
+from .backend.backends import TestProtocol, Backend
 
 from pathlib import Path, PurePath
 import os
@@ -673,22 +674,22 @@ class MachineHolder(InterpreterObject, ObjectHolder):
 
     @noPosargs
     @permittedKwargs({})
-    def cpu_family_method(self, args, kwargs):
+    def cpu_family_method(self, args: T.List[TYPE_var], kwargs: TYPE_nkwargs) -> str:
         return self.held_object.cpu_family
 
     @noPosargs
     @permittedKwargs({})
-    def cpu_method(self, args, kwargs):
+    def cpu_method(self, args: T.List[TYPE_var], kwargs: TYPE_nkwargs) -> str:
         return self.held_object.cpu
 
     @noPosargs
     @permittedKwargs({})
-    def system_method(self, args, kwargs):
+    def system_method(self, args: T.List[TYPE_var], kwargs: TYPE_nkwargs) -> str:
         return self.held_object.system
 
     @noPosargs
     @permittedKwargs({})
-    def endian_method(self, args, kwargs):
+    def endian_method(self, args: T.List[TYPE_var], kwargs: TYPE_nkwargs) -> str:
         return self.held_object.endian
 
 class IncludeDirsHolder(InterpreterObject, ObjectHolder):
@@ -2333,8 +2334,18 @@ permitted_kwargs = {'add_global_arguments': {'language', 'native'},
 
 class Interpreter(InterpreterBase):
 
-    def __init__(self, build, backend=None, subproject='', subdir='', subproject_dir='subprojects',
-                 modules = None, default_project_options=None, mock=False, ast=None):
+    def __init__(
+                self,
+                build: build.Build,
+                backend: T.Optional[Backend] = None,
+                subproject: str = '',
+                subdir: str = '',
+                subproject_dir: str = 'subprojects',
+                modules: T.Optional[T.Dict[str, ExtensionModule]] = None,
+                default_project_options: T.Optional[T.Dict[str, str]] = None,
+                mock: bool = False,
+                ast: T.Optional[mparser.CodeBlockNode] = None,
+            ) -> None:
         super().__init__(build.environment.get_source_dir(), subdir, subproject)
         self.an_unpicklable_object = mesonlib.an_unpicklable_object
         self.build = build
@@ -2396,7 +2407,8 @@ class Interpreter(InterpreterBase):
         self.builtin['target_machine'] = \
             MachineHolder(self.build.environment.machines.target)
 
-    def get_non_matching_default_options(self):
+    # TODO: Why is this in interpreter.py and not CoreData or Environment?
+    def get_non_matching_default_options(self) -> T.Iterator[T.Tuple[str, str, coredata.UserOption]]:
         env = self.environment
         for def_opt_name, def_opt_value in self.project_default_options.items():
             for opts in env.coredata.get_all_options():
@@ -2530,7 +2542,7 @@ class Interpreter(InterpreterBase):
         self.process_new_values(invalues)
         return self.holderify(return_object.return_value)
 
-    def get_build_def_files(self):
+    def get_build_def_files(self) -> T.List[str]:
         return self.build_def_files
 
     def add_build_def_file(self, f):
@@ -2599,7 +2611,9 @@ class Interpreter(InterpreterBase):
             module = importlib.import_module('mesonbuild.modules.' + modname)
         except ImportError:
             raise InvalidArguments('Module "%s" does not exist' % (modname, ))
-        self.modules[modname] = module.initialize(self)
+        ext_module = module.initialize(self)
+        assert isinstance(ext_module, ExtensionModule)
+        self.modules[modname] = ext_module
 
     @stringArgs
     @noKwargs
@@ -4598,7 +4612,7 @@ different subdirectory.
     def func_join_paths(self, node, args, kwargs):
         return self.join_path_strings(args)
 
-    def run(self):
+    def run(self) -> None:
         super().run()
         mlog.log('Build targets in project:', mlog.bold(str(len(self.build.targets))))
         FeatureNew.report(self.subproject)
@@ -4608,14 +4622,14 @@ different subdirectory.
         if self.subproject == '':
             self._print_summary()
 
-    def print_extra_warnings(self):
+    def print_extra_warnings(self) -> None:
         # TODO cross compilation
         for c in self.coredata.compilers.host.values():
             if c.get_id() == 'clang':
                 self.check_clang_asan_lundef()
                 break
 
-    def check_clang_asan_lundef(self):
+    def check_clang_asan_lundef(self) -> None:
         if 'b_lundef' not in self.coredata.base_options:
             return
         if 'b_sanitize' not in self.coredata.base_options:
diff --git a/mesonbuild/mcompile.py b/mesonbuild/mcompile.py
index 5466ad2..3267529 100644
--- a/mesonbuild/mcompile.py
+++ b/mesonbuild/mcompile.py
@@ -32,7 +32,7 @@ if T.TYPE_CHECKING:
     import argparse
 
 def array_arg(value: str) -> T.List[str]:
-    return T.cast(T.List[str], UserArrayOption(None, value, allow_dups=True, user_input=True).value)
+    return UserArrayOption(None, value, allow_dups=True, user_input=True).value
 
 def validate_builddir(builddir: Path) -> None:
     if not (builddir / 'meson-private' / 'coredata.dat' ).is_file():
@@ -45,7 +45,9 @@ def get_backend_from_coredata(builddir: Path) -> str:
     """
     Gets `backend` option value from coredata
     """
-    return T.cast(str, coredata.load(str(builddir)).get_builtin_option('backend'))
+    backend = coredata.load(str(builddir)).get_builtin_option('backend')
+    assert isinstance(backend, str)
+    return backend
 
 def parse_introspect_data(builddir: Path) -> T.Dict[str, T.List[dict]]:
     """
diff --git a/mesonbuild/mconf.py b/mesonbuild/mconf.py
index f070355..0245155 100644
--- a/mesonbuild/mconf.py
+++ b/mesonbuild/mconf.py
@@ -15,15 +15,19 @@
 import os
 from . import coredata, environment, mesonlib, build, mintro, mlog
 from .ast import AstIDGenerator
+import typing as T
 
-def add_arguments(parser):
+if T.TYPE_CHECKING:
+    import argparse
+
+def add_arguments(parser: 'argparse.ArgumentParser') -> None:
     coredata.register_builtin_arguments(parser)
     parser.add_argument('builddir', nargs='?', default='.')
     parser.add_argument('--clearcache', action='store_true', default=False,
                         help='Clear cached state (e.g. found dependencies)')
 
 
-def make_lower_case(val):
+def make_lower_case(val: T.Any) -> T.Union[str, T.List[T.Any]]:  # T.Any because of recursion...
     if isinstance(val, bool):
         return str(val).lower()
     elif isinstance(val, list):
diff --git a/mesonbuild/mintro.py b/mesonbuild/mintro.py
index d60386e..1e400b0 100644
--- a/mesonbuild/mintro.py
+++ b/mesonbuild/mintro.py
@@ -254,11 +254,9 @@ def list_buildoptions(coredata: cdata.CoreData, subprojects: T.Optional[T.List[s
         'compiler',
         machine='host',
     )
+    tmp_dict = dict(coredata.flatten_lang_iterator(coredata.compiler_options.build.items()))  # type: T.Dict[str, cdata.UserOption]
     add_keys(
-        {
-            'build.' + k: o for k, o in
-            coredata.flatten_lang_iterator(coredata.compiler_options.build.items())
-        },
+        {'build.' + k: o for k, o in tmp_dict.items()},
         'compiler',
         machine='build',
     )
@@ -305,10 +303,10 @@ def list_deps(coredata: cdata.CoreData) -> T.List[T.Dict[str, T.Union[str, T.Lis
                         'link_args': d.get_link_args()}]
     return result
 
-def get_test_list(testdata: backends.TestSerialisation) -> T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]:
+def get_test_list(testdata: T.List[backends.TestSerialisation]) -> T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]:
     result = []  # type: T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]
     for t in testdata:
-        to = {}
+        to = {}  # type: T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]
         if isinstance(t.fname, str):
             fname = [t.fname]
         else:
@@ -329,21 +327,21 @@ def get_test_list(testdata: backends.TestSerialisation) -> T.List[T.Dict[str, T.
         result.append(to)
     return result
 
-def list_tests(testdata: backends.TestSerialisation) -> T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]:
+def list_tests(testdata: T.List[backends.TestSerialisation]) -> T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]:
     return get_test_list(testdata)
 
-def list_benchmarks(benchdata: backends.TestSerialisation) -> T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]:
+def list_benchmarks(benchdata: T.List[backends.TestSerialisation]) -> T.List[T.Dict[str, T.Union[str, int, T.List[str], T.Dict[str, str]]]]:
     return get_test_list(benchdata)
 
 def list_projinfo(builddata: build.Build) -> T.Dict[str, T.Union[str, T.List[T.Dict[str, str]]]]:
     result = {'version': builddata.project_version,
               'descriptive_name': builddata.project_name,
-              'subproject_dir': builddata.subproject_dir}
+              'subproject_dir': builddata.subproject_dir}    # type: T.Dict[str, T.Union[str, T.List[T.Dict[str, str]]]]
     subprojects = []
     for k, v in builddata.subprojects.items():
         c = {'name': k,
              'version': v,
-             'descriptive_name': builddata.projects.get(k)}
+             'descriptive_name': builddata.projects.get(k)}  # type: T.Dict[str, str]
         subprojects.append(c)
     result['subprojects'] = subprojects
     return result
@@ -391,6 +389,7 @@ def run(options: argparse.Namespace) -> int:
         # Make sure that log entries in other parts of meson don't interfere with the JSON output
         mlog.disable()
         backend = backends.get_backend_from_name(options.backend)
+        assert backend is not None
         intr = IntrospectionInterpreter(sourcedir, '', backend.name, visitors = [AstIDGenerator(), AstIndentationGenerator(), AstConditionLevel()])
         intr.analyze()
         # Re-enable logging just in case
diff --git a/mesonbuild/modules/__init__.py b/mesonbuild/modules/__init__.py
index 47be039..ff27a11 100644
--- a/mesonbuild/modules/__init__.py
+++ b/mesonbuild/modules/__init__.py
@@ -19,14 +19,18 @@ import os
 
 from .. import build
 from ..mesonlib import unholder
+import typing as T
 
+if T.TYPE_CHECKING:
+    from ..interpreter import Interpreter
+    from ..interpreterbase import TYPE_var
 
 class ExtensionModule:
-    def __init__(self, interpreter):
+    def __init__(self, interpreter: 'Interpreter') -> None:
         self.interpreter = interpreter
-        self.snippets = set() # List of methods that operate only on the interpreter.
+        self.snippets = set()  # type: T.Set[str] # List of methods that operate only on the interpreter.
 
-    def is_snippet(self, funcname):
+    def is_snippet(self, funcname: str) -> bool:
         return funcname in self.snippets
 
 
@@ -69,7 +73,7 @@ def is_module_library(fname):
 
 
 class ModuleReturnValue:
-    def __init__(self, return_value, new_objects):
+    def __init__(self, return_value: 'TYPE_var', new_objects: T.List['TYPE_var']) -> None:
         self.return_value = return_value
         assert(isinstance(new_objects, list))
         self.new_objects = new_objects
diff --git a/mesonbuild/msetup.py b/mesonbuild/msetup.py
index f940a58..5fd88d7 100644
--- a/mesonbuild/msetup.py
+++ b/mesonbuild/msetup.py
@@ -178,12 +178,18 @@ class MesonApp:
             logger_fun = mlog.log
         else:
             logger_fun = mlog.debug
-        logger_fun('Build machine cpu family:', mlog.bold(intr.builtin['build_machine'].cpu_family_method([], {})))
-        logger_fun('Build machine cpu:', mlog.bold(intr.builtin['build_machine'].cpu_method([], {})))
-        mlog.log('Host machine cpu family:', mlog.bold(intr.builtin['host_machine'].cpu_family_method([], {})))
-        mlog.log('Host machine cpu:', mlog.bold(intr.builtin['host_machine'].cpu_method([], {})))
-        logger_fun('Target machine cpu family:', mlog.bold(intr.builtin['target_machine'].cpu_family_method([], {})))
-        logger_fun('Target machine cpu:', mlog.bold(intr.builtin['target_machine'].cpu_method([], {})))
+        build_machine = intr.builtin['build_machine']
+        host_machine = intr.builtin['build_machine']
+        target_machine = intr.builtin['target_machine']
+        assert isinstance(build_machine, interpreter.MachineHolder)
+        assert isinstance(host_machine, interpreter.MachineHolder)
+        assert isinstance(target_machine, interpreter.MachineHolder)
+        logger_fun('Build machine cpu family:', mlog.bold(build_machine.cpu_family_method([], {})))
+        logger_fun('Build machine cpu:', mlog.bold(build_machine.cpu_method([], {})))
+        mlog.log('Host machine cpu family:', mlog.bold(host_machine.cpu_family_method([], {})))
+        mlog.log('Host machine cpu:', mlog.bold(host_machine.cpu_method([], {})))
+        logger_fun('Target machine cpu family:', mlog.bold(target_machine.cpu_family_method([], {})))
+        logger_fun('Target machine cpu:', mlog.bold(target_machine.cpu_method([], {})))
         try:
             if self.options.profile:
                 fname = os.path.join(self.build_dir, 'meson-private', 'profile-interpreter.log')
diff --git a/mesonbuild/mtest.py b/mesonbuild/mtest.py
index cc82bd0..86d7384 100644
--- a/mesonbuild/mtest.py
+++ b/mesonbuild/mtest.py
@@ -43,10 +43,7 @@ from . import environment
 from . import mlog
 from .dependencies import ExternalProgram
 from .mesonlib import MesonException, get_wine_shortpath, split_args, join_args
-from .backend.backends import TestProtocol
-
-if T.TYPE_CHECKING:
-    from .backend.backends import TestSerialisation
+from .backend.backends import TestProtocol, TestSerialisation
 
 # GNU autotools interprets a return code of 77 from tests it executes to
 # mean that the test should be skipped.
@@ -445,7 +442,7 @@ class JunitBuilder:
 class TestRun:
 
     @classmethod
-    def make_gtest(cls, test: 'TestSerialisation', test_env: T.Dict[str, str],
+    def make_gtest(cls, test: TestSerialisation, test_env: T.Dict[str, str],
                    returncode: int, starttime: float, duration: float,
                    stdo: T.Optional[str], stde: T.Optional[str],
                    cmd: T.Optional[T.List[str]]) -> 'TestRun':
@@ -459,7 +456,7 @@ class TestRun:
             junit=tree)
 
     @classmethod
-    def make_exitcode(cls, test: 'TestSerialisation', test_env: T.Dict[str, str],
+    def make_exitcode(cls, test: TestSerialisation, test_env: T.Dict[str, str],
                       returncode: int, starttime: float, duration: float,
                       stdo: T.Optional[str], stde: T.Optional[str],
                       cmd: T.Optional[T.List[str]], **kwargs: T.Any) -> 'TestRun':
@@ -474,7 +471,7 @@ class TestRun:
         return cls(test, test_env, res, [], returncode, starttime, duration, stdo, stde, cmd, **kwargs)
 
     @classmethod
-    def make_tap(cls, test: 'TestSerialisation', test_env: T.Dict[str, str],
+    def make_tap(cls, test: TestSerialisation, test_env: T.Dict[str, str],
                  returncode: int, starttime: float, duration: float,
                  stdo: str, stde: str,
                  cmd: T.Optional[T.List[str]]) -> 'TestRun':
@@ -511,7 +508,7 @@ class TestRun:
 
         return cls(test, test_env, res, results, returncode, starttime, duration, stdo, stde, cmd)
 
-    def __init__(self, test: 'TestSerialisation', test_env: T.Dict[str, str],
+    def __init__(self, test: TestSerialisation, test_env: T.Dict[str, str],
                  res: TestResult, results: T.List[TestResult], returncode:
                  int, starttime: float, duration: float,
                  stdo: T.Optional[str], stde: T.Optional[str],
@@ -577,26 +574,32 @@ def write_json_log(jsonlogfile: T.TextIO, test_name: str, result: TestRun) -> No
 def run_with_mono(fname: str) -> bool:
     return fname.endswith('.exe') and not (is_windows() or is_cygwin())
 
-def load_benchmarks(build_dir: str) -> T.List['TestSerialisation']:
+def load_benchmarks(build_dir: str) -> T.List[TestSerialisation]:
     datafile = Path(build_dir) / 'meson-private' / 'meson_benchmark_setup.dat'
     if not datafile.is_file():
         raise TestException('Directory {!r} does not seem to be a Meson build directory.'.format(build_dir))
     with datafile.open('rb') as f:
-        obj = T.cast(T.List['TestSerialisation'], pickle.load(f))
+        obj = pickle.load(f)
+        assert isinstance(obj, list)
+        for i in obj:
+            assert isinstance(i, TestSerialisation)
     return obj
 
-def load_tests(build_dir: str) -> T.List['TestSerialisation']:
+def load_tests(build_dir: str) -> T.List[TestSerialisation]:
     datafile = Path(build_dir) / 'meson-private' / 'meson_test_setup.dat'
     if not datafile.is_file():
         raise TestException('Directory {!r} does not seem to be a Meson build directory.'.format(build_dir))
     with datafile.open('rb') as f:
-        obj = T.cast(T.List['TestSerialisation'], pickle.load(f))
+        obj = pickle.load(f)
+        assert isinstance(obj, list)
+        for i in obj:
+            assert isinstance(i, TestSerialisation)
     return obj
 
 
 class SingleTestRunner:
 
-    def __init__(self, test: 'TestSerialisation', test_env: T.Dict[str, str],
+    def __init__(self, test: TestSerialisation, test_env: T.Dict[str, str],
                  env: T.Dict[str, str], options: argparse.Namespace):
         self.test = test
         self.test_env = test_env
@@ -605,9 +608,9 @@ class SingleTestRunner:
 
     def _get_cmd(self) -> T.Optional[T.List[str]]:
         if self.test.fname[0].endswith('.jar'):
-            return ['java', '-jar'] + T.cast(T.List[str], self.test.fname)
+            return ['java', '-jar'] + self.test.fname
         elif not self.test.is_cross_built and run_with_mono(self.test.fname[0]):
-            return ['mono'] + T.cast(T.List[str], self.test.fname)
+            return ['mono'] + self.test.fname
         elif self.test.cmd_is_built and self.test.needs_exe_wrapper:
             if self.test.exe_runner is None:
                 # Can not run test on cross compiled executable
@@ -620,8 +623,8 @@ class SingleTestRunner:
                     msg = ('The exe_wrapper defined in the cross file {!r} was not '
                            'found. Please check the command and/or add it to PATH.')
                     raise TestException(msg.format(self.test.exe_runner.name))
-                return T.cast(T.List[str], self.test.exe_runner.get_command()) + T.cast(T.List[str], self.test.fname)
-        return T.cast(T.List[str], self.test.fname)
+                return self.test.exe_runner.get_command() + self.test.fname
+        return self.test.fname
 
     def run(self) -> TestRun:
         cmd = self._get_cmd()
@@ -833,7 +836,7 @@ class TestHarness:
                 lfile.close()
                 setattr(self, f, None)
 
-    def merge_suite_options(self, options: argparse.Namespace, test: 'TestSerialisation') -> T.Dict[str, str]:
+    def merge_suite_options(self, options: argparse.Namespace, test: TestSerialisation) -> T.Dict[str, str]:
         if ':' in options.setup:
             if options.setup not in self.build_data.test_setups:
                 sys.exit("Unknown test setup '{}'.".format(options.setup))
@@ -855,9 +858,9 @@ class TestHarness:
             sys.exit('Conflict: both test setup and command line specify an exe wrapper.')
         if options.wrapper is None:
             options.wrapper = current.exe_wrapper
-        return T.cast(T.Dict[str, str], current.env.get_env(os.environ.copy()))
+        return current.env.get_env(os.environ.copy())
 
-    def get_test_runner(self, test: 'TestSerialisation') -> SingleTestRunner:
+    def get_test_runner(self, test: TestSerialisation) -> SingleTestRunner:
         options = deepcopy(self.options)
         if not options.setup:
             options.setup = self.build_data.test_setup_default_name
@@ -889,7 +892,7 @@ class TestHarness:
             sys.exit('Unknown test result encountered: {}'.format(result.res))
 
     def print_stats(self, test_count: int, name_max_len: int,
-                    tests: T.List['TestSerialisation'],
+                    tests: T.List[TestSerialisation],
                     name: str, result: TestRun, i: int) -> None:
         ok_statuses = (TestResult.OK, TestResult.EXPECTEDFAIL)
         bad_statuses = (TestResult.FAIL, TestResult.TIMEOUT,
@@ -983,14 +986,14 @@ class TestHarness:
     @staticmethod
     def split_suite_string(suite: str) -> T.Tuple[str, str]:
         if ':' in suite:
-            # mypy can't figure out that str.split(n, 1) will return a list of
-            # length 2, so we have to help it.
-            return T.cast(T.Tuple[str, str], tuple(suite.split(':', 1)))
+            split = suite.split(':', 1)
+            assert len(split) == 2
+            return split[0], split[1]
         else:
             return suite, ""
 
     @staticmethod
-    def test_in_suites(test: 'TestSerialisation', suites: T.List[str]) -> bool:
+    def test_in_suites(test: TestSerialisation, suites: T.List[str]) -> bool:
         for suite in suites:
             (prj_match, st_match) = TestHarness.split_suite_string(suite)
             for prjst in test.suite:
@@ -1021,12 +1024,12 @@ class TestHarness:
                 return True
         return False
 
-    def test_suitable(self, test: 'TestSerialisation') -> bool:
+    def test_suitable(self, test: TestSerialisation) -> bool:
         return ((not self.options.include_suites or
                 TestHarness.test_in_suites(test, self.options.include_suites)) and not
                 TestHarness.test_in_suites(test, self.options.exclude_suites))
 
-    def get_tests(self) -> T.List['TestSerialisation']:
+    def get_tests(self) -> T.List[TestSerialisation]:
         if not self.tests:
             print('No tests defined.')
             return []
@@ -1089,17 +1092,17 @@ class TestHarness:
             wrap += options.wrapper
         return wrap
 
-    def get_pretty_suite(self, test: 'TestSerialisation') -> str:
+    def get_pretty_suite(self, test: TestSerialisation) -> str:
         if len(self.suites) > 1 and test.suite:
             rv = TestHarness.split_suite_string(test.suite[0])[0]
             s = "+".join(TestHarness.split_suite_string(s)[1] for s in test.suite)
             if s:
                 rv += ":"
-            return rv + s + " / " + T.cast(str, test.name)
+            return rv + s + " / " + test.name
         else:
-            return T.cast(str, test.name)
+            return test.name
 
-    def run_tests(self, tests: T.List['TestSerialisation']) -> None:
+    def run_tests(self, tests: T.List[TestSerialisation]) -> None:
         executor = None
         futures = []  # type: T.List[T.Tuple[conc.Future[TestRun], int, int, T.List[TestSerialisation], str, int]]
         test_count = len(tests)
@@ -1141,7 +1144,7 @@ class TestHarness:
         finally:
             os.chdir(startdir)
 
-    def drain_futures(self, futures: T.List[T.Tuple['conc.Future[TestRun]', int, int, T.List['TestSerialisation'], str, int]]) -> None:
+    def drain_futures(self, futures: T.List[T.Tuple['conc.Future[TestRun]', int, int, T.List[TestSerialisation], str, int]]) -> None:
         for x in futures:
             (result, test_count, name_max_len, tests, name, i) = x
             if self.options.repeat > 1 and self.fail_count:
diff --git a/mesonbuild/optinterpreter.py b/mesonbuild/optinterpreter.py
index a6fd503..75011e7 100644
--- a/mesonbuild/optinterpreter.py
+++ b/mesonbuild/optinterpreter.py
@@ -62,7 +62,6 @@ optname_regex = re.compile('[^a-zA-Z0-9_-]')
 def StringParser(description, kwargs):
     return coredata.UserStringOption(description,
                                      kwargs.get('value', ''),
-                                     kwargs.get('choices', []),
                                      kwargs.get('yield', coredata.default_yielding))
 
 @permitted_kwargs({'value', 'yield'})
@@ -134,11 +133,11 @@ option_types = {'string': StringParser,
                 } # type: T.Dict[str, T.Callable[[str, T.Dict], coredata.UserOption]]
 
 class OptionInterpreter:
-    def __init__(self, subproject):
+    def __init__(self, subproject: str) -> None:
         self.options = {}
         self.subproject = subproject
 
-    def process(self, option_file):
+    def process(self, option_file: str) -> None:
         try:
             with open(option_file, 'r', encoding='utf8') as f:
                 ast = mparser.Parser(f.read(), option_file).parse()
@@ -159,7 +158,7 @@ class OptionInterpreter:
                 e.file = option_file
                 raise e
 
-    def reduce_single(self, arg):
+    def reduce_single(self, arg: T.Union[str, mparser.BaseNode]) -> T.Union[str, int, bool]:
         if isinstance(arg, str):
             return arg
         elif isinstance(arg, (mparser.StringNode, mparser.BooleanNode,
@@ -189,7 +188,7 @@ class OptionInterpreter:
         else:
             raise OptionException('Arguments may only be string, int, bool, or array of those.')
 
-    def reduce_arguments(self, args):
+    def reduce_arguments(self, args: mparser.ArgumentNode) -> T.Tuple[T.List[T.Union[str, int, bool]], T.Dict[str, T.Union[str, int, bool]]]:
         assert(isinstance(args, mparser.ArgumentNode))
         if args.incorrect_order():
             raise OptionException('All keyword arguments must be after positional arguments.')
@@ -202,7 +201,7 @@ class OptionInterpreter:
             reduced_kw[key.value] = self.reduce_single(a)
         return reduced_pos, reduced_kw
 
-    def evaluate_statement(self, node):
+    def evaluate_statement(self, node: mparser.BaseNode) -> None:
         if not isinstance(node, mparser.FunctionNode):
             raise OptionException('Option file may only contain option definitions')
         func_name = node.func_name
diff --git a/mesonbuild/scripts/depfixer.py b/mesonbuild/scripts/depfixer.py
index a64caca..76cf2b5 100644
--- a/mesonbuild/scripts/depfixer.py
+++ b/mesonbuild/scripts/depfixer.py
@@ -311,7 +311,7 @@ class Elf(DataSizes):
         old_rpath = self.read_str()
         # Some rpath entries may come from multiple sources.
         # Only add each one once.
-        new_rpaths = OrderedSet()
+        new_rpaths = OrderedSet()  # type: OrderedSet[bytes]
         if new_rpath:
             new_rpaths.add(new_rpath)
         if old_rpath:
diff --git a/mesonbuild/scripts/meson_exe.py b/mesonbuild/scripts/meson_exe.py
index 68ba120..df54b47 100644
--- a/mesonbuild/scripts/meson_exe.py
+++ b/mesonbuild/scripts/meson_exe.py
@@ -57,7 +57,7 @@ def run_exe(exe: ExecutableSerialisation) -> int:
 
     if p.returncode == 0xc0000135:
         # STATUS_DLL_NOT_FOUND on Windows indicating a common problem that is otherwise hard to diagnose
-        raise FileNotFoundError('Missing DLLs on calling {!r}'.format(exe.name))
+        raise FileNotFoundError('Missing DLLs on calling {!r}'.format(cmd_args))
 
     if exe.capture and p.returncode == 0:
         skip_write = False
diff --git a/run_mypy.py b/run_mypy.py
index 759d667..d5ec55d 100755
--- a/run_mypy.py
+++ b/run_mypy.py
@@ -15,6 +15,7 @@ modules = [
   # specific files
   'mesonbuild/arglist.py',
   # 'mesonbuild/compilers/mixins/intel.py',
+  # 'mesonbuild/coredata.py',
   'mesonbuild/dependencies/boost.py',
   'mesonbuild/dependencies/hdf5.py',
   'mesonbuild/dependencies/mpi.py',