Merge pull request #6736 from dcbaker/mesonlib-type-annotations

Mesonlib type annotations

20 files changed, 355 insertions, 325 deletions

diff --git a/.github/workflows/lint_mypy.yml b/.github/workflows/lint_mypy.yml
index 7c83714..c826729 100644
--- a/.github/workflows/lint_mypy.yml
+++ b/.github/workflows/lint_mypy.yml
@@ -30,4 +30,4 @@ jobs:
       with:
         python-version: '3.x'
     - run: python -m pip install mypy
-    - run: mypy --follow-imports=skip mesonbuild/interpreterbase.py mesonbuild/mtest.py mesonbuild/minit.py mesonbuild/mintro.py mesonbuild/mparser.py mesonbuild/msetup.py mesonbuild/ast mesonbuild/wrap tools/ mesonbuild/modules/fs.py mesonbuild/dependencies/boost.py mesonbuild/dependencies/mpi.py mesonbuild/dependencies/hdf5.py mesonbuild/compilers/mixins/intel.py mesonbuild/mlog.py mesonbuild/mcompile.py
+    - run: mypy --follow-imports=skip mesonbuild/interpreterbase.py mesonbuild/mtest.py mesonbuild/minit.py mesonbuild/mintro.py mesonbuild/mparser.py mesonbuild/msetup.py mesonbuild/ast mesonbuild/wrap tools/ mesonbuild/modules/fs.py mesonbuild/dependencies/boost.py mesonbuild/dependencies/mpi.py mesonbuild/dependencies/hdf5.py mesonbuild/compilers/mixins/intel.py mesonbuild/mlog.py mesonbuild/mcompile.py mesonbuild/mesonlib.py
diff --git a/mesonbuild/ast/introspection.py b/mesonbuild/ast/introspection.py
index c9a51bf..142c219 100644
--- a/mesonbuild/ast/introspection.py
+++ b/mesonbuild/ast/introspection.py
@@ -101,8 +101,8 @@ class IntrospectionInterpreter(AstInterpreter):
             self.coredata.merge_user_options(oi.options)
 
         def_opts = self.flatten_args(kwargs.get('default_options', []))
-        self.project_default_options = mesonlib.stringlistify(def_opts)
-        self.project_default_options = cdata.create_options_dict(self.project_default_options)
+        _project_default_options = mesonlib.stringlistify(def_opts)
+        self.project_default_options = cdata.create_options_dict(_project_default_options)
         self.default_options.update(self.project_default_options)
         self.coredata.set_default_options(self.default_options, self.subproject, self.environment)
 
diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index a8f4789..e9ab9f4 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -21,7 +21,7 @@ from .. import mlog
 import json
 import subprocess
 from ..mesonlib import MachineChoice, MesonException, OrderedSet, OptionOverrideProxy
-from ..mesonlib import classify_unity_sources
+from ..mesonlib import classify_unity_sources, unholder
 from ..mesonlib import File
 from ..compilers import CompilerArgs, VisualStudioLikeCompiler
 from ..interpreter import Interpreter
@@ -748,9 +748,7 @@ class Backend:
             else:
                 extra_paths = []
             cmd_args = []
-            for a in t.cmd_args:
-                if hasattr(a, 'held_object'):
-                    a = a.held_object
+            for a in unholder(t.cmd_args):
                 if isinstance(a, build.BuildTarget):
                     extra_paths += self.determine_windows_extra_paths(a, [])
                 if isinstance(a, mesonlib.File):
@@ -868,14 +866,10 @@ class Backend:
         # also be built by default. XXX: Sometime in the future these should be
         # built only before running tests.
         for t in self.build.get_tests():
-            exe = t.exe
-            if hasattr(exe, 'held_object'):
-                exe = exe.held_object
+            exe = unholder(t.exe)
             if isinstance(exe, (build.CustomTarget, build.BuildTarget)):
                 result[exe.get_id()] = exe
-            for arg in t.cmd_args:
-                if hasattr(arg, 'held_object'):
-                    arg = arg.held_object
+            for arg in unholder(t.cmd_args):
                 if not isinstance(arg, (build.CustomTarget, build.BuildTarget)):
                     continue
                 result[arg.get_id()] = arg
@@ -915,9 +909,7 @@ class Backend:
         Returns the path to them relative to the build root directory.
         '''
         srcs = []
-        for i in target.get_sources():
-            if hasattr(i, 'held_object'):
-                i = i.held_object
+        for i in unholder(target.get_sources()):
             if isinstance(i, str):
                 fname = [os.path.join(self.build_to_src, target.subdir, i)]
             elif isinstance(i, build.BuildTarget):
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 24c91f3..c80d832 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -32,7 +32,8 @@ from ..compilers import (Compiler, CompilerArgs, CCompiler, FortranCompiler,
                          PGICCompiler, VisualStudioLikeCompiler)
 from ..linkers import ArLinker
 from ..mesonlib import (
-    File, LibType, MachineChoice, MesonException, OrderedSet, PerMachine, ProgressBar, quote_arg
+    File, LibType, MachineChoice, MesonException, OrderedSet, PerMachine,
+    ProgressBar, quote_arg, unholder,
 )
 from ..mesonlib import get_compiler_for_source, has_path_sep
 from .backends import CleanTrees
@@ -648,9 +649,7 @@ int dummy;
                 self.generate_target(t)
 
     def custom_target_generator_inputs(self, target):
-        for s in target.sources:
-            if hasattr(s, 'held_object'):
-                s = s.held_object
+        for s in unholder(target.sources):
             if isinstance(s, build.GeneratedList):
                 self.generate_genlist_for_target(s, target)
 
diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 2b5c0ea..6755dca 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -26,7 +26,7 @@ from . import mlog
 from .mesonlib import (
     File, MesonException, MachineChoice, PerMachine, OrderedSet, listify,
     extract_as_list, typeslistify, stringlistify, classify_unity_sources,
-    get_filenames_templates_dict, substitute_values, has_path_sep,
+    get_filenames_templates_dict, substitute_values, has_path_sep, unholder
 )
 from .compilers import Compiler, is_object, clink_langs, sort_clink, lang_suffixes
 from .linkers import StaticLinker
@@ -541,9 +541,7 @@ class BuildTarget(Target):
 
     def process_objectlist(self, objects):
         assert(isinstance(objects, list))
-        for s in objects:
-            if hasattr(s, 'held_object'):
-                s = s.held_object
+        for s in unholder(objects):
             if isinstance(s, (str, File, ExtractedObjects)):
                 self.objects.append(s)
             elif isinstance(s, (GeneratedList, CustomTarget)):
@@ -559,10 +557,7 @@ class BuildTarget(Target):
     def process_sourcelist(self, sources):
         sources = listify(sources)
         added_sources = {} # If the same source is defined multiple times, use it only once.
-        for s in sources:
-            # Holder unpacking. Ugly.
-            if hasattr(s, 'held_object'):
-                s = s.held_object
+        for s in unholder(sources):
             if isinstance(s, File):
                 if s not in added_sources:
                     self.sources.append(s)
@@ -639,9 +634,7 @@ class BuildTarget(Target):
                 # which is what we need.
                 if not is_object(s):
                     sources.append(s)
-        for d in self.external_deps:
-            if hasattr(d, 'held_object'):
-                d = d.held_object
+        for d in unholder(self.external_deps):
             for s in d.sources:
                 if isinstance(s, (str, File)):
                     sources.append(s)
@@ -703,10 +696,7 @@ class BuildTarget(Target):
         link_depends.
         """
         sources = listify(sources)
-        for s in sources:
-            if hasattr(s, 'held_object'):
-                s = s.held_object
-
+        for s in unholder(sources):
             if isinstance(s, File):
                 self.link_depends.append(s)
             elif isinstance(s, str):
@@ -817,11 +807,7 @@ class BuildTarget(Target):
         kwargs.get('modules', [])
         self.need_install = kwargs.get('install', self.need_install)
         llist = extract_as_list(kwargs, 'link_with')
-        for linktarget in llist:
-            # Sorry for this hack. Keyword targets are kept in holders
-            # in kwargs. Unpack here without looking at the exact type.
-            if hasattr(linktarget, "held_object"):
-                linktarget = linktarget.held_object
+        for linktarget in unholder(llist):
             if isinstance(linktarget, dependencies.ExternalLibrary):
                 raise MesonException('''An external library was used in link_with keyword argument, which
 is reserved for libraries built as part of this project. External
@@ -834,8 +820,7 @@ just like those detected with the dependency() function.''')
             self.link_whole(linktarget)
 
         c_pchlist, cpp_pchlist, clist, cpplist, cudalist, cslist, valalist,  objclist, objcpplist, fortranlist, rustlist \
-            = extract_as_list(kwargs, 'c_pch', 'cpp_pch', 'c_args', 'cpp_args', 'cuda_args', 'cs_args', 'vala_args', 'objc_args',
-                              'objcpp_args', 'fortran_args', 'rust_args')
+            = [extract_as_list(kwargs, c) for c in ['c_pch', 'cpp_pch', 'c_args', 'cpp_args', 'cuda_args', 'cs_args', 'vala_args', 'objc_args', 'objcpp_args', 'fortran_args', 'rust_args']]
 
         self.add_pch('c', c_pchlist)
         self.add_pch('cpp', cpp_pchlist)
@@ -863,7 +848,7 @@ just like those detected with the dependency() function.''')
         if dfeature_debug:
             dfeatures['debug'] = dfeature_debug
         if 'd_import_dirs' in kwargs:
-            dfeature_import_dirs = extract_as_list(kwargs, 'd_import_dirs', unholder=True)
+            dfeature_import_dirs = unholder(extract_as_list(kwargs, 'd_import_dirs'))
             for d in dfeature_import_dirs:
                 if not isinstance(d, IncludeDirs):
                     raise InvalidArguments('Arguments to d_import_dirs must be include_directories.')
@@ -1041,9 +1026,7 @@ This will become a hard error in a future Meson release.''')
 
     def add_deps(self, deps):
         deps = listify(deps)
-        for dep in deps:
-            if hasattr(dep, 'held_object'):
-                dep = dep.held_object
+        for dep in unholder(deps):
             if isinstance(dep, dependencies.InternalDependency):
                 # Those parts that are internal.
                 self.process_sourcelist(dep.sources)
@@ -1090,7 +1073,7 @@ You probably should put it in link_with instead.''')
         return isinstance(self, StaticLibrary) and not self.need_install
 
     def link(self, target):
-        for t in listify(target, unholder=True):
+        for t in unholder(listify(target)):
             if isinstance(self, StaticLibrary) and self.need_install and t.is_internal():
                 # When we're a static library and we link_with to an
                 # internal/convenience library, promote to link_whole.
@@ -1112,7 +1095,7 @@ You probably should put it in link_with instead.''')
             self.link_targets.append(t)
 
     def link_whole(self, target):
-        for t in listify(target, unholder=True):
+        for t in unholder(listify(target)):
             if isinstance(t, (CustomTarget, CustomTargetIndex)):
                 if not t.is_linkable_target():
                     raise InvalidArguments('Custom target {!r} is not linkable.'.format(t))
@@ -1178,10 +1161,7 @@ You probably should put it in link_with instead.''')
 
     def add_include_dirs(self, args, set_is_system: T.Optional[str] = None):
         ids = []
-        for a in args:
-            # FIXME same hack, forcibly unpack from holder.
-            if hasattr(a, 'held_object'):
-                a = a.held_object
+        for a in unholder(args):
             if not isinstance(a, IncludeDirs):
                 raise InvalidArguments('Include directory to be added is not an include directory object.')
             ids.append(a)
@@ -1268,7 +1248,7 @@ You probably should put it in link_with instead.''')
                     if dl != linker.language:
                         stdlib_args += all_compilers[dl].language_stdlib_only_link_flags()
                         added_languages.add(dl)
-                # Type of var 'linker' is Compiler. 
+                # Type of var 'linker' is Compiler.
                 # Pretty hard to fix because the return value is passed everywhere
                 return linker, stdlib_args
 
@@ -1321,9 +1301,7 @@ class Generator:
     def __init__(self, args, kwargs):
         if len(args) != 1:
             raise InvalidArguments('Generator requires exactly one positional argument: the executable')
-        exe = args[0]
-        if hasattr(exe, 'held_object'):
-            exe = exe.held_object
+        exe = unholder(args[0])
         if not isinstance(exe, (Executable, dependencies.ExternalProgram)):
             raise InvalidArguments('First generator argument must be an executable.')
         self.exe = exe
@@ -1379,7 +1357,7 @@ class Generator:
                 raise InvalidArguments('Capture must be boolean.')
             self.capture = capture
         if 'depends' in kwargs:
-            depends = listify(kwargs['depends'], unholder=True)
+            depends = unholder(listify(kwargs['depends']))
             for d in depends:
                 if not isinstance(d, BuildTarget):
                     raise InvalidArguments('Depends entries must be build targets.')
@@ -1424,9 +1402,7 @@ class Generator:
 
 class GeneratedList:
     def __init__(self, generator, subdir, preserve_path_from=None, extra_args=None):
-        if hasattr(generator, 'held_object'):
-            generator = generator.held_object
-        self.generator = generator
+        self.generator = unholder(generator)
         self.name = self.generator.exe
         self.subdir = subdir
         self.infilelist = []
@@ -1436,10 +1412,10 @@ class GeneratedList:
         self.depend_files = []
         self.preserve_path_from = preserve_path_from
         self.extra_args = extra_args if extra_args is not None else []
-        if isinstance(generator.exe, dependencies.ExternalProgram):
-            if not generator.exe.found():
+        if isinstance(self.generator.exe, dependencies.ExternalProgram):
+            if not self.generator.exe.found():
                 raise InvalidArguments('Tried to use not-found external program as generator')
-            path = generator.exe.get_path()
+            path = self.generator.exe.get_path()
             if os.path.isabs(path):
                 # Can only add a dependency on an external program which we
                 # know the absolute path of
@@ -1881,9 +1857,7 @@ class SharedLibrary(BuildTarget):
 
         # Visual Studio module-definitions file
         if 'vs_module_defs' in kwargs:
-            path = kwargs['vs_module_defs']
-            if hasattr(path, 'held_object'):
-                path = path.held_object
+            path = unholder(kwargs['vs_module_defs'])
             if isinstance(path, str):
                 if os.path.isabs(path):
                     self.vs_module_defs = File.from_absolute_file(path)
@@ -2035,9 +2009,7 @@ class CustomTarget(Target):
     def get_target_dependencies(self):
         deps = self.dependencies[:]
         deps += self.extra_depends
-        for c in self.sources:
-            if hasattr(c, 'held_object'):
-                c = c.held_object
+        for c in unholder(self.sources):
             if isinstance(c, (BuildTarget, CustomTarget)):
                 deps.append(c)
         return deps
@@ -2061,7 +2033,7 @@ class CustomTarget(Target):
         return bdeps
 
     def flatten_command(self, cmd):
-        cmd = listify(cmd, unholder=True)
+        cmd = unholder(listify(cmd))
         final_cmd = []
         for c in cmd:
             if isinstance(c, str):
@@ -2089,7 +2061,7 @@ class CustomTarget(Target):
 
     def process_kwargs(self, kwargs, backend):
         self.process_kwargs_base(kwargs)
-        self.sources = extract_as_list(kwargs, 'input', unholder=True)
+        self.sources = unholder(extract_as_list(kwargs, 'input'))
         if 'output' not in kwargs:
             raise InvalidArguments('Missing keyword argument "output".')
         self.outputs = listify(kwargs['output'])
@@ -2168,10 +2140,8 @@ class CustomTarget(Target):
             self.build_always_stale = kwargs['build_always_stale']
         if not isinstance(self.build_always_stale, bool):
             raise InvalidArguments('Argument build_always_stale must be a boolean.')
-        extra_deps, depend_files = extract_as_list(kwargs, 'depends', 'depend_files', pop = False)
-        for ed in extra_deps:
-            while hasattr(ed, 'held_object'):
-                ed = ed.held_object
+        extra_deps, depend_files = [extract_as_list(kwargs, c, pop=False) for c in ['depends', 'depend_files']]
+        for ed in unholder(extra_deps):
             if not isinstance(ed, (CustomTarget, BuildTarget)):
                 raise InvalidArguments('Can only depend on toplevel targets: custom_target or build_target (executable or a library) got: {}({})'
                                       .format(type(ed), ed))
@@ -2206,9 +2176,7 @@ class CustomTarget(Target):
 
     def get_generated_lists(self):
         genlists = []
-        for c in self.sources:
-            if hasattr(c, 'held_object'):
-                c = c.held_object
+        for c in unholder(self.sources):
             if isinstance(c, GeneratedList):
                 genlists.append(c)
         return genlists
@@ -2416,20 +2384,20 @@ class ConfigureFile:
         return self.targetname
 
 class ConfigurationData:
-    def __init__(self):
+    def __init__(self) -> None:
         super().__init__()
-        self.values = {}
+        self.values = {}  # T.Dict[str, T.Union[str, int, bool]]
 
     def __repr__(self):
         return repr(self.values)
 
-    def __contains__(self, value):
+    def __contains__(self, value: str) -> bool:
         return value in self.values
 
-    def get(self, name):
+    def get(self, name: str) -> T.Tuple[T.Union[str, int, bool], T.Optional[str]]:
         return self.values[name] # (val, desc)
 
-    def keys(self):
+    def keys(self) -> T.Iterator[str]:
         return self.values.keys()
 
 # A bit poorly named, but this represents plain data files to copy
@@ -2471,9 +2439,7 @@ def get_sources_string_names(sources, backend):
     get all the output basenames.
     '''
     names = []
-    for s in sources:
-        if hasattr(s, 'held_object'):
-            s = s.held_object
+    for s in unholder(sources):
         if isinstance(s, str):
             names.append(s)
         elif isinstance(s, (BuildTarget, CustomTarget, CustomTargetIndex, GeneratedList)):
diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 1c0adff..caa8600 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -35,6 +35,8 @@ if T.TYPE_CHECKING:
     from ..environment import Environment
     from ..linkers import DynamicLinker  # noqa: F401
 
+    CompilerType = T.TypeVar('CompilerType', bound=Compiler)
+
 """This file contains the data files of all compilers Meson knows
 about. To support a new compiler, add its information below.
 Also add corresponding autodetection code in environment.py."""
diff --git a/mesonbuild/dependencies/base.py b/mesonbuild/dependencies/base.py
index 6f8181d..5f212e6 100644
--- a/mesonbuild/dependencies/base.py
+++ b/mesonbuild/dependencies/base.py
@@ -38,7 +38,7 @@ from ..mesonlib import Popen_safe, version_compare_many, version_compare, listif
 from ..mesonlib import Version, LibType
 
 if T.TYPE_CHECKING:
-    from ..compilers.compilers import Compiler  # noqa: F401
+    from ..compilers.compilers import CompilerType  # noqa: F401
     DependencyType = T.TypeVar('DependencyType', bound='Dependency')
 
 # These must be defined in this file to avoid cyclical references.
@@ -2488,7 +2488,7 @@ def factory_methods(methods: T.Set[DependencyMethods]) -> 'FactoryType':
 
 
 def detect_compiler(name: str, env: Environment, for_machine: MachineChoice,
-                    language: T.Optional[str]) -> T.Optional['Compiler']:
+                    language: T.Optional[str]) -> T.Optional['CompilerType']:
     """Given a language and environment find the compiler used."""
     compilers = env.coredata.compilers[for_machine]
 
diff --git a/mesonbuild/dependencies/boost.py b/mesonbuild/dependencies/boost.py
index 53a0cfb..d083672 100644
--- a/mesonbuild/dependencies/boost.py
+++ b/mesonbuild/dependencies/boost.py
@@ -272,7 +272,7 @@ class BoostDependency(ExternalDependency):
         self.boost_root = None
 
         # Extract and validate modules
-        self.modules = mesonlib.extract_as_list(kwargs, 'modules')
+        self.modules = mesonlib.extract_as_list(kwargs, 'modules')  # type: T.List[str]
         for i in self.modules:
             if not isinstance(i, str):
                 raise DependencyException('Boost module argument is not a string.')
@@ -299,9 +299,9 @@ class BoostDependency(ExternalDependency):
             mlog.debug('  - BOOST_LIBRARYDIR = {}'.format(lib_dir))
 
             boost_inc_dir = None
-            for i in [inc_dir / 'version.hpp', inc_dir / 'boost' / 'version.hpp']:
-                if i.is_file():
-                    boost_inc_dir = self._include_dir_from_version_header(i)
+            for j in [inc_dir / 'version.hpp', inc_dir / 'boost' / 'version.hpp']:
+                if j.is_file():
+                    boost_inc_dir = self._include_dir_from_version_header(j)
                     break
             if not boost_inc_dir:
                 self.is_found = False
@@ -317,20 +317,20 @@ class BoostDependency(ExternalDependency):
         roots = list(mesonlib.OrderedSet(roots))
 
         # B) Foreach candidate
-        for i in roots:
+        for j in roots:
             #   1. Look for the boost headers (boost/version.pp)
-            mlog.debug('Checking potential boost root {}'.format(i.as_posix()))
-            inc_dirs = self.detect_inc_dirs(i)
+            mlog.debug('Checking potential boost root {}'.format(j.as_posix()))
+            inc_dirs = self.detect_inc_dirs(j)
             inc_dirs = sorted(inc_dirs, reverse=True)  # Prefer the newer versions
 
             # Early abort when boost is not found
             if not inc_dirs:
                 continue
 
-            lib_dirs = self.detect_lib_dirs(i)
+            lib_dirs = self.detect_lib_dirs(j)
             self.is_found = self.run_check(inc_dirs, lib_dirs)
             if self.is_found:
-                self.boost_root = i
+                self.boost_root = j
                 break
 
     def run_check(self, inc_dirs: T.List[BoostIncludeDir], lib_dirs: T.List[Path]) -> bool:
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 77d64c4..e8284bb 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -21,7 +21,7 @@ from . import optinterpreter
 from . import compilers
 from .wrap import wrap, WrapMode
 from . import mesonlib
-from .mesonlib import FileMode, MachineChoice, Popen_safe, listify, extract_as_list, has_path_sep
+from .mesonlib import FileMode, MachineChoice, Popen_safe, listify, extract_as_list, has_path_sep, unholder
 from .dependencies import ExternalProgram
 from .dependencies import InternalDependency, Dependency, NotFoundDependency, DependencyException
 from .depfile import DepFile
@@ -2484,11 +2484,11 @@ class Interpreter(InterpreterBase):
         if not isinstance(version, str):
             raise InterpreterException('Version must be a string.')
         incs = self.extract_incdirs(kwargs)
-        libs = extract_as_list(kwargs, 'link_with', unholder=True)
-        libs_whole = extract_as_list(kwargs, 'link_whole', unholder=True)
+        libs = unholder(extract_as_list(kwargs, 'link_with'))
+        libs_whole = unholder(extract_as_list(kwargs, 'link_whole'))
         sources = extract_as_list(kwargs, 'sources')
-        sources = listify(self.source_strings_to_files(sources), unholder=True)
-        deps = extract_as_list(kwargs, 'dependencies', unholder=True)
+        sources = unholder(listify(self.source_strings_to_files(sources)))
+        deps = unholder(extract_as_list(kwargs, 'dependencies'))
         compile_args = mesonlib.stringlistify(kwargs.get('compile_args', []))
         link_args = mesonlib.stringlistify(kwargs.get('link_args', []))
         variables = kwargs.get('variables', {})
@@ -3111,9 +3111,7 @@ external dependencies (including libraries) must go to "dependencies".''')
 
     def program_from_file_for(self, for_machine, prognames, silent):
         bins = self.environment.binaries[for_machine]
-        for p in prognames:
-            if hasattr(p, 'held_object'):
-                p = p.held_object
+        for p in unholder(prognames):
             if isinstance(p, mesonlib.File):
                 continue # Always points to a local (i.e. self generated) file.
             if not isinstance(p, str):
@@ -3655,12 +3653,12 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
             if 'command' not in kwargs:
                 raise InterpreterException('Missing "command" keyword argument')
             all_args = extract_as_list(kwargs, 'command')
-            deps = extract_as_list(kwargs, 'depends', unholder=True)
+            deps = unholder(extract_as_list(kwargs, 'depends'))
         else:
             raise InterpreterException('Run_target needs at least one positional argument.')
 
         cleaned_args = []
-        for i in listify(all_args, unholder=True):
+        for i in unholder(listify(all_args)):
             if not isinstance(i, (str, build.BuildTarget, build.CustomTarget, dependencies.ExternalProgram, mesonlib.File)):
                 mlog.debug('Wrong type:', str(i))
                 raise InterpreterException('Invalid argument to run_target.')
@@ -3691,7 +3689,7 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
         name = args[0]
         if not isinstance(name, str):
             raise InterpreterException('First argument must be a string.')
-        deps = listify(args[1:], unholder=True)
+        deps = unholder(listify(args[1:]))
         for d in deps:
             if not isinstance(d, (build.BuildTarget, build.CustomTarget)):
                 raise InterpreterException('Depends items must be build targets.')
@@ -3749,7 +3747,7 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
         par = kwargs.get('is_parallel', True)
         if not isinstance(par, bool):
             raise InterpreterException('Keyword argument is_parallel must be a boolean.')
-        cmd_args = extract_as_list(kwargs, 'args', unholder=True)
+        cmd_args = unholder(extract_as_list(kwargs, 'args'))
         for i in cmd_args:
             if not isinstance(i, (str, mesonlib.File, build.Target)):
                 raise InterpreterException('Command line arguments must be strings, files or targets.')
@@ -3777,7 +3775,7 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
             if len(s) > 0:
                 s = ':' + s
             suite.append(prj.replace(' ', '_').replace(':', '_') + s)
-        depends = extract_as_list(kwargs, 'depends', unholder=True)
+        depends = unholder(extract_as_list(kwargs, 'depends'))
         for dep in depends:
             if not isinstance(dep, (build.CustomTarget, build.BuildTarget)):
                 raise InterpreterException('Depends items must be build targets.')
@@ -4140,7 +4138,7 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
         return mesonlib.File.from_built_file(self.subdir, output)
 
     def extract_incdirs(self, kwargs):
-        prospectives = listify(kwargs.get('include_directories', []), unholder=True)
+        prospectives = unholder(extract_as_list(kwargs, 'include_directories'))
         result = []
         for p in prospectives:
             if isinstance(p, build.IncludeDirs):
@@ -4201,7 +4199,7 @@ different subdirectory.
         if ":" not in setup_name:
             setup_name = (self.subproject if self.subproject else self.build.project_name) + ":" + setup_name
         try:
-            inp = extract_as_list(kwargs, 'exe_wrapper', unholder=True)
+            inp = unholder(extract_as_list(kwargs, 'exe_wrapper'))
             exe_wrapper = []
             for i in inp:
                 if isinstance(i, str):
diff --git a/mesonbuild/interpreterbase.py b/mesonbuild/interpreterbase.py
index 8071136..b8008b0 100644
--- a/mesonbuild/interpreterbase.py
+++ b/mesonbuild/interpreterbase.py
@@ -446,7 +446,7 @@ class InterpreterBase:
                 self.current_lineno = cur.lineno
                 self.evaluate_statement(cur)
             except Exception as e:
-                if not hasattr(e, 'lineno'):
+                if getattr(e, 'lineno') 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
diff --git a/mesonbuild/mesonlib.py b/mesonbuild/mesonlib.py
index 2298c86..e215dcd 100644
--- a/mesonbuild/mesonlib.py
+++ b/mesonbuild/mesonlib.py
@@ -1,4 +1,4 @@
-# Copyright 2012-2019 The Meson development team
+# Copyright 2012-2020 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.
@@ -20,20 +20,27 @@ import time
 import platform, subprocess, operator, os, shlex, shutil, re
 import collections
 from enum import Enum
-from functools import lru_cache, update_wrapper
+from functools import lru_cache, wraps
 from itertools import tee, filterfalse
 import typing as T
 import uuid
+import textwrap
 
 from mesonbuild import mlog
 
+if T.TYPE_CHECKING:
+    from .build import ConfigurationData
+    from .coredata import OptionDictType, UserOption
+    from .compilers.compilers import CompilerType
+    from .interpreterbase import ObjectHolder
+
 _T = T.TypeVar('_T')
 _U = T.TypeVar('_U')
 
 have_fcntl = False
 have_msvcrt = False
 # {subproject: project_meson_version}
-project_meson_versions = {}
+project_meson_versions = {}  # type: T.Dict[str, str]
 
 try:
     import fcntl
@@ -57,7 +64,7 @@ else:
 meson_command = None
 
 GIT = shutil.which('git')
-def git(cmd: T.List[str], workingdir: str, **kwargs) -> subprocess.CompletedProcess:
+def git(cmd: T.List[str], workingdir: str, **kwargs: T.Any) -> subprocess.CompletedProcess:
     pc = subprocess.run([GIT, '-C', workingdir] + cmd,
                         # Redirect stdin to DEVNULL otherwise git messes up the
                         # console and ANSI colors stop working on Windows.
@@ -69,7 +76,7 @@ def git(cmd: T.List[str], workingdir: str, **kwargs) -> subprocess.CompletedProc
     return pc
 
 
-def set_meson_command(mainfile):
+def set_meson_command(mainfile: str) -> None:
     global python_command
     global meson_command
     # On UNIX-like systems `meson` is a Python script
@@ -86,7 +93,8 @@ def set_meson_command(mainfile):
     if 'MESON_COMMAND_TESTS' in os.environ:
         mlog.log('meson_command is {!r}'.format(meson_command))
 
-def is_ascii_string(astring) -> bool:
+
+def is_ascii_string(astring: T.Union[str, bytes]) -> bool:
     try:
         if isinstance(astring, str):
             astring.encode('ascii')
@@ -96,7 +104,8 @@ def is_ascii_string(astring) -> bool:
         return False
     return True
 
-def check_direntry_issues(direntry_array):
+
+def check_direntry_issues(direntry_array: T.Union[T.List[T.Union[str, bytes]], str, bytes]) -> None:
     import locale
     # Warn if the locale is not UTF-8. This can cause various unfixable issues
     # such as os.stat not being able to decode filenames with unicode in them.
@@ -109,22 +118,31 @@ def check_direntry_issues(direntry_array):
         for de in direntry_array:
             if is_ascii_string(de):
                 continue
-            mlog.warning('''You are using {!r} which is not a Unicode-compatible '
-locale but you are trying to access a file system entry called {!r} which is
-not pure ASCII. This may cause problems.
-'''.format(e, de), file=sys.stderr)
+            mlog.warning(textwrap.dedent('''
+                You are using {!r} which is not a Unicode-compatible
+                locale but you are trying to access a file system entry called {!r} which is
+                not pure ASCII. This may cause problems.
+                '''.format(e, de)), file=sys.stderr)
+
 
 # Put this in objects that should not get dumped to pickle files
 # by accident.
 import threading
 an_unpicklable_object = threading.Lock()
 
+
 class MesonException(Exception):
     '''Exceptions thrown by Meson'''
 
+    file = None    # type: T.Optional[str]
+    lineno = None  # type: T.Optional[int]
+    colno = None   # type: T.Optional[int]
+
+
 class EnvironmentException(MesonException):
     '''Exceptions thrown while processing and creating the build environment'''
 
+
 class FileMode:
     # The first triad is for owner permissions, the second for group permissions,
     # and the third for others (everyone else).
@@ -151,18 +169,19 @@ class FileMode:
                                       '[r-][w-][xsS-]' # Group perms
                                       '[r-][w-][xtT-]') # Others perms
 
-    def __init__(self, perms=None, owner=None, group=None):
+    def __init__(self, perms: T.Optional[str] = None, owner: T.Optional[str] = None,
+                 group: T.Optional[str] = None):
         self.perms_s = perms
         self.perms = self.perms_s_to_bits(perms)
         self.owner = owner
         self.group = group
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         ret = '<FileMode: {!r} owner={} group={}'
         return ret.format(self.perms_s, self.owner, self.group)
 
     @classmethod
-    def perms_s_to_bits(cls, perms_s):
+    def perms_s_to_bits(cls, perms_s: T.Optional[str]) -> int:
         '''
         Does the opposite of stat.filemode(), converts strings of the form
         'rwxr-xr-x' to st_mode enums which can be passed to os.chmod()
@@ -221,8 +240,6 @@ class File:
         self.is_built = is_built
         self.subdir = subdir
         self.fname = fname
-        assert(isinstance(self.subdir, str))
-        assert(isinstance(self.fname, str))
 
     def __str__(self) -> str:
         return self.relative_name()
@@ -236,17 +253,17 @@ class File:
 
     @staticmethod
     @lru_cache(maxsize=None)
-    def from_source_file(source_root: str, subdir: str, fname: str):
+    def from_source_file(source_root: str, subdir: str, fname: str) -> 'File':
         if not os.path.isfile(os.path.join(source_root, subdir, fname)):
             raise MesonException('File %s does not exist.' % fname)
         return File(False, subdir, fname)
 
     @staticmethod
-    def from_built_file(subdir: str, fname: str):
+    def from_built_file(subdir: str, fname: str) -> 'File':
         return File(True, subdir, fname)
 
     @staticmethod
-    def from_absolute_file(fname: str):
+    def from_absolute_file(fname: str) -> 'File':
         return File(False, '', fname)
 
     @lru_cache(maxsize=None)
@@ -270,6 +287,8 @@ class File:
         return self.fname.split(s)
 
     def __eq__(self, other) -> bool:
+        if not isinstance(other, File):
+            return NotImplemented
         return (self.fname, self.subdir, self.is_built) == (other.fname, other.subdir, other.is_built)
 
     def __hash__(self) -> int:
@@ -280,14 +299,16 @@ class File:
         return os.path.join(self.subdir, self.fname)
 
 
-def get_compiler_for_source(compilers, src):
+def get_compiler_for_source(compilers: T.Iterable['CompilerType'], src: str) -> 'CompilerType':
+    """Given a set of compilers and a source, find the compiler for that source type."""
     for comp in compilers:
         if comp.can_compile(src):
             return comp
     raise MesonException('No specified compiler can handle file {!s}'.format(src))
 
-def classify_unity_sources(compilers, sources):
-    compsrclist = {}
+
+def classify_unity_sources(compilers: T.Iterable['CompilerType'], sources: T.Iterable[str]) -> T.Dict['CompilerType', T.List[str]]:
+    compsrclist = {}  # type: T.Dict[CompilerType, T.List[str]]
     for src in sources:
         comp = get_compiler_for_source(compilers, src)
         if comp not in compsrclist:
@@ -296,6 +317,7 @@ def classify_unity_sources(compilers, sources):
             compsrclist[comp].append(src)
     return compsrclist
 
+
 class OrderedEnum(Enum):
     """
     An Enum which additionally offers homogeneous ordered comparison.
@@ -330,10 +352,10 @@ class MachineChoice(OrderedEnum):
     BUILD = 0
     HOST = 1
 
-    def get_lower_case_name(self):
+    def get_lower_case_name(self) -> str:
         return PerMachine('build', 'host')[self]
 
-    def get_prefix(self):
+    def get_prefix(self) -> str:
         return PerMachine('build.', '')[self]
 
 
@@ -397,6 +419,7 @@ class PerThreeMachine(PerMachine[_T]):
     def matches_build_machine(self, machine: MachineChoice) -> bool:
         return self.build == self[machine]
 
+
 class PerMachineDefaultable(PerMachine[T.Optional[_T]]):
     """Extends `PerMachine` with the ability to default from `None`s.
     """
@@ -439,40 +462,52 @@ class PerThreeMachineDefaultable(PerMachineDefaultable, PerThreeMachine[T.Option
 def is_sunos() -> bool:
     return platform.system().lower() == 'sunos'
 
+
 def is_osx() -> bool:
     return platform.system().lower() == 'darwin'
 
+
 def is_linux() -> bool:
     return platform.system().lower() == 'linux'
 
+
 def is_android() -> bool:
     return platform.system().lower() == 'android'
 
+
 def is_haiku() -> bool:
     return platform.system().lower() == 'haiku'
 
+
 def is_openbsd() -> bool:
     return platform.system().lower() == 'openbsd'
 
+
 def is_windows() -> bool:
     platname = platform.system().lower()
     return platname == 'windows' or 'mingw' in platname
 
+
 def is_cygwin() -> bool:
     return platform.system().lower().startswith('cygwin')
 
+
 def is_debianlike() -> bool:
     return os.path.isfile('/etc/debian_version')
 
+
 def is_dragonflybsd() -> bool:
     return platform.system().lower() == 'dragonfly'
 
+
 def is_netbsd() -> bool:
     return platform.system().lower() == 'netbsd'
 
+
 def is_freebsd() -> bool:
     return platform.system().lower() == 'freebsd'
 
+
 def exe_exists(arglist: T.List[str]) -> bool:
     try:
         if subprocess.run(arglist, timeout=10).returncode == 0:
@@ -481,8 +516,9 @@ def exe_exists(arglist: T.List[str]) -> bool:
         pass
     return False
 
+
 @lru_cache(maxsize=None)
-def darwin_get_object_archs(objpath):
+def darwin_get_object_archs(objpath: str) -> T.List[str]:
     '''
     For a specific object (executable, static library, dylib, etc), run `lipo`
     to fetch the list of archs supported by it. Supports both thin objects and
@@ -501,7 +537,8 @@ def darwin_get_object_archs(objpath):
         stdo += ' arm'
     return stdo.split()
 
-def detect_vcs(source_dir):
+
+def detect_vcs(source_dir: str) -> T.Optional[T.Dict[str, str]]:
     vcs_systems = [
         dict(name = 'git',        cmd = 'git', repo_dir = '.git', get_rev = 'git describe --dirty=+', rev_regex = '(.*)', dep = '.git/logs/HEAD'),
         dict(name = 'mercurial',  cmd = 'hg',  repo_dir = '.hg',  get_rev = 'hg id -i',               rev_regex = '(.*)', dep = '.hg/dirstate'),
@@ -520,17 +557,19 @@ def detect_vcs(source_dir):
 
 # a helper class which implements the same version ordering as RPM
 class Version:
-    def __init__(self, s):
+    def __init__(self, s: str):
         self._s = s
 
         # split into numeric, alphabetic and non-alphanumeric sequences
-        sequences = re.finditer(r'(\d+|[a-zA-Z]+|[^a-zA-Z\d]+)', s)
+        sequences1 = re.finditer(r'(\d+|[a-zA-Z]+|[^a-zA-Z\d]+)', s)
+
         # non-alphanumeric separators are discarded
-        sequences = [m for m in sequences if not re.match(r'[^a-zA-Z\d]+', m.group(1))]
+        sequences2 = [m for m in sequences1 if not re.match(r'[^a-zA-Z\d]+', m.group(1))]
+
         # numeric sequences are converted from strings to ints
-        sequences = [int(m.group(1)) if m.group(1).isdigit() else m.group(1) for m in sequences]
+        sequences3 = [int(m.group(1)) if m.group(1).isdigit() else m.group(1) for m in sequences2]
 
-        self._v = sequences
+        self._v = sequences3
 
     def __str__(self):
         return '%s (V=%s)' % (self._s, str(self._v))
@@ -568,7 +607,7 @@ class Version:
             return self._v != other._v
         return NotImplemented
 
-    def __cmp(self, other, comparator):
+    def __cmp(self, other: 'Version', comparator: T.Callable[[T.Any, T.Any], bool]) -> bool:
         # compare each sequence in order
         for ours, theirs in zip(self._v, other._v):
             # sort a non-digit sequence before a digit sequence
@@ -584,6 +623,7 @@ class Version:
         # otherwise, the version with a suffix remaining is greater
         return comparator(len(self._v), len(other._v))
 
+
 def _version_extract_cmpop(vstr2: str) -> T.Tuple[T.Callable[[T.Any, T.Any], bool], str]:
     if vstr2.startswith('>='):
         cmpop = operator.ge
@@ -611,12 +651,14 @@ def _version_extract_cmpop(vstr2: str) -> T.Tuple[T.Callable[[T.Any, T.Any], boo
 
     return (cmpop, vstr2)
 
+
 def version_compare(vstr1: str, vstr2: str) -> bool:
     (cmpop, vstr2) = _version_extract_cmpop(vstr2)
     return cmpop(Version(vstr1), Version(vstr2))
 
-def version_compare_many(vstr1, conditions):
-    if not isinstance(conditions, (list, tuple, frozenset)):
+
+def version_compare_many(vstr1: str, conditions: T.Union[str, T.Iterable[str]]) -> T.Tuple[bool, T.List[str], T.List[str]]:
+    if isinstance(conditions, str):
         conditions = [conditions]
     found = []
     not_found = []
@@ -627,6 +669,7 @@ def version_compare_many(vstr1, conditions):
             found.append(req)
     return not_found == [], not_found, found
 
+
 # determine if the minimum version satisfying the condition |condition| exceeds
 # the minimum version for a feature |minimum|
 def version_compare_condition_with_min(condition: str, minimum: str) -> bool:
@@ -667,7 +710,8 @@ def version_compare_condition_with_min(condition: str, minimum: str) -> bool:
 
     return cmpop(Version(minimum), Version(condition))
 
-def default_libdir():
+
+def default_libdir() -> str:
     if is_debianlike():
         try:
             pc = subprocess.Popen(['dpkg-architecture', '-qDEB_HOST_MULTIARCH'],
@@ -685,13 +729,16 @@ def default_libdir():
         return 'lib64'
     return 'lib'
 
-def default_libexecdir():
+
+def default_libexecdir() -> str:
     # There is no way to auto-detect this, so it must be set at build time
     return 'libexec'
 
-def default_prefix():
+
+def default_prefix() -> str:
     return 'c:/' if is_windows() else '/usr/local'
 
+
 def get_library_dirs() -> T.List[str]:
     if is_windows():
         return ['C:/mingw/lib'] # TODO: get programmatically
@@ -737,7 +784,8 @@ def get_library_dirs() -> T.List[str]:
 
     return unixdirs
 
-def has_path_sep(name, sep='/\\'):
+
+def has_path_sep(name: str, sep: str = '/\\') -> bool:
     'Checks if any of the specified @sep path separators are in @name'
     for each in sep:
         if each in name:
@@ -754,7 +802,7 @@ if is_windows():
     _whitespace = ' \t\n\r'
     _find_unsafe_char = re.compile(r'[{}"]'.format(_whitespace)).search
 
-    def quote_arg(arg):
+    def quote_arg(arg: str) -> str:
         if arg and not _find_unsafe_char(arg):
             return arg
 
@@ -776,7 +824,7 @@ if is_windows():
         result += (num_backslashes * 2) * '\\' + '"'
         return result
 
-    def split_args(cmd: T.Sequence[str]) -> T.List[str]:
+    def split_args(cmd: str) -> T.List[str]:
         result = []
         arg = ''
         num_backslashes = 0
@@ -811,26 +859,29 @@ if is_windows():
 
         return result
 else:
-    def quote_arg(arg):
+    def quote_arg(arg: str) -> str:
         return shlex.quote(arg)
 
-    def split_args(cmd):
+    def split_args(cmd: str) -> T.List[str]:
         return shlex.split(cmd)
 
 
-def join_args(args):
+def join_args(args: T.Iterable[str]) -> str:
     return ' '.join([quote_arg(x) for x in args])
 
 
-def do_replacement(regex, line, variable_format, confdata):
-    missing_variables = set()
-    start_tag = '@'
-    backslash_tag = '\\@'
+def do_replacement(regex: T.Pattern[str], line: str, variable_format: str,
+                   confdata: 'ConfigurationData') -> T.Tuple[str, T.Set[str]]:
+    missing_variables = set()  # type: T.Set[str]
     if variable_format == 'cmake':
         start_tag = '${'
         backslash_tag = '\\${'
+    else:
+        assert variable_format == 'meson'
+        start_tag = '@'
+        backslash_tag = '\\@'
 
-    def variable_replace(match):
+    def variable_replace(match: T.Match[str]) -> str:
         # Pairs of escape characters before '@' or '\@'
         if match.group(0).endswith('\\'):
             num_escapes = match.end(0) - match.start(0)
@@ -857,7 +908,8 @@ def do_replacement(regex, line, variable_format, confdata):
             return var
     return re.sub(regex, variable_replace, line), missing_variables
 
-def do_mesondefine(line, confdata):
+
+def do_mesondefine(line: str, confdata: 'ConfigurationData') -> str:
     arr = line.split()
     if len(arr) != 2:
         raise MesonException('#mesondefine does not contain exactly two tokens: %s' % line.strip())
@@ -879,7 +931,8 @@ def do_mesondefine(line, confdata):
         raise MesonException('#mesondefine argument "%s" is of unknown type.' % varname)
 
 
-def do_conf_file(src, dst, confdata, variable_format, encoding='utf-8'):
+def do_conf_file(src: str, dst: str, confdata: 'ConfigurationData', variable_format: str,
+                 encoding: str = 'utf-8') -> T.Tuple[T.Set[str], bool]:
     try:
         with open(src, encoding=encoding, newline='') as f:
             data = f.readlines()
@@ -937,7 +990,7 @@ CONF_NASM_PRELUDE = '''; Autogenerated by the Meson build system.
 
 '''
 
-def dump_conf_header(ofilename, cdata, output_format):
+def dump_conf_header(ofilename: str, cdata: 'ConfigurationData', output_format: str) -> None:
     if output_format == 'c':
         prelude = CONF_C_PRELUDE
         prefix = '#'
@@ -967,7 +1020,8 @@ def dump_conf_header(ofilename, cdata, output_format):
                 raise MesonException('Unknown data type in configuration file entry: ' + k)
     replace_if_different(ofilename, ofilename_tmp)
 
-def replace_if_different(dst, dst_tmp):
+
+def replace_if_different(dst: str, dst_tmp: str) -> None:
     # If contents are identical, don't touch the file to prevent
     # unnecessary rebuilds.
     different = True
@@ -982,49 +1036,57 @@ def replace_if_different(dst, dst_tmp):
     else:
         os.unlink(dst_tmp)
 
-def listify(item: T.Any,
-            flatten: bool = True,
-            unholder: bool = False) -> T.List[T.Any]:
+
+@T.overload
+def unholder(item: 'ObjectHolder[_T]') -> _T: ...
+
+@T.overload
+def unholder(item: T.List['ObjectHolder[_T]']) -> T.List[_T]: ...
+
+@T.overload
+def unholder(item: T.List[_T]) -> T.List[_T]: ...
+
+@T.overload
+def unholder(item: T.List[T.Union[_T, 'ObjectHolder[_T]']]) -> T.List[_T]: ...
+
+def unholder(item):
+    """Get the held item of an object holder or list of object holders."""
+    if isinstance(item, list):
+        return [i.held_object if hasattr(i, 'held_object') else i for i in item]
+    if hasattr(item, 'held_object'):
+        return item.held_object
+    return item
+
+
+def listify(item: T.Any, flatten: bool = True) -> T.List[T.Any]:
     '''
     Returns a list with all args embedded in a list if they are not a list.
     This function preserves order.
     @flatten: Convert lists of lists to a flat list
-    @unholder: Replace each item with the object it holds, if required
-
-    Note: unholding only works recursively when flattening
     '''
     if not isinstance(item, list):
-        if unholder and hasattr(item, 'held_object'):
-            item = item.held_object
         return [item]
-    result = []
+    result = []  # type: T.List[T.Any]
     for i in item:
-        if unholder and hasattr(i, 'held_object'):
-            i = i.held_object
         if flatten and isinstance(i, list):
-            result += listify(i, flatten=True, unholder=unholder)
+            result += listify(i, flatten=True)
         else:
             result.append(i)
     return result
 
 
-def extract_as_list(dict_object, *keys, pop=False, **kwargs):
+def extract_as_list(dict_object: T.Dict[_T, _U], key: _T, pop: bool = False) -> T.List[_U]:
     '''
     Extracts all values from given dict_object and listifies them.
     '''
-    result = []
     fetch = dict_object.get
     if pop:
         fetch = dict_object.pop
     # If there's only one key, we don't return a list with one element
-    if len(keys) == 1:
-        return listify(fetch(keys[0], []), **kwargs)
-    # Return a list of values corresponding to *keys
-    for key in keys:
-        result.append(listify(fetch(key, []), **kwargs))
-    return result
+    return listify(fetch(key, []), flatten=True)
+
 
-def typeslistify(item: 'T.Union[_T, T.List[_T]]',
+def typeslistify(item: 'T.Union[_T, T.Sequence[_T]]',
                  types: 'T.Union[T.Type[_T], T.Tuple[T.Type[_T]]]') -> T.List[_T]:
     '''
     Ensure that type(@item) is one of @types or a
@@ -1039,11 +1101,13 @@ def typeslistify(item: 'T.Union[_T, T.List[_T]]',
             raise MesonException('List item must be one of {!r}'.format(types))
     return item
 
-def stringlistify(item: T.Union[str, T.List[str]]) -> T.List[str]:
+
+def stringlistify(item: T.Union[T.Any, T.Sequence[T.Any]]) -> T.List[str]:
     return typeslistify(item, str)
 
-def expand_arguments(args):
-    expended_args = []
+
+def expand_arguments(args: T.Iterable[str]) -> T.Optional[T.List[str]]:
+    expended_args = []  # type: T.List[str]
     for arg in args:
         if not arg.startswith('@'):
             expended_args.append(arg)
@@ -1055,17 +1119,24 @@ def expand_arguments(args):
                 extended_args = f.read().split()
             expended_args += extended_args
         except Exception as e:
-            print('Error expanding command line arguments, %s not found' % args_file)
-            print(e)
+            mlog.error('Expanding command line arguments:',  args_file, 'not found')
+            mlog.exception(e)
             return None
     return expended_args
 
-def partition(pred, iterable):
-    'Use a predicate to partition entries into false entries and true entries'
-    # partition(is_odd, range(10)) --> 0 2 4 6 8   and  1 3 5 7 9
+
+def partition(pred: T.Callable[[_T], object], iterable: T.Iterator[_T]) -> T.Tuple[T.Iterator[_T], T.Iterator[_T]]:
+    """Use a predicate to partition entries into false entries and true
+    entries.
+
+    >>> x, y = partition(is_odd, range(10))
+    >>> (list(x), list(y))
+    ([0, 2, 4, 6, 8], [1, 3, 5, 7, 9])
+    """
     t1, t2 = tee(iterable)
     return filterfalse(pred, t1), filter(pred, t2)
 
+
 def Popen_safe(args: T.List[str], write: T.Optional[str] = None,
                stdout: T.Union[T.BinaryIO, int] = subprocess.PIPE,
                stderr: T.Union[T.BinaryIO, int] = subprocess.PIPE,
@@ -1088,6 +1159,7 @@ def Popen_safe(args: T.List[str], write: T.Optional[str] = None,
     mlog.setup_console()
     return p, o, e
 
+
 def Popen_safe_legacy(args: T.List[str], write: T.Optional[str] = None,
                       stdout: T.Union[T.BinaryIO, int] = subprocess.PIPE,
                       stderr: T.Union[T.BinaryIO, int] = subprocess.PIPE,
@@ -1110,7 +1182,8 @@ def Popen_safe_legacy(args: T.List[str], write: T.Optional[str] = None,
             e = e.decode(errors='replace').replace('\r\n', '\n')
     return p, o, e
 
-def iter_regexin_iter(regexiter, initer):
+
+def iter_regexin_iter(regexiter: T.Iterable[str], initer: T.Iterable[str]) -> T.Optional[str]:
     '''
     Takes each regular expression in @regexiter and tries to search for it in
     every item in @initer. If there is a match, returns that match.
@@ -1123,12 +1196,13 @@ def iter_regexin_iter(regexiter, initer):
             match = re.search(regex, ii)
             if match:
                 return match.group()
-    return False
+    return None
+
 
-def _substitute_values_check_errors(command, values):
+def _substitute_values_check_errors(command: T.List[str], values: T.Dict[str, str]) -> None:
     # Error checking
-    inregex = ('@INPUT([0-9]+)?@', '@PLAINNAME@', '@BASENAME@')
-    outregex = ('@OUTPUT([0-9]+)?@', '@OUTDIR@')
+    inregex = ['@INPUT([0-9]+)?@', '@PLAINNAME@', '@BASENAME@']  # type: T.List[str]
+    outregex = ['@OUTPUT([0-9]+)?@', '@OUTDIR@']                 # type: T.List[str]
     if '@INPUT@' not in values:
         # Error out if any input-derived templates are present in the command
         match = iter_regexin_iter(inregex, command)
@@ -1146,10 +1220,10 @@ def _substitute_values_check_errors(command, values):
         for each in command:
             if not isinstance(each, str):
                 continue
-            match = re.search(inregex[0], each)
-            if match and match.group() not in values:
+            match2 = re.search(inregex[0], each)
+            if match2 and match2.group() not in values:
                 m = 'Command cannot have {!r} since there are only {!r} inputs'
-                raise MesonException(m.format(match.group(), len(values['@INPUT@'])))
+                raise MesonException(m.format(match2.group(), len(values['@INPUT@'])))
     if '@OUTPUT@' not in values:
         # Error out if any output-derived templates are present in the command
         match = iter_regexin_iter(outregex, command)
@@ -1161,12 +1235,13 @@ def _substitute_values_check_errors(command, values):
         for each in command:
             if not isinstance(each, str):
                 continue
-            match = re.search(outregex[0], each)
-            if match and match.group() not in values:
+            match2 = re.search(outregex[0], each)
+            if match2 and match2.group() not in values:
                 m = 'Command cannot have {!r} since there are only {!r} outputs'
-                raise MesonException(m.format(match.group(), len(values['@OUTPUT@'])))
+                raise MesonException(m.format(match2.group(), len(values['@OUTPUT@'])))
 
-def substitute_values(command, values):
+
+def substitute_values(command: T.List[str], values: T.Dict[str, str]) -> T.List[str]:
     '''
     Substitute the template strings in the @values dict into the list of
     strings @command and return a new list. For a full list of the templates,
@@ -1179,7 +1254,7 @@ def substitute_values(command, values):
     # Error checking
     _substitute_values_check_errors(command, values)
     # Substitution
-    outcmd = []
+    outcmd = []  # type: T.List[str]
     rx_keys = [re.escape(key) for key in values if key not in ('@INPUT@', '@OUTPUT@')]
     value_rx = re.compile('|'.join(rx_keys)) if rx_keys else None
     for vv in command:
@@ -1214,7 +1289,8 @@ def substitute_values(command, values):
             outcmd.append(vv)
     return outcmd
 
-def get_filenames_templates_dict(inputs, outputs):
+
+def get_filenames_templates_dict(inputs: T.List[str], outputs: T.List[str]) -> T.Dict[str, T.Union[str, T.List[str]]]:
     '''
     Create a dictionary with template strings as keys and values as values for
     the following templates:
@@ -1236,7 +1312,7 @@ def get_filenames_templates_dict(inputs, outputs):
 
     @OUTPUT0@, @OUTPUT1@, ... one for each output file
     '''
-    values = {}
+    values = {}  # type: T.Dict[str, T.Union[str, T.List[str]]]
     # Gather values derived from the input
     if inputs:
         # We want to substitute all the inputs.
@@ -1261,7 +1337,7 @@ def get_filenames_templates_dict(inputs, outputs):
     return values
 
 
-def _make_tree_writable(topdir):
+def _make_tree_writable(topdir: str) -> None:
     # Ensure all files and directories under topdir are writable
     # (and readable) by owner.
     for d, _, files in os.walk(topdir):
@@ -1272,7 +1348,7 @@ def _make_tree_writable(topdir):
                 os.chmod(fpath, os.stat(fpath).st_mode | stat.S_IWRITE | stat.S_IREAD)
 
 
-def windows_proof_rmtree(f):
+def windows_proof_rmtree(f: str) -> None:
     # On Windows if anyone is holding a file open you can't
     # delete it. As an example an anti virus scanner might
     # be scanning files you are trying to delete. The only
@@ -1292,7 +1368,7 @@ def windows_proof_rmtree(f):
     shutil.rmtree(f)
 
 
-def windows_proof_rm(fpath):
+def windows_proof_rm(fpath: str) -> None:
     """Like windows_proof_rmtree, but for a single file."""
     if os.path.isfile(fpath):
         os.chmod(fpath, os.stat(fpath).st_mode | stat.S_IWRITE | stat.S_IREAD)
@@ -1308,7 +1384,8 @@ def windows_proof_rm(fpath):
     os.unlink(fpath)
 
 
-def detect_subprojects(spdir_name, current_dir='', result=None):
+def detect_subprojects(spdir_name: str, current_dir: str = '',
+                       result: T.Optional[T.Dict[str, T.List[str]]] = None) -> T.Optional[T.Dict[str, T.List[str]]]:
     if result is None:
         result = {}
     spdir = os.path.join(current_dir, spdir_name)
@@ -1332,57 +1409,63 @@ def detect_subprojects(spdir_name, current_dir='', result=None):
                 result[basename] = [trial]
     return result
 
+
 def substring_is_in_list(substr: str, strlist: T.List[str]) -> bool:
     for s in strlist:
         if substr in s:
             return True
     return False
 
-class OrderedSet(collections.abc.MutableSet):
+
+class OrderedSet(T.MutableSet[_T]):
     """A set that preserves the order in which items are added, by first
     insertion.
     """
-    def __init__(self, iterable=None):
-        self.__container = collections.OrderedDict()
+    def __init__(self, iterable: T.Optional[T.Iterable[_T]] = None):
+        # typing.OrderedDict is new in 3.7.2, so we can't use that, but we can
+        # use MutableMapping, which is fine in this case.
+        self.__container = collections.OrderedDict()  # type: T.MutableMapping[_T, None]
         if iterable:
             self.update(iterable)
 
-    def __contains__(self, value):
+    def __contains__(self, value: object) -> bool:
         return value in self.__container
 
-    def __iter__(self):
+    def __iter__(self) -> T.Iterator[_T]:
         return iter(self.__container.keys())
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self.__container)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         # Don't print 'OrderedSet("")' for an empty set.
         if self.__container:
             return 'OrderedSet("{}")'.format(
                 '", "'.join(repr(e) for e in self.__container.keys()))
         return 'OrderedSet()'
 
-    def __reversed__(self):
-        return reversed(self.__container)
+    def __reversed__(self) -> T.Iterator[_T]:
+        # Mypy is complaining that sets cant be reversed, which is true for
+        # unordered sets, but this is an ordered, set so reverse() makes sense.
+        return reversed(self.__container.keys())  # type: ignore
 
-    def add(self, value):
+    def add(self, value: _T) -> None:
         self.__container[value] = None
 
-    def discard(self, value):
+    def discard(self, value: _T) -> None:
         if value in self.__container:
             del self.__container[value]
 
-    def update(self, iterable):
+    def update(self, iterable: T.Iterable[_T]) -> None:
         for item in iterable:
             self.__container[item] = None
 
-    def difference(self, set_):
+    def difference(self, set_: T.Union[T.Set[_T], 'OrderedSet[_T]']) -> 'OrderedSet[_T]':
         return type(self)(e for e in self if e not in set_)
 
 class BuildDirLock:
 
-    def __init__(self, builddir):
+    def __init__(self, builddir: str):
         self.lockfilename = os.path.join(builddir, 'meson-private/meson.lock')
 
     def __enter__(self):
@@ -1431,7 +1514,8 @@ class ProgressBarFallback:  # lgtm [py/iter-returns-non-self]
     fallback, it is safe to ignore the 'Iterator does not return self from
     __iter__ method' warning.
     '''
-    def __init__(self, iterable=None, total=None, bar_type=None, desc=None):
+    def __init__(self, iterable: T.Optional[T.Iterable[str]] = None, total: T.Optional[int] = None,
+                 bar_type: T.Optional[str] = None, desc: T.Optional[str] = None):
         if iterable is not None:
             self.iterable = iter(iterable)
             return
@@ -1445,18 +1529,18 @@ class ProgressBarFallback:  # lgtm [py/iter-returns-non-self]
 
     # Pretend to be an iterator when called as one and don't print any
     # progress
-    def __iter__(self):
+    def __iter__(self) -> T.Iterator[str]:
         return self.iterable
 
-    def __next__(self):
+    def __next__(self) -> str:
         return next(self.iterable)
 
-    def print_dot(self):
+    def print_dot(self) -> None:
         print('.', end='')
         sys.stdout.flush()
         self.printed_dots += 1
 
-    def update(self, progress):
+    def update(self, progress: int) -> None:
         self.done += progress
         if not self.total:
             # Just print one dot per call if we don't have a total length
@@ -1466,32 +1550,33 @@ class ProgressBarFallback:  # lgtm [py/iter-returns-non-self]
         while self.printed_dots < ratio:
             self.print_dot()
 
-    def close(self):
+    def close(self) -> None:
         print('')
 
 try:
     from tqdm import tqdm
-
-    class ProgressBar(tqdm):
-        def __init__(self, *args, bar_type=None, **kwargs):
+except ImportError:
+    # ideally we would use a typing.Protocol here, but it's part of typing_extensions until 3.8
+    ProgressBar = ProgressBarFallback  # type: T.Union[T.Type[ProgressBarFallback], T.Type[ProgressBarTqdm]]
+else:
+    class ProgressBarTqdm(tqdm):
+        def __init__(self, *args, bar_type: T.Optional[str] = None, **kwargs):
             if bar_type == 'download':
                 kwargs.update({'unit': 'bytes', 'leave': True})
             else:
                 kwargs.update({'leave': False})
             kwargs['ncols'] = 100
             super().__init__(*args, **kwargs)
-except ImportError:
-    ProgressBar = ProgressBarFallback
 
+    ProgressBar = ProgressBarTqdm
 
-def get_wine_shortpath(winecmd, wine_paths):
 
-    """ Get A short version of @wine_paths to avoid
-    reaching WINEPATH number of char limit.
+def get_wine_shortpath(winecmd: T.List[str], wine_paths: T.Sequence[str]) -> str:
+    """Get A short version of @wine_paths to avoid reaching WINEPATH number
+    of char limit.
     """
 
-    seen = set()
-    wine_paths = [p for p in wine_paths if not (p in seen or seen.add(p))]
+    wine_paths = list(OrderedSet(wine_paths))
 
     getShortPathScript = '%s.bat' % str(uuid.uuid4()).lower()[:5]
     with open(getShortPathScript, mode='w') as f:
@@ -1516,10 +1601,12 @@ def get_wine_shortpath(winecmd, wine_paths):
 
     return wine_path.strip(';')
 
-def run_once(func):
-    ret = []
 
-    def wrapper(*args, **kwargs):
+def run_once(func: T.Callable[..., _T]) -> T.Callable[..., _T]:
+    ret = []  # type: T.List[_T]
+
+    @wraps(func)
+    def wrapper(*args: T.Any, **kwargs: T.Any) -> _T:
         if ret:
             return ret[0]
 
@@ -1527,33 +1614,40 @@ def run_once(func):
         ret.append(val)
         return val
 
-    return update_wrapper(wrapper, func)
+    return wrapper
 
 
-class OptionProxy:
-    def __init__(self, value):
+class OptionProxy(T.Generic[_T]):
+    def __init__(self, value: _T):
         self.value = value
 
+
 class OptionOverrideProxy:
-    '''Mimic an option list but transparently override
-    selected option values.'''
-    def __init__(self, overrides, *options):
+
+    '''Mimic an option list but transparently override selected option
+    values.
+    '''
+
+    # TODO: the typing here could be made more explicit using a TypeDict from
+    # python 3.8 or typing_extensions
+
+    def __init__(self, overrides: T.Dict[str, T.Any], *options: 'OptionDictType'):
         self.overrides = overrides
         self.options = options
 
-    def __getitem__(self, option_name):
+    def __getitem__(self, option_name: str) -> T.Any:
         for opts in self.options:
             if option_name in opts:
                 return self._get_override(option_name, opts[option_name])
         raise KeyError('Option not found', option_name)
 
-    def _get_override(self, option_name, base_opt):
+    def _get_override(self, option_name: str, base_opt: 'UserOption[T.Any]') -> T.Union[OptionProxy[T.Any], 'UserOption[T.Any]']:
         if option_name in self.overrides:
             return OptionProxy(base_opt.validate_value(self.overrides[option_name]))
         return base_opt
 
-    def copy(self):
-        result = {}
+    def copy(self) -> T.Dict[str, T.Any]:
+        result = {}  # type: T.Dict[str, T.Any]
         for opts in self.options:
             for option_name in opts:
                 result[option_name] = self._get_override(option_name, opts[option_name])
diff --git a/mesonbuild/mlog.py b/mesonbuild/mlog.py
index ea75bd0..88a47e7 100644
--- a/mesonbuild/mlog.py
+++ b/mesonbuild/mlog.py
@@ -298,7 +298,7 @@ def exception(e: Exception, prefix: T.Optional[AnsiDecorator] = None) -> None:
         prefix = red('ERROR:')
     log()
     args = []  # type: T.List[T.Union[AnsiDecorator, str]]
-    if hasattr(e, 'file') and hasattr(e, 'lineno') and hasattr(e, 'colno'):
+    if getattr(e, 'file') is not None and getattr(e, 'lineno') is not None and getattr(e, 'colno') is not None:
         # Mypy doesn't follow hasattr, and it's pretty easy to visually inspect
         # that this is correct, so we'll just ignore it.
         path = get_relative_path(Path(e.file), Path(os.getcwd()))  # type: ignore
diff --git a/mesonbuild/modules/__init__.py b/mesonbuild/modules/__init__.py
index dc3c786..dc86a1b 100644
--- a/mesonbuild/modules/__init__.py
+++ b/mesonbuild/modules/__init__.py
@@ -18,6 +18,7 @@
 import os
 
 from .. import build
+from ..mesonlib import unholder
 
 
 class ExtensionModule:
@@ -38,12 +39,7 @@ def get_include_args(include_dirs, prefix='-I'):
         return []
 
     dirs_str = []
-    for incdirs in include_dirs:
-        if hasattr(incdirs, "held_object"):
-            dirs = incdirs.held_object
-        else:
-            dirs = incdirs
-
+    for dirs in unholder(include_dirs):
         if isinstance(dirs, str):
             dirs_str += ['%s%s' % (prefix, dirs)]
             continue
diff --git a/mesonbuild/modules/gnome.py b/mesonbuild/modules/gnome.py
index 1743b59..4ddde28 100644
--- a/mesonbuild/modules/gnome.py
+++ b/mesonbuild/modules/gnome.py
@@ -29,7 +29,8 @@ from . import get_include_args
 from . import ExtensionModule
 from . import ModuleReturnValue
 from ..mesonlib import (
-    MachineChoice, MesonException, OrderedSet, Popen_safe, extract_as_list, join_args
+    MachineChoice, MesonException, OrderedSet, Popen_safe, extract_as_list,
+    join_args, unholder,
 )
 from ..dependencies import Dependency, PkgConfigDependency, InternalDependency
 from ..interpreterbase import noKwargs, permittedKwargs, FeatureNew, FeatureNewKwargs
@@ -83,11 +84,11 @@ class GnomeModule(ExtensionModule):
                          mlog.bold('https://bugzilla.gnome.org/show_bug.cgi?id=774368'))
 
     @staticmethod
-    @mesonlib.run_once
     def _print_gdbus_warning():
         mlog.warning('Code generated with gdbus_codegen() requires the root directory be added to\n'
                      '  include_directories of targets with GLib < 2.51.3:',
-                     mlog.bold('https://github.com/mesonbuild/meson/issues/1387'))
+                     mlog.bold('https://github.com/mesonbuild/meson/issues/1387'),
+                     once=True)
 
     @FeatureNewKwargs('gnome.compile_resources', '0.37.0', ['gresource_bundle', 'export', 'install_header'])
     @permittedKwargs({'source_dir', 'c_name', 'dependencies', 'export', 'gresource_bundle', 'install_header',
@@ -98,7 +99,7 @@ class GnomeModule(ExtensionModule):
 
         cmd = ['glib-compile-resources', '@INPUT@']
 
-        source_dirs, dependencies = mesonlib.extract_as_list(kwargs, 'source_dir', 'dependencies', pop=True)
+        source_dirs, dependencies = [mesonlib.extract_as_list(kwargs, c, pop=True) for c in  ['source_dir', 'dependencies']]
 
         if len(args) < 2:
             raise MesonException('Not enough arguments; the name of the resource '
@@ -238,9 +239,7 @@ class GnomeModule(ExtensionModule):
         subdirs = []
         for resfile in dep_files[:]:
             resbasename = os.path.basename(resfile)
-            for dep in dependencies:
-                if hasattr(dep, 'held_object'):
-                    dep = dep.held_object
+            for dep in unholder(dependencies):
                 if isinstance(dep, mesonlib.File):
                     if dep.fname != resbasename:
                         continue
@@ -316,15 +315,13 @@ class GnomeModule(ExtensionModule):
         # require two args in order, such as -framework AVFoundation
         external_ldflags_nodedup = []
         gi_includes = OrderedSet()
-        deps = mesonlib.listify(deps, unholder=True)
+        deps = mesonlib.unholder(mesonlib.listify(deps))
 
         for dep in deps:
             if isinstance(dep, InternalDependency):
                 cflags.update(dep.get_compile_args())
                 cflags.update(get_include_args(dep.include_directories))
-                for lib in dep.libraries:
-                    if hasattr(lib, 'held_object'):
-                        lib = lib.held_object
+                for lib in unholder(dep.libraries):
                     if isinstance(lib, build.SharedLibrary):
                         internal_ldflags.update(self._get_link_args(state, lib, depends, include_rpath))
                         libdepflags = self._get_dependencies_flags(lib.get_external_deps(), state, depends, include_rpath,
@@ -341,9 +338,7 @@ class GnomeModule(ExtensionModule):
                 external_ldflags.update(extdepflags[2])
                 external_ldflags_nodedup += extdepflags[3]
                 gi_includes.update(extdepflags[4])
-                for source in dep.sources:
-                    if hasattr(source, 'held_object'):
-                        source = source.held_object
+                for source in unholder(dep.sources):
                     if isinstance(source, GirTarget):
                         gi_includes.update([os.path.join(state.environment.get_build_dir(),
                                             source.get_subdir())])
@@ -457,9 +452,7 @@ class GnomeModule(ExtensionModule):
 
         if 'includes' in kwargs:
             includes = mesonlib.extract_as_list(kwargs, 'includes', pop = True)
-            for inc in includes:
-                if hasattr(inc, 'held_object'):
-                    inc = inc.held_object
+            for inc in unholder(includes):
                 if isinstance(inc, str):
                     ret += ['--include=%s' % (inc, )]
                 elif isinstance(inc, GirTarget):
@@ -616,9 +609,7 @@ class GnomeModule(ExtensionModule):
         gir_filelist_filename = os.path.join(gir_filelist_dir, '%s_%s_gir_filelist' % (ns, nsversion))
 
         with open(gir_filelist_filename, 'w', encoding='utf-8') as gir_filelist:
-            for s in libsources:
-                if hasattr(s, 'held_object'):
-                    s = s.held_object
+            for s in unholder(libsources):
                 if isinstance(s, (build.CustomTarget, build.CustomTargetIndex)):
                     for custom_output in s.get_outputs():
                         gir_filelist.write(os.path.join(state.environment.get_build_dir(),
@@ -671,15 +662,11 @@ class GnomeModule(ExtensionModule):
         # dependencies and also find the include directories needed for the
         # typelib generation custom target below.
         typelib_includes = []
-        for dep in deps:
-            if hasattr(dep, 'held_object'):
-                dep = dep.held_object
+        for dep in unholder(deps):
             # Add a dependency on each GirTarget listed in dependencies and add
             # the directory where it will be generated to the typelib includes
             if isinstance(dep, InternalDependency):
-                for source in dep.sources:
-                    if hasattr(source, 'held_object'):
-                        source = source.held_object
+                for source in unholder(dep.sources):
                     if isinstance(source, GirTarget) and source not in depends:
                         depends.append(source)
                         subdir = os.path.join(state.environment.get_build_dir(),
@@ -776,7 +763,7 @@ class GnomeModule(ExtensionModule):
         langs_compilers = self._get_girtargets_langs_compilers(girtargets)
         cflags, internal_ldflags, external_ldflags = self._get_langs_compilers_flags(state, langs_compilers)
         deps = self._get_gir_targets_deps(girtargets)
-        deps += extract_as_list(kwargs, 'dependencies', pop=True, unholder=True)
+        deps += mesonlib.unholder(extract_as_list(kwargs, 'dependencies', pop=True))
         typelib_includes = self._gather_typelib_includes_and_update_depends(state, deps, depends)
         # ldflags will be misinterpreted by gir scanner (showing
         # spurious dependencies) but building GStreamer fails if they
@@ -1003,9 +990,7 @@ This will become a hard error in the future.''')
 
         depends = []
         content_files = []
-        for s in mesonlib.extract_as_list(kwargs, 'content_files'):
-            if hasattr(s, 'held_object'):
-                s = s.held_object
+        for s in unholder(mesonlib.extract_as_list(kwargs, 'content_files')):
             if isinstance(s, (build.CustomTarget, build.CustomTargetIndex)):
                 depends.append(s)
                 for o in s.get_outputs():
@@ -1057,7 +1042,7 @@ This will become a hard error in the future.''')
 
     def _get_build_args(self, kwargs, state, depends):
         args = []
-        deps = extract_as_list(kwargs, 'dependencies', unholder=True)
+        deps = mesonlib.unholder(extract_as_list(kwargs, 'dependencies'))
         cflags = []
         cflags.extend(mesonlib.stringlistify(kwargs.pop('c_args', [])))
         deps_cflags, internal_ldflags, external_ldflags, gi_includes = \
@@ -1601,9 +1586,7 @@ G_END_DECLS'''
         vapi_includes = []
         ret = []
         remaining_args = []
-        for arg in arg_list:
-            if hasattr(arg, 'held_object'):
-                arg = arg.held_object
+        for arg in unholder(arg_list):
             if isinstance(arg, InternalDependency):
                 targets = [t for t in arg.sources if isinstance(t, VapiTarget)]
                 for target in targets:
diff --git a/mesonbuild/modules/i18n.py b/mesonbuild/modules/i18n.py
index 80ec8bd..2652e7d 100644
--- a/mesonbuild/modules/i18n.py
+++ b/mesonbuild/modules/i18n.py
@@ -16,7 +16,7 @@ import shutil
 
 from os import path
 from .. import coredata, mesonlib, build, mlog
-from ..mesonlib import MesonException, run_once
+from ..mesonlib import MesonException
 from . import ModuleReturnValue
 from . import ExtensionModule
 from ..interpreterbase import permittedKwargs, FeatureNew, FeatureNewKwargs
@@ -59,9 +59,8 @@ PRESET_ARGS = {
 class I18nModule(ExtensionModule):
 
     @staticmethod
-    @run_once
     def nogettext_warning():
-        mlog.warning('Gettext not found, all translation targets will be ignored.')
+        mlog.warning('Gettext not found, all translation targets will be ignored.', once=True)
         return ModuleReturnValue(None, [])
 
     @staticmethod
diff --git a/mesonbuild/modules/pkgconfig.py b/mesonbuild/modules/pkgconfig.py
index 8de88c9..1d8e8a9 100644
--- a/mesonbuild/modules/pkgconfig.py
+++ b/mesonbuild/modules/pkgconfig.py
@@ -74,7 +74,7 @@ class DependenciesHelper:
     def _process_reqs(self, reqs):
         '''Returns string names of requirements'''
         processed_reqs = []
-        for obj in mesonlib.listify(reqs, unholder=True):
+        for obj in mesonlib.unholder(mesonlib.listify(reqs)):
             if not isinstance(obj, str):
                 FeatureNew('pkgconfig.generate requirement from non-string object', '0.46.0').use(self.state.subproject)
             if hasattr(obj, 'generated_pc'):
@@ -108,7 +108,7 @@ class DependenciesHelper:
         self.cflags += mesonlib.stringlistify(cflags)
 
     def _process_libs(self, libs, public):
-        libs = mesonlib.listify(libs, unholder=True)
+        libs = mesonlib.unholder(mesonlib.listify(libs))
         processed_libs = []
         processed_reqs = []
         processed_cflags = []
diff --git a/mesonbuild/modules/qt.py b/mesonbuild/modules/qt.py
index 76edb7e..c7da530 100644
--- a/mesonbuild/modules/qt.py
+++ b/mesonbuild/modules/qt.py
@@ -15,7 +15,7 @@
 import os
 from .. import mlog
 from .. import build
-from ..mesonlib import MesonException, Popen_safe, extract_as_list, File
+from ..mesonlib import MesonException, Popen_safe, extract_as_list, File, unholder
 from ..dependencies import Dependency, Qt4Dependency, Qt5Dependency
 import xml.etree.ElementTree as ET
 from . import ModuleReturnValue, get_include_args, ExtensionModule
@@ -142,7 +142,7 @@ class QtBaseModule(ExtensionModule):
     @permittedKwargs({'moc_headers', 'moc_sources', 'uic_extra_arguments', 'moc_extra_arguments', 'rcc_extra_arguments', 'include_directories', 'dependencies', 'ui_files', 'qresources', 'method'})
     def preprocess(self, state, args, kwargs):
         rcc_files, ui_files, moc_headers, moc_sources, uic_extra_arguments, moc_extra_arguments, rcc_extra_arguments, sources, include_directories, dependencies \
-            = extract_as_list(kwargs, 'qresources', 'ui_files', 'moc_headers', 'moc_sources', 'uic_extra_arguments', 'moc_extra_arguments', 'rcc_extra_arguments', 'sources', 'include_directories', 'dependencies', pop = True)
+            = [extract_as_list(kwargs, c, pop=True) for c in ['qresources', 'ui_files', 'moc_headers', 'moc_sources', 'uic_extra_arguments', 'moc_extra_arguments', 'rcc_extra_arguments', 'sources', 'include_directories', 'dependencies']]
         sources += args[1:]
         method = kwargs.get('method', 'auto')
         self._detect_tools(state.environment, method)
@@ -190,9 +190,7 @@ class QtBaseModule(ExtensionModule):
             sources.append(ui_output)
         inc = get_include_args(include_dirs=include_directories)
         compile_args = []
-        for dep in dependencies:
-            if hasattr(dep, 'held_object'):
-                dep = dep.held_object
+        for dep in unholder(dependencies):
             if isinstance(dep, Dependency):
                 for arg in dep.get_compile_args():
                     if arg.startswith('-I') or arg.startswith('-D'):
@@ -221,7 +219,7 @@ class QtBaseModule(ExtensionModule):
     @FeatureNew('qt.compile_translations', '0.44.0')
     @permittedKwargs({'ts_files', 'install', 'install_dir', 'build_by_default', 'method'})
     def compile_translations(self, state, args, kwargs):
-        ts_files, install_dir = extract_as_list(kwargs, 'ts_files', 'install_dir', pop=True)
+        ts_files, install_dir = [extract_as_list(kwargs, c, pop=True) for c in  ['ts_files', 'install_dir']]
         self._detect_tools(state.environment, kwargs.get('method', 'auto'))
         translations = []
         for ts in ts_files:
diff --git a/mesonbuild/modules/windows.py b/mesonbuild/modules/windows.py
index 7cf46f7..8589adc 100644
--- a/mesonbuild/modules/windows.py
+++ b/mesonbuild/modules/windows.py
@@ -18,7 +18,7 @@ import re
 
 from .. import mlog
 from .. import mesonlib, build
-from ..mesonlib import MachineChoice, MesonException, extract_as_list
+from ..mesonlib import MachineChoice, MesonException, extract_as_list, unholder
 from . import get_include_args
 from . import ModuleReturnValue
 from . import ExtensionModule
@@ -116,9 +116,7 @@ class WindowsModule(ExtensionModule):
                 for subsrc in src:
                     add_target(subsrc)
                 return
-
-            if hasattr(src, 'held_object'):
-                src = src.held_object
+            src = unholder(src)
 
             if isinstance(src, str):
                 name_format = 'file {!r}'
diff --git a/mypy.ini b/mypy.ini
index aacfb3f..b8dad03 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -1,4 +1,5 @@
 [mypy]
 strict_optional = False
 show_error_context = False
-show_column_numbers = True
\ No newline at end of file
+show_column_numbers = True
+ignore_missing_imports = True
diff --git a/run_unittests.py b/run_unittests.py
index e254d4a..c9b7563 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -686,12 +686,17 @@ class InternalTests(unittest.TestCase):
         self.assertEqual([holder1], listify([holder1]))
         self.assertEqual([holder1, 2], listify([holder1, 2]))
         self.assertEqual([holder1, 2, 3], listify([holder1, 2, [3]]))
-        self.assertEqual([1], listify(holder1, unholder=True))
-        self.assertEqual([1], listify([holder1], unholder=True))
-        self.assertEqual([1, 2], listify([holder1, 2], unholder=True))
-        self.assertEqual([1, 2, 3], listify([holder1, 2, [holder3]], unholder=True))
-        # Unholding doesn't work recursively when not flattening
-        self.assertEqual([1, [2], [holder3]], listify([holder1, [2], [holder3]], unholder=True, flatten=False))
+
+    def test_unholder(self):
+        unholder = mesonbuild.mesonlib.unholder
+
+        holder1 = ObjectHolder(1)
+        holder3 = ObjectHolder(3)
+        holders = [holder1, holder3]
+
+        self.assertEqual(1, unholder(holder1))
+        self.assertEqual([1], unholder([holder1]))
+        self.assertEqual([1, 3], unholder(holders))
 
     def test_extract_as_list(self):
         extract = mesonbuild.mesonlib.extract_as_list
@@ -701,16 +706,15 @@ class InternalTests(unittest.TestCase):
         self.assertEqual(kwargs, {'sources': [1, 2, 3]})
         self.assertEqual([1, 2, 3], extract(kwargs, 'sources', pop=True))
         self.assertEqual(kwargs, {})
+
         # Test unholding
         holder3 = ObjectHolder(3)
         kwargs = {'sources': [1, 2, holder3]}
-        self.assertEqual([1, 2, 3], extract(kwargs, 'sources', unholder=True))
         self.assertEqual(kwargs, {'sources': [1, 2, holder3]})
-        self.assertEqual([1, 2, 3], extract(kwargs, 'sources', unholder=True, pop=True))
-        self.assertEqual(kwargs, {})
-        # Test listification
-        kwargs = {'sources': [1, 2, 3], 'pch_sources': [4, 5, 6]}
-        self.assertEqual([[1, 2, 3], [4, 5, 6]], extract(kwargs, 'sources', 'pch_sources'))
+
+        # flatten nested lists
+        kwargs = {'sources': [1, [2, [3]]]}
+        self.assertEqual([1, 2, 3], extract(kwargs, 'sources'))
 
     def test_pkgconfig_module(self):