diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 4987d75a9087..d729d19892a8 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -855,8 +855,8 @@ def generate_target(self, target):
             self.generate_custom_target(target)
         if isinstance(target, build.RunTarget):
             self.generate_run_target(target)
-        compiled_sources = []
-        source2object = {}
+        compiled_sources: T.List[str] = []
+        source2object: T.Dict[str, str] = {}
         name = target.get_id()
         if name in self.processed_targets:
             return
@@ -939,7 +939,7 @@ def generate_target(self, target):
         # this target's sources (generated sources and preexisting sources).
         # This will be set as dependencies of all the target's sources. At the
         # same time, also deal with generated sources that need to be compiled.
-        generated_source_files = []
+        generated_source_files: T.List[File] = []
         for rel_src in generated_sources.keys():
             raw_src = File.from_built_relative(rel_src)
             if self.environment.is_source(rel_src):
@@ -1084,7 +1084,10 @@ def should_use_dyndeps_for_target(self, target: 'build.BuildTarget') -> bool:
             return False
         return True
 
-    def generate_dependency_scan_target(self, target: build.BuildTarget, compiled_sources, source2object, generated_source_files: T.List[mesonlib.File],
+    def generate_dependency_scan_target(self, target: build.BuildTarget,
+                                        compiled_sources: T.List[str],
+                                        source2object: T.Dict[str, str],
+                                        generated_source_files: T.List[mesonlib.File],
                                         object_deps: T.List['mesonlib.FileOrString']) -> None:
         if not self.should_use_dyndeps_for_target(target):
             return
@@ -1113,7 +1116,7 @@ def generate_dependency_scan_target(self, target: build.BuildTarget, compiled_so
             pickle.dump(scaninfo, p)
         self.add_build(elem)
 
-    def select_sources_to_scan(self, compiled_sources):
+    def select_sources_to_scan(self, compiled_sources: T.List[str]) -> T.List[str]:
         # in practice pick up C++ and Fortran files. If some other language
         # requires scanning (possibly Java to deal with inner class files)
         # then add them here.
@@ -2763,7 +2766,7 @@ def get_link_debugfile_name(self, linker, target) -> T.Optional[str]:
     def get_link_debugfile_args(self, linker, target):
         return linker.get_link_debugfile_args(self.get_target_debug_filename(target))
 
-    def generate_llvm_ir_compile(self, target, src):
+    def generate_llvm_ir_compile(self, target, src: mesonlib.FileOrString):
         base_proxy = target.get_options()
         compiler = get_compiler_for_source(target.compilers.values(), src)
         commands = compiler.compiler_args()
@@ -2925,7 +2928,8 @@ def generate_single_compile(self, target: build.BuildTarget, src,
                                 is_generated: bool = False, header_deps=None,
                                 order_deps: T.Optional[T.List['mesonlib.FileOrString']] = None,
                                 extra_args: T.Optional[T.List[str]] = None,
-                                unity_sources: T.Optional[T.List[mesonlib.FileOrString]] = None) -> None:
+                                unity_sources: T.Optional[T.List[mesonlib.FileOrString]] = None,
+                                ) -> T.Tuple[str, str]:
         """
         Compiles C/C++, ObjC/ObjC++, Fortran, and D sources
         """