build: Use a tuple for pch data

This really isn't a list because it's not homogenous data, it's really
`tuple[str, str | None] | None`, but we're using list length to decide
what to do with it, and that makes for strict null issues, as an
accurate annotation would be `list[str | None]`, which would require a
lot of `is not None` checking. By using a tuple we don't need to keep
checking length, which is more expensive than null checking.

To ensure correctness I annotated some things in the VS backend

7 files changed, 53 insertions, 49 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index 76bd20932..7fba3f3fb 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -824,10 +824,11 @@ class Backend:
         # MSVC generate an object file for PCH
         if extobj.pch and self.target_uses_pch(extobj.target):
             for lang, pch in extobj.target.pch.items():
-                compiler = extobj.target.compilers[lang]
-                if compiler.get_argument_syntax() == 'msvc':
-                    objname = self.get_msvc_pch_objname(lang, pch)
-                    result.append(os.path.join(targetdir, objname))
+                if pch:
+                    compiler = extobj.target.compilers[lang]
+                    if compiler.get_argument_syntax() == 'msvc':
+                        objname = self.get_msvc_pch_objname(lang, pch)
+                        result.append(os.path.join(targetdir, objname))
 
         # extobj could contain only objects and no sources
         if not sources:
@@ -862,13 +863,13 @@ class Backend:
         args: T.List[str] = []
         pchpath = self.get_target_private_dir(target)
         includeargs = compiler.get_include_args(pchpath, False)
-        p = target.get_pch(compiler.get_language())
+        p = target.pch.get(compiler.get_language())
         if p:
             args += compiler.get_pch_use_args(pchpath, p[0])
         return includeargs + args
 
-    def get_msvc_pch_objname(self, lang: str, pch: T.List[str]) -> str:
-        if len(pch) == 1:
+    def get_msvc_pch_objname(self, lang: str, pch: T.Tuple[str, T.Optional[str]]) -> str:
+        if pch[1] is None:
             # Same name as in create_msvc_pch_implementation() below.
             return f'meson_pch-{lang}.obj'
         return os.path.splitext(pch[1])[0] + '.obj'
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 3510d19b4..90c61a26b 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -3180,11 +3180,11 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         # Add MSVC debug file generation compile flags: /Fd /FS
         commands += self.get_compile_debugfile_args(compiler, target, rel_obj)
 
-        # PCH handling
-        if self.target_uses_pch(target):
-            pchlist = target.get_pch(compiler.language)
+        # PCH handling. We only support PCH for C and C++
+        if compiler.language in {'c', 'cpp'} and target.has_pch() and self.target_uses_pch(target):
+            pchlist = target.pch[compiler.language]
         else:
-            pchlist = []
+            pchlist = None
         if not pchlist:
             pch_dep = []
         elif compiler.id == 'intel':
@@ -3328,7 +3328,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
             for lt in itertools.chain(target.link_targets, target.link_whole_targets)
         ]
 
-    def generate_msvc_pch_command(self, target, compiler, pch):
+    def generate_msvc_pch_command(self, target, compiler, pch: T.Tuple[str, T.Optional[str]]):
         header = pch[0]
         pchname = compiler.get_pch_name(header)
         dst = os.path.join(self.get_target_private_dir(target), pchname)
@@ -3336,7 +3336,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         commands = []
         commands += self.generate_basic_compiler_args(target, compiler)
 
-        if len(pch) == 1:
+        if pch[1] is None:
             # Auto generate PCH.
             source = self.create_msvc_pch_implementation(target, compiler.get_language(), pch[0])
             pch_header_dir = os.path.dirname(os.path.join(self.build_to_src, target.get_source_subdir(), header))
@@ -3355,7 +3355,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
 
         return commands, dep, dst, link_objects, source
 
-    def generate_gcc_pch_command(self, target, compiler, pch):
+    def generate_gcc_pch_command(self, target, compiler, pch: str):
         commands = self._generate_single_compile(target, compiler)
         if pch.split('.')[-1] == 'h' and compiler.language == 'cpp':
             # Explicitly compile pch headers as C++. If Clang is invoked in C++ mode, it actually warns if
@@ -3366,21 +3366,21 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         dep = dst + '.' + compiler.get_depfile_suffix()
         return commands, dep, dst, []  # Gcc does not create an object file during pch generation.
 
-    def generate_mwcc_pch_command(self, target, compiler, pch):
+    def generate_mwcc_pch_command(self, target, compiler, pch: str):
         commands = self._generate_single_compile(target, compiler)
         dst = os.path.join(self.get_target_private_dir(target),
                            os.path.basename(pch) + '.' + compiler.get_pch_suffix())
         dep = os.path.splitext(dst)[0] + '.' + compiler.get_depfile_suffix()
         return commands, dep, dst, []  # mwcc compilers do not create an object file during pch generation.
 
-    def generate_pch(self, target, header_deps=None):
+    def generate_pch(self, target: build.BuildTarget, header_deps=None):
         header_deps = header_deps if header_deps is not None else []
         pch_objects = []
         for lang in ['c', 'cpp']:
-            pch = target.get_pch(lang)
+            pch = target.pch[lang]
             if not pch:
                 continue
-            if not has_path_sep(pch[0]) or not has_path_sep(pch[-1]):
+            if not has_path_sep(pch[0]) or (pch[1] and has_path_sep(pch[1])):
                 msg = f'Precompiled header of {target.get_basename()!r} must not be in the same ' \
                       'directory as source, please put it in a subdirectory.'
                 raise InvalidArguments(msg)
diff --git a/mesonbuild/backend/vs2010backend.py b/mesonbuild/backend/vs2010backend.py
index 9c9f61259..1753ef002 100644
--- a/mesonbuild/backend/vs2010backend.py
+++ b/mesonbuild/backend/vs2010backend.py
@@ -820,23 +820,27 @@ class Vs2010Backend(backends.Backend):
             return 'masm'
         raise MesonException(f'Could not guess language from source file {src}.')
 
-    def add_pch(self, pch_sources, lang, inc_cl):
+    def add_pch(self, pch_sources: T.Dict[str, T.Tuple[str, T.Optional[str], str, T.Optional[str]]],
+                lang, inc_cl) -> None:
         if lang in pch_sources:
             self.use_pch(pch_sources, lang, inc_cl)
 
-    def create_pch(self, pch_sources, lang, inc_cl):
+    def create_pch(self, pch_sources: T.Dict[str, T.Tuple[str, T.Optional[str], str, T.Optional[str]]],
+                   lang, inc_cl) -> None:
         pch = ET.SubElement(inc_cl, 'PrecompiledHeader')
         pch.text = 'Create'
         self.add_pch_files(pch_sources, lang, inc_cl)
 
-    def use_pch(self, pch_sources, lang, inc_cl):
+    def use_pch(self, pch_sources: T.Dict[str, T.Tuple[str, T.Optional[str], str, T.Optional[str]]],
+                lang, inc_cl) -> None:
         pch = ET.SubElement(inc_cl, 'PrecompiledHeader')
         pch.text = 'Use'
         header = self.add_pch_files(pch_sources, lang, inc_cl)
         pch_include = ET.SubElement(inc_cl, 'ForcedIncludeFiles')
         pch_include.text = header + ';%(ForcedIncludeFiles)'
 
-    def add_pch_files(self, pch_sources, lang, inc_cl):
+    def add_pch_files(self, pch_sources: T.Dict[str, T.Tuple[str, T.Optional[str], str, T.Optional[str]]],
+                      lang, inc_cl) -> str:
         header = os.path.basename(pch_sources[lang][0])
         pch_file = ET.SubElement(inc_cl, 'PrecompiledHeaderFile')
         # When USING PCHs, MSVC will not do the regular include
@@ -1687,25 +1691,25 @@ class Vs2010Backend(backends.Backend):
             else:
                 return False
 
-        pch_sources = {}
+        pch_sources: T.Dict[str, T.Tuple[str, T.Optional[str], str, T.Optional[str]]] = {}
         if self.target_uses_pch(target):
             for lang in ['c', 'cpp']:
-                pch = target.get_pch(lang)
+                pch = target.pch[lang]
                 if not pch:
                     continue
                 if compiler.id == 'msvc':
-                    if len(pch) == 1:
+                    if pch[1] is None:
                         # Auto generate PCH.
                         src = os.path.join(proj_to_build_root, self.create_msvc_pch_implementation(target, lang, pch[0]))
                         pch_header_dir = os.path.dirname(os.path.join(proj_to_src_dir, pch[0]))
                     else:
                         src = os.path.join(proj_to_src_dir, pch[1])
                         pch_header_dir = None
-                    pch_sources[lang] = [pch[0], src, lang, pch_header_dir]
+                    pch_sources[lang] = (pch[0], src, lang, pch_header_dir)
                 else:
                     # I don't know whether its relevant but let's handle other compilers
                     # used with a vs backend
-                    pch_sources[lang] = [pch[0], None, lang, None]
+                    pch_sources[lang] = (pch[0], None, lang, None)
 
         previous_includes = []
         if len(headers) + len(gen_hdrs) + len(target.extra_files) + len(pch_sources) > 0:
diff --git a/mesonbuild/backend/xcodebackend.py b/mesonbuild/backend/xcodebackend.py
index 87e1309a6..95a62a02a 100644
--- a/mesonbuild/backend/xcodebackend.py
+++ b/mesonbuild/backend/xcodebackend.py
@@ -1802,9 +1802,7 @@ class XCodeBackend(backends.Backend):
                 # Xcode uses GCC_PREFIX_HEADER which only allows one file per target/executable. Precompiling various header files and
                 # applying a particular pch to each source file will require custom scripts (as a build phase) and build flags per each
                 # file. Since Xcode itself already discourages precompiled headers in favor of modules we don't try much harder here.
-                pchs = target.get_pch('c') + target.get_pch('cpp') + target.get_pch('objc') + target.get_pch('objcpp')
-                # Make sure to use headers (other backends require implementation files like *.c *.cpp, etc; these should not be used here)
-                pchs = [pch for pch in pchs if pch.endswith('.h') or pch.endswith('.hh') or pch.endswith('hpp')]
+                pchs = [t[0] for t in [target.pch['c'], target.pch['cpp']] if t is not None]
                 if pchs:
                     if len(pchs) > 1:
                         mlog.warning(f'Unsupported Xcode configuration: More than 1 precompiled header found "{pchs!s}". Target "{target.name}" might not compile correctly.')
diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index c2b218955..4721df113 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -70,8 +70,8 @@ if T.TYPE_CHECKING:
 
         build_by_default: bool
         build_rpath: str
-        c_pch: T.List[str]
-        cpp_pch: T.List[str]
+        c_pch: T.Optional[T.Tuple[str, T.Optional[str]]]
+        cpp_pch: T.Optional[T.Tuple[str, T.Optional[str]]]
         d_debug: T.List[T.Union[str, int]]
         d_import_dirs: T.List[IncludeDirs]
         d_module_versions: T.List[T.Union[str, int]]
@@ -807,7 +807,7 @@ class BuildTarget(Target):
         # The list of all files outputted by this target. Useful in cases such
         # as Vala which generates .vapi and .h besides the compiled output.
         self.outputs = [self.filename]
-        self.pch: T.Dict[str, T.List[str]] = {}
+        self.pch: T.Dict[str, T.Optional[T.Tuple[str, T.Optional[str]]]] = {}
         self.extra_args: T.DefaultDict[str, T.List[str]] = kwargs.get('language_args', defaultdict(list))
         self.sources: T.List[File] = []
         # If the same source is defined multiple times, use it only once.
@@ -1266,8 +1266,8 @@ class BuildTarget(Target):
 
         self.raw_overrides = kwargs.get('override_options', {})
 
-        self.add_pch('c', kwargs.get('c_pch', []))
-        self.add_pch('cpp', kwargs.get('cpp_pch', []))
+        self.pch['c'] = kwargs.get('c_pch')
+        self.pch['cpp'] = kwargs.get('cpp_pch')
 
         self.link_args = extract_as_list(kwargs, 'link_args')
         for i in self.link_args:
@@ -1416,10 +1416,7 @@ class BuildTarget(Target):
         return self.install
 
     def has_pch(self) -> bool:
-        return bool(self.pch)
-
-    def get_pch(self, language: str) -> T.List[str]:
-        return self.pch.get(language, [])
+        return any(x is not None for x in self.pch.values())
 
     def get_include_dirs(self) -> T.List['IncludeDirs']:
         return self.include_dirs
@@ -1590,10 +1587,6 @@ class BuildTarget(Target):
             else:
                 mlog.warning(msg + ' This will fail in cross build.')
 
-    def add_pch(self, language: str, pchlist: T.List[str]) -> None:
-        if pchlist:
-            self.pch[language] = pchlist
-
     def add_include_dirs(self, args: T.Sequence['IncludeDirs'], set_is_system: T.Optional[str] = None) -> None:
         ids: T.List['IncludeDirs'] = []
         for a in args:
diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index 36647f32c..a3cdde04a 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -3479,7 +3479,7 @@ class Interpreter(InterpreterBase, HoldableObject):
             self.check_for_jar_sources(sources, targetclass)
             kwargs['d_import_dirs'] = self.extract_incdirs(kwargs, 'd_import_dirs')
             missing: T.List[str] = []
-            for each in itertools.chain(kwargs['c_pch'], kwargs['cpp_pch']):
+            for each in itertools.chain(kwargs['c_pch'] or [], kwargs['cpp_pch'] or []):
                 if each is not None:
                     if not os.path.isfile(os.path.join(self.environment.source_dir, self.subdir, each)):
                         missing.append(os.path.join(self.subdir, each))
diff --git a/mesonbuild/interpreter/type_checking.py b/mesonbuild/interpreter/type_checking.py
index 5dc9127f3..6cd43b7c6 100644
--- a/mesonbuild/interpreter/type_checking.py
+++ b/mesonbuild/interpreter/type_checking.py
@@ -679,11 +679,19 @@ def _pch_feature_validator(args: T.List[str]) -> T.Iterable[FeatureCheckBase]:
         yield FeatureDeprecated('PCH source files', '0.50.0', 'Only a single header file should be used.')
 
 
-def _pch_convertor(args: T.List[str]) -> T.List[str]:
-    # Flip so that we always have [header, src]
-    if len(args) == 2 and compilers.is_source(args[0]):
-        return [args[1], args[0]]
-    return args
+def _pch_convertor(args: T.List[str]) -> T.Optional[T.Tuple[str, T.Optional[str]]]:
+    num_args = len(args)
+
+    if num_args == 1:
+        return (args[0], None)
+
+    if num_args == 2:
+        if compilers.is_source(args[0]):
+            # Flip so that we always have [header, src]
+            return (args[1], args[0])
+        return (args[0], args[1])
+
+    return None
 
 
 _PCH_ARGS: KwargInfo[T.List[str]] = KwargInfo(