swift: Add swift_implicit_main target kwarg

The '-parse-as-library' compiler option forbids emitting top-level
statements as part of an implicitly generated main function. This
happens either in Swift modules consisting of only one file, or
modules containing a 'main.swift' file. As a consequence, library
targets should usually have this argument applied in order to not
cause duplicate main symbols when linking the final executable. This
is the new default behavior. Sometimes it is necessary to override
this behavior, for example in case your Swift executable has a struct
annotated with @main, or you have an executable Swift target that gets
its main function from elsewhere (for example, linked C code). For
this, targets can be passed the 'swift_implicit_main' kwarg, either
with 'auto', which specifies Swift's default behavior, or 'never',
which passes the '-parse-as-library' flag.

Author: 2xsaiko

mesonbuild/backend/ninjabackend.py

@@ -2220,6 +2220,8 @@ def generate_swift_target(self, target) -> None:
         compile_args = self.generate_basic_compiler_args(target, swiftc)
         compile_args += swiftc.get_compile_only_args()
         compile_args += swiftc.get_module_args(module_name)
+        if target.uses_swift_parse_as_library_args():
+            compile_args += swiftc.get_library_args()
         for i in reversed(target.get_include_dirs()):
             basedir = i.get_curdir()
             for d in i.get_incdirs():

mesonbuild/build.py

@@ -75,6 +75,7 @@ class DFeatures(TypedDict):
 vala_kwargs = {'vala_header', 'vala_gir', 'vala_vapi'}
 rust_kwargs = {'rust_crate_type', 'rust_dependency_map'}
 cs_kwargs = {'resources', 'cs_args'}
+swift_kwargs = {'swift_implicit_main'}
 
 buildtarget_kwargs = {
     'build_by_default',
@@ -110,7 +111,8 @@ class DFeatures(TypedDict):
     pch_kwargs |
     vala_kwargs |
     rust_kwargs |
-    cs_kwargs)
+    cs_kwargs |
+    swift_kwargs)
 
 known_exe_kwargs = known_build_target_kwargs | {'implib', 'export_dynamic', 'pie', 'vs_module_defs'}
 known_shlib_kwargs = known_build_target_kwargs | {'version', 'soversion', 'vs_module_defs', 'darwin_versions', 'rust_abi'}
@@ -1233,6 +1235,13 @@ def process_kwargs(self, kwargs):
             raise InvalidArguments(f'Invalid rust_dependency_map "{rust_dependency_map}": must be a dictionary with string values.')
         self.rust_dependency_map = rust_dependency_map
 
+        self.swift_implicit_main = kwargs.get('swift_implicit_main', '')
+        permitted = {'', 'auto', 'never'}
+        if self.swift_implicit_main not in permitted:
+            raise InvalidArguments(
+                'Invalid value for swift_implicit_main {!r}: must be one of {}'
+                .format(self.swift_implicit_main, ', '.join(repr(x) for x in permitted)))
+
     def _extract_pic_pie(self, kwargs: T.Dict[str, T.Any], arg: str, option: str) -> bool:
         # Check if we have -fPIC, -fpic, -fPIE, or -fpie in cflags
         all_flags = self.extra_args['c'] + self.extra_args['cpp']
@@ -1649,6 +1658,12 @@ def uses_rust_abi(self) -> bool:
     def uses_fortran(self) -> bool:
         return 'fortran' in self.compilers
 
+    def uses_swift_parse_as_library_args(self) -> bool:
+        if self.swift_implicit_main == '':
+            return isinstance(self, (StaticLibrary, SharedLibrary))
+        else:
+            return self.swift_implicit_main == 'never'
+
     def get_using_msvc(self) -> bool:
         '''
         Check if the dynamic linker is MSVC. Used by Executable, StaticLibrary,

mesonbuild/compilers/swift.py

@@ -121,6 +121,9 @@ def get_working_directory_args(self, path: str) -> T.Optional[T.List[str]]:
 
         return ['-working-directory', path]
 
+    def get_library_args(self) -> T.List[str]:
+        return ['-parse-as-library']
+
     def compute_parameters_with_absolute_paths(self, parameter_list: T.List[str],
                                                build_dir: str) -> T.List[str]:
         for idx, i in enumerate(parameter_list):

mesonbuild/interpreter/type_checking.py

@@ -613,6 +613,7 @@ def _name_suffix_validator(arg: T.Optional[T.Union[str, T.List]]) -> T.Optional[
         default={},
         since='1.2.0',
     ),
+    KwargInfo('swift_implicit_main', str, default='', validator=in_set_validator({'', 'auto', 'never'}), since='1.7.99'),
     KwargInfo('build_rpath', str, default='', since='0.42.0'),
     KwargInfo(
         'gnu_symbol_visibility',

test cases/swift/12 single-file library/main.swift

@@ -0,0 +1,3 @@
+import SingleFile
+
+callMe()

test cases/swift/12 single-file library/meson.build

@@ -0,0 +1,4 @@
+project('single-file library', 'swift')
+
+lib = static_library('SingleFile', 'singlefile.swift')
+executable('program', 'main.swift', link_with: [lib])

test cases/swift/12 single-file library/singlefile.swift

@@ -0,0 +1 @@
+public func callMe() {}

test cases/swift/8 extra args/main.swift

@@ -1 +1,3 @@
-print("test")
+public func callMe() {
+    print("test")
+}