swift: Fix duplicate SDK include paths causing a compile error

Some dependencies can bring include paths pointing to older macOS SDK's.
In this case, it was libffi pointing to SDK from 12.0. When the Foundation
framework is imported in Swift, swiftc attempts to import the FFI module
from the most recent version of the SDK, which causes a compilation error
because of conflicting definitions between the two SDK versions.

SwiftPM also had this problem:
swiftlang/swift-package-manager#6772

The solution on our side is a simplified version of what SwiftPM did.
Let's naively look for .sdk paths in the compile args of our dependencies
and replace them with the most recent one.

I included a test which is confirmed to fail without the workaround
added in this patch. This was not tested on anything else than macOS,
but I don't expect it to make the situation worse in any case.

mesonbuild/compilers/swift.py

@@ -11,6 +11,7 @@
 from .compilers import Compiler, clike_debug_args
 
 if T.TYPE_CHECKING:
+    from ..dependencies import Dependency
     from ..envconfig import MachineInfo
     from ..environment import Environment
     from ..linkers.linkers import DynamicLinker
@@ -39,6 +40,10 @@ def __init__(self, exelist: T.List[str], version: str, for_machine: MachineChoic
                          is_cross=is_cross, full_version=full_version,
                          linker=linker)
         self.version = version
+        if self.info.is_darwin():
+            self.sdk_path = subprocess.check_output(['xcrun', '--show-sdk-path'],
+                                                    universal_newlines=True,
+                                                    encoding='utf-8').strip()
 
     def get_pic_args(self) -> T.List[str]:
         return []
@@ -55,6 +60,22 @@ def get_werror_args(self) -> T.List[str]:
     def get_dependency_gen_args(self, outtarget: str, outfile: str) -> T.List[str]:
         return ['-emit-dependencies']
 
+    def get_dependency_compile_args(self, dep: Dependency) -> T.List[str]:
+        args = dep.get_compile_args()
+        # Some deps might sneak in a hardcoded path to an older macOS SDK, which can
+        # cause compilation errors. Let's replace all .sdk paths with the current one.
+        # SwiftPM does it this way: https://github.com/swiftlang/swift-package-manager/pull/6772
+        # Not tested on anything else than macOS for now.
+        if not self.info.is_darwin():
+            return args
+        suffix = '.sdk'
+        for idx, arg in enumerate(args):
+            if suffix not in arg:
+                continue
+            sdk_end = arg.index(suffix) + len(suffix)
+            args[idx] = arg[:2] + self.sdk_path + arg[sdk_end:]
+        return args
+
     def depfile_for_object(self, objfile: str) -> T.Optional[str]:
         return os.path.splitext(objfile)[0] + '.' + self.get_depfile_suffix()
 

test cases/swift/9 sdk path from dep/foo.swift

@@ -0,0 +1,4 @@
+// This import is needed for swiftc to implictly import the FFI module
+// which will in turn conflict with the dependency's include path and error out
+// if we don't manually replace all SDK paths with the newest one.
+import Foundation

test cases/swift/9 sdk path from dep/meson.build

@@ -0,0 +1,12 @@
+project('swift sdk include dir test', 'swift')
+
+bar_dep = declare_dependency(
+  # Simulates including 'libffi' from brew as a dep via pkg-config
+  # Without a workaround that replaces all SDK paths with the most recent one,
+  # a compile error will occur due to conflicting definitions of the FFI module.
+  compile_args: '-I/Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk/usr/include/ffi',
+)
+
+foo = static_library('foo', 'foo.swift',
+  dependencies: [bar_dep],
+)