Fix missing weak linking flags

Author: adincebic

swift/toolchains/xcode_swift_toolchain.bzl

@@ -117,23 +117,6 @@ _DEVELOPER_DIR_SYMLINKS = [
     "/var/db/xcode_select_link",
 ]
 
-def _swift_developer_lib_dir(platform_framework_dir):
-    """Returns the directory containing extra Swift developer libraries.
-
-    Args:
-        platform_framework_dir: The developer platform framework directory for
-            the current platform.
-
-    Returns:
-        The directory containing extra Swift-specific development libraries and
-        swiftmodules.
-    """
-    return paths.join(
-        paths.dirname(paths.dirname(platform_framework_dir)),
-        "usr",
-        "lib",
-    )
-
 def _platform_developer_framework_dir(developer_dir, target_triple):
     """Returns the Developer framework directory for the platform.
 
@@ -294,11 +277,16 @@ def _swift_linkopts_cc_info(
     # Add the linker path to the directory containing the dylib with Swift
     # extensions for the XCTest module.
     if platform_developer_framework_dir:
-        linkopts.extend([
-            "-L{}".format(
-                _swift_developer_lib_dir(platform_developer_framework_dir),
+        swift_lib_dir_path = swift_developer_lib_dir([
+            struct(
+                developer_path_label = "platform",
+                path = platform_developer_framework_dir,
             ),
         ])
+        if swift_lib_dir_path:
+            linkopts.extend([
+                "-L{}".format(swift_lib_dir_path),
+            ])
 
     return CcInfo(
         linking_context = cc_common.create_linking_context(
@@ -311,23 +299,34 @@ def _swift_linkopts_cc_info(
         ),
     )
 
-def _test_linking_context(target_triple, toolchain_label):
+def _test_linking_context(target_triple, toolchain_label, xcode_config):
     """Returns a `CcLinkingContext` containing linker flags for test binaries.
 
     Args:
         target_triple: The target triple `struct`.
         toolchain_label: The label of the Swift toolchain that will act as the
             owner of the linker input propagating the flags.
+        xcode_config: The Xcode configuration.
 
     Returns:
         A `CcLinkingContext` that will provide linker flags to `swift_test`
         binaries.
     """
 
+    # Weakly link to XCTest. It's possible that machine that links the test
+    # binary will have Xcode installed at a different path than the machine that
+    # runs the binary. To handle this, the binary `dlopen`s XCTest at startup
+    # using the path Bazel passes in the test action's environment.
+    linkopts = [
+        "-Wl,-weak_framework,XCTest",
+        "-Wl,-weak-lXCTestSwiftSupport",
+    ]
+    if _is_xcode_at_least_version(xcode_config, "16.0"):
+        linkopts.append("-Wl,-weak_framework,Testing")
+
     # We use these as the rpaths for linking tests so that the required
     # libraries are found if Xcode is installed in a different location on the
     # machine that runs the tests than the machine used to link them.
-    linkopts = []
     for developer_dir in _DEVELOPER_DIR_SYMLINKS:
         platform_developer_framework_dir = _platform_developer_framework_dir(
             developer_dir,
@@ -735,6 +734,7 @@ def _xcode_swift_toolchain_impl(ctx):
     test_linking_context = _test_linking_context(
         target_triple = target_triple,
         toolchain_label = ctx.label,
+        xcode_config = xcode_config,
     )
 
     # `--define=SWIFT_USE_TOOLCHAIN_ROOT=<path>` is a rapid development feature