Add swiftmodule outputs to the linking step.

* This change enables most of LLDB functionality on binaries built by swift_library. 
It makes the linker record references to the .swiftmodule outputs, which then can be read by LLDB to deserialize Swift modules and provide debug info.

--
MOS_MIGRATED_REVID=136620475

2 files changed, 49 insertions, 1 deletions

diff --git a/src/test/shell/bazel/apple/bazel_apple_test.sh b/src/test/shell/bazel/apple/bazel_apple_test.sh
index a2546b3de7..9831b319f7 100755
--- a/src/test/shell/bazel/apple/bazel_apple_test.sh
+++ b/src/test/shell/bazel/apple/bazel_apple_test.sh
@@ -740,4 +740,42 @@ EOF
   expect_log "Error in target '//ios:bad-dep'"
 }
 
+function test_swift_ast_is_recorded() {
+  rm -rf ios
+  mkdir -p ios
+
+  touch ios/main.swift
+  touch ios/dep.swift
+
+  cat >ios/main.m <<EOF
+#import <UIKit/UIKit.h>
+
+int main(int argc, char *argv[]) {
+  @autoreleasepool {
+    return UIApplicationMain(argc, argv, nil, nil);
+  }
+}
+EOF
+
+  cat >ios/BUILD <<EOF
+load("//tools/build_defs/apple:swift.bzl", "swift_library")
+
+swift_library(name = "dep",
+              srcs = ["dep.swift"])
+
+swift_library(name = "swift_lib",
+              srcs = ["main.swift"],
+              deps = [":dep"])
+objc_binary(name = "bin",
+            srcs = ["main.m"],
+            deps = [":swift_lib"])
+EOF
+
+  bazel build --verbose_failures --ios_sdk_version=$IOS_SDK_VERSION \
+      --xcode_version=$XCODE_VERSION -s \
+      //ios:bin >$TEST_log 2>&1 || fail "should build"
+  expect_log "-Xlinker -add_ast_path -Xlinker bazel-out/local-fastbuild/genfiles/ios/dep/_objs/ios_dep.swiftmodule"
+  expect_log "-Xlinker -add_ast_path -Xlinker bazel-out/local-fastbuild/genfiles/ios/swift_lib/_objs/ios_swift_lib.swiftmodule"
+}
+
 run_suite "apple_tests"
diff --git a/tools/build_defs/apple/swift.bzl b/tools/build_defs/apple/swift.bzl
index 48596f5665..deb8027647 100644
--- a/tools/build_defs/apple/swift.bzl
+++ b/tools/build_defs/apple/swift.bzl
@@ -247,6 +247,15 @@ def _swift_library_impl(ctx):
   framework_args = ["-F%s" % x for x in framework_dirs]
   define_args = ["-D%s" % x for x in swiftc_defines]
 
+  # This tells the linker to write a reference to .swiftmodule as an AST symbol
+  # in the final binary.
+  # With dSYM enabled, this results in a __DWARF,__swift_ast section added to
+  # the dSYM binary, from where LLDB is able deserialize module information.
+  # Without dSYM, LLDB will follow the AST references, however there is a bug
+  # where it follows only the first one https://bugs.swift.org/browse/SR-2637
+  # This means that dSYM is required for debugging until that is resolved.
+  extra_linker_args = ["-Xlinker -add_ast_path -Xlinker " + output_module.path]
+
   clang_args = _intersperse(
       "-Xcc",
 
@@ -317,7 +326,8 @@ def _swift_library_impl(ctx):
       library=set([output_lib] + dep_libs),
       header=set([output_header]),
       providers=objc_providers,
-      linkopt=_swift_linkopts(ctx),
+      linkopt=_swift_linkopts(ctx) + extra_linker_args,
+      link_inputs=set([output_module]),
       uses_swift=True,)
 
   return struct(