Allow swiftc to correctly recognize main.swift as script target.

* This change removes forced -parse-as-library mode in favor of the driver figuring out how to parse each source. This means that main.swift will be parsed as a script and can contain top-level expressions.

--
MOS_MIGRATED_REVID=138285669

2 files changed, 73 insertions, 4 deletions

diff --git a/src/test/shell/bazel/apple/bazel_apple_test.sh b/src/test/shell/bazel/apple/bazel_apple_test.sh
index 667e98e174..1f8f9ea485 100755
--- a/src/test/shell/bazel/apple/bazel_apple_test.sh
+++ b/src/test/shell/bazel/apple/bazel_apple_test.sh
@@ -222,7 +222,7 @@ function test_swift_import_objc_framework() {
   # Copy the prebuilt framework into app's directory.
   cp -RL "${BAZEL_RUNFILES}/tools/build_defs/apple/test/testdata/BlazeFramework.framework" ios
 
-  cat >ios/main.swift <<EOF
+  cat >ios/app.swift <<EOF
 import UIKit
 
 import BlazeFramework
@@ -248,7 +248,7 @@ objc_binary(name = "bin",
             deps = [":swift_lib"])
 
 swift_library(name = "swift_lib",
-              srcs = ["main.swift"],
+              srcs = ["app.swift"],
               deps = [":dylib"])
 
 objc_framework(name = "dylib",
@@ -752,4 +752,56 @@ EOF
   expect_log "-Xlinker -add_ast_path -Xlinker bazel-out/local-fastbuild/genfiles/ios/swift_lib/_objs/ios_swift_lib.swiftmodule"
 }
 
+function test_swiftc_script_mode() {
+  rm -rf ios
+  mkdir -p ios
+  touch ios/foo.swift
+
+  cat >ios/top.swift <<EOF
+print() // Top level expression outside of main.swift, should fail.
+EOF
+
+  cat >ios/main.swift <<EOF
+import UIKit
+
+class AppDelegate: UIResponder, UIApplicationDelegate {}
+
+#if swift(>=3)
+UIApplicationMain(
+  CommandLine.argc,
+  UnsafeMutableRawPointer(CommandLine.unsafeArgv)
+    .bindMemory(
+      to: UnsafeMutablePointer<Int8>.self,
+      capacity: Int(CommandLine.argc)),
+  nil,
+  NSStringFromClass(AppDelegate.self)
+)
+#else
+UIApplicationMain(
+  Process.argc, UnsafeMutablePointer<UnsafeMutablePointer<CChar>>(Process.unsafeArgv),
+  nil, NSStringFromClass(AppDelegate)
+)
+#endif
+EOF
+
+cat >ios/BUILD <<EOF
+load("//tools/build_defs/apple:swift.bzl", "swift_library")
+
+swift_library(name = "main_should_compile_as_script",
+              srcs = ["main.swift", "foo.swift"])
+swift_library(name = "top_should_not_compile_as_script",
+              srcs = ["top.swift"])
+swift_library(name = "single_source_should_compile_as_library",
+              srcs = ["foo.swift"])
+EOF
+
+  bazel build --verbose_failures --xcode_version=$XCODE_VERSION \
+      //ios:single_source_should_compile_as_library \
+      //ios:main_should_compile_as_script >$TEST_log 2>&1 || fail "should build"
+
+  ! bazel build --verbose_failures --xcode_version=$XCODE_VERSION \
+      //ios:top_should_not_compile_as_script >$TEST_log 2>&1 || fail "should not build"
+  expect_log "ios/top.swift:1:1: error: expressions are not allowed at the top level"
+}
+
 run_suite "apple_tests"
diff --git a/tools/build_defs/apple/swift.bzl b/tools/build_defs/apple/swift.bzl
index 336329f537..447134377e 100644
--- a/tools/build_defs/apple/swift.bzl
+++ b/tools/build_defs/apple/swift.bzl
@@ -116,6 +116,21 @@ def _swift_xcrun_args(ctx):
   return []
 
 
+def _swift_parsing_flags(ctx):
+  """Returns additional parsing flags for swiftc."""
+  srcs = ctx.files.srcs
+
+  # swiftc has two different parsing modes: script and library.
+  # The difference is that in script mode top-level expressions are allowed.
+  # This mode is triggered when the file compiled is called main.swift.
+  # Additionally, script mode is used when there's just one file in the
+  # compilation. we would like to avoid that and therefore force library mode
+  # when there's only one source and it's not called main.
+  if len(srcs) == 1 and srcs[0].basename != "main.swift":
+    return ["-parse-as-library"]
+  return []
+
+
 def _is_valid_swift_module_name(string):
   """Returns True if the string is a valid Swift module name."""
   if not string:
@@ -288,7 +303,6 @@ def _swift_library_impl(ctx):
       module_name,
       "-emit-objc-header-path",
       output_header.path,
-      "-parse-as-library",
       "-target",
       target,
       "-sdk",
@@ -297,11 +311,14 @@ def _swift_library_impl(ctx):
       module_cache_path(ctx),
       "-output-file-map",
       swiftc_output_map_file.path,
-  ] + _swift_compilation_mode_flags(ctx) + _swift_bitcode_flags(ctx)
+  ]
 
   if ctx.configuration.coverage_enabled:
     args.extend(["-profile-generate", "-profile-coverage-mapping"])
 
+  args.extend(_swift_compilation_mode_flags(ctx))
+  args.extend(_swift_bitcode_flags(ctx))
+  args.extend(_swift_parsing_flags(ctx))
   args.extend(srcs_args)
   args.extend(include_args)
   args.extend(framework_args)