Add special -parse-as-library handling for swift_binary

This matches the behavior implemented in
swiftlang/swift-package-manager#3410 for single
file modules.

swift/internal/swift_binary_test.bzl

@@ -140,6 +140,25 @@ def _configure_features_for_binary(
         unsupported_features = unsupported_features,
     )
 
+def _maybe_parse_as_library_copts(srcs):
+    """Returns a list of compiler flags depending on `main.swift`'s presence.
+
+    Now that the `@main` attribute exists and is becoming more common, in the
+    case there is a single file not named `main.swift`, we assume that it has a
+    `@main` annotation, in which case it needs to be parsed as a library, not
+    as if it has top level code. In the case this is the wrong assumption,
+    compilation or linking will fail.
+
+    Args:
+        srcs: A list of source files to check for the presence of `main.swift`.
+
+    Returns:
+        A list of compiler flags to add to `copts`
+    """
+    use_parse_as_library = len(srcs) == 1 and \
+                           srcs[0].basename != "main.swift"
+    return ["-parse-as-library"] if use_parse_as_library else []
+
 def _swift_linking_rule_impl(
         ctx,
         binary_path,
@@ -184,7 +203,8 @@ def _swift_linking_rule_impl(
         if not module_name:
             module_name = swift_common.derive_module_name(ctx.label)
 
-        copts = expand_locations(ctx, ctx.attr.copts, ctx.attr.swiftc_inputs)
+        copts = expand_locations(ctx, ctx.attr.copts, ctx.attr.swiftc_inputs) + \
+                _maybe_parse_as_library_copts(srcs)
 
         module_context, cc_compilation_outputs, other_compilation_outputs = swift_common.compile(
             actions = ctx.actions,

test/BUILD

@@ -5,6 +5,7 @@ load(":debug_settings_tests.bzl", "debug_settings_test_suite")
 load(":features_tests.bzl", "features_test_suite")
 load(":generated_header_tests.bzl", "generated_header_test_suite")
 load(":linking_tests.bzl", "linking_test_suite")
+load(":mainattr_tests.bzl", "mainattr_test_suite")
 load(":module_cache_settings_tests.bzl", "module_cache_settings_test_suite")
 load(":output_file_map_tests.bzl", "output_file_map_test_suite")
 load(":private_deps_tests.bzl", "private_deps_test_suite")
@@ -28,6 +29,8 @@ generated_header_test_suite(name = "generated_header")
 
 linking_test_suite(name = "linking")
 
+mainattr_test_suite(name = "mainattr")
+
 module_cache_settings_test_suite(name = "module_cache_settings")
 
 output_file_map_test_suite(name = "output_file_map")

test/fixtures/mainattr/BUILD

@@ -0,0 +1,23 @@
+load("//swift:swift.bzl", "swift_binary")
+
+package(
+    default_visibility = ["//test:__subpackages__"],
+)
+
+swift_binary(
+    name = "main",
+    srcs = ["main.swift"],
+)
+
+swift_binary(
+    name = "custommain",
+    srcs = ["custommain.swift"],
+)
+
+swift_binary(
+    name = "multiplefiles",
+    srcs = [
+        "file1.swift",
+        "file2.swift",
+    ],
+)

test/fixtures/mainattr/custommain.swift

@@ -0,0 +1,6 @@
+@main
+struct Foo {
+    static func main() {
+        print("Hello, world!")
+    }
+}

test/fixtures/mainattr/file1.swift

@@ -0,0 +1 @@
+print(foo())

test/fixtures/mainattr/file2.swift

@@ -0,0 +1,3 @@
+func foo() -> String {
+    return "foo"
+}

test/fixtures/mainattr/main.swift

@@ -0,0 +1 @@
+print("hi")

test/mainattr_tests.bzl

@@ -0,0 +1,38 @@
+"""Tests for validating @main related usage"""
+
+load(
+    "@build_bazel_rules_swift//test/rules:action_command_line_test.bzl",
+    "make_action_command_line_test_rule",
+)
+
+mainattr_test = make_action_command_line_test_rule()
+
+def mainattr_test_suite(name):
+    mainattr_test(
+        name = "{}_single_main".format(name),
+        not_expected_argv = ["-parse-as-library"],
+        mnemonic = "SwiftCompile",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/mainattr:main",
+    )
+
+    mainattr_test(
+        name = "{}_single_custom_main".format(name),
+        expected_argv = ["-parse-as-library"],
+        mnemonic = "SwiftCompile",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/mainattr:custommain",
+    )
+
+    mainattr_test(
+        name = "{}_multiple_files".format(name),
+        not_expected_argv = ["-parse-as-library"],
+        mnemonic = "SwiftCompile",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/mainattr:multiplefiles",
+    )
+
+    native.test_suite(
+        name = name,
+        tags = [name],
+    )