Add the module_mapping flag to the Swift rules.

The value of this flag should be the label a `swift_module_mapping` target, which provides the aliases to use during the build.

At this time, module aliases are only being supported as a globally-applied build flag. This can be revisited in the future if aliases are needed for individual subgraphs, but this is much more complex to represent in BUILD.

This change only adds the flag and hooks it into the toolchain; the compilation changes will be added in a subsequent change.

PiperOrigin-RevId: 486659250

swift/BUILD

@@ -5,6 +5,7 @@ load(
     "per_module_swiftcopt_flag",
 )
 load(":swift_interop_hint.bzl", "swift_interop_hint")
+load(":swift_module_mapping.bzl", "swift_module_mapping")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -242,6 +243,20 @@ filegroup(
     visibility = ["//visibility:private"],
 )
 
+# Provides the mapping that will be passed to the compiler as module aliases.
+label_flag(
+    name = "module_mapping",
+    build_setting_default = ":empty_module_mapping",
+)
+
+# The default empty module mapping used when the `:module_mapping` flag is not
+# set.
+swift_module_mapping(
+    name = "empty_module_mapping",
+    aliases = {},
+    visibility = ["//visibility:private"],
+)
+
 # Consumed by Bazel integration tests.
 filegroup(
     name = "for_bazel_tests",

swift/providers.bzl

@@ -202,6 +202,10 @@ linking target (but not to precompiled explicit C/Objective-C modules):
 
 For ease of use, this field is never `None`; it will always be a valid `struct`
 containing the fields described above, even if those lists are empty.
+""",
+        "module_aliases": """\
+A `SwiftModuleAliasesInfo` provider that defines the module aliases to use
+during compilation.
 """,
         "package_configurations": """\
 A list of `SwiftPackageConfigurationInfo` providers that specify additional

swift/swift_module_mapping.bzl

@@ -29,7 +29,7 @@ def _swift_module_mapping_impl(ctx):
 
     aliases = ctx.attr.aliases
     new_names_seen = dict()
-    for original_name, new_name in aliases:
+    for original_name, new_name in aliases.items():
         previous_new_name = new_names_seen.get(new_name, None)
         if previous_new_name:
             fail((

swift/toolchains/BUILD

@@ -12,6 +12,7 @@ bzl_library(
         "//swift/internal:action_names",
         "//swift/internal:feature_names",
         "//swift/internal:features",
+        "//swift/internal:providers",
         "//swift/internal:target_triples",
         "//swift/internal:utils",
         "//swift/internal:wmo",
@@ -36,6 +37,7 @@ bzl_library(
         "//swift/internal:attrs",
         "//swift/internal:feature_names",
         "//swift/internal:features",
+        "//swift/internal:providers",
         "//swift/internal:target_triples",
         "//swift/internal:utils",
         "//swift/internal:wmo",

swift/toolchains/xcode_swift_toolchain.bzl

@@ -51,6 +51,10 @@ load(
     "@build_bazel_rules_swift//swift/internal:features.bzl",
     "features_for_build_modes",
 )
+load(
+    "@build_bazel_rules_swift//swift/internal:providers.bzl",
+    "SwiftModuleAliasesInfo",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:target_triples.bzl",
     "target_triples",
@@ -791,6 +795,13 @@ def _xcode_swift_toolchain_impl(ctx):
         xcode_config = xcode_config,
     )
 
+    module_aliases = ctx.attr._module_mapping[SwiftModuleAliasesInfo].aliases
+    if module_aliases and not _is_xcode_at_least_version(xcode_config, "14.0"):
+        fail(
+            "Module mappings are only supported by Swift 5.7 (Xcode 14.0) " +
+            "and higher.",
+        )
+
     swift_toolchain_info = SwiftToolchainInfo(
         action_configs = all_action_configs,
         cc_toolchain_info = cc_toolchain,
@@ -818,6 +829,7 @@ def _xcode_swift_toolchain_impl(ctx):
                 swift_linkopts_providers.objc_info,
             ],
         ),
+        module_aliases = module_aliases,
         package_configurations = [
             target[SwiftPackageConfigurationInfo]
             for target in ctx.attr.package_configurations
@@ -919,6 +931,12 @@ The C++ toolchain from which linking flags and other tools needed by the Swift
 toolchain (such as `clang`) will be retrieved.
 """,
             ),
+            "_module_mapping": attr.label(
+                default = Label(
+                    "@build_bazel_rules_swift//swift:module_mapping",
+                ),
+                providers = [[SwiftModuleAliasesInfo]],
+            ),
             "_worker": attr.label(
                 cfg = "exec",
                 allow_files = True,