Add the swift_common.use_toolchain() helper.

This function currently returns an empty list so that adding it to the rules/aspects is a no-op. A future change will have it return a list containing the Swift toolchain type, enabling new toolchain resolution.

PiperOrigin-RevId: 471075965

swift/internal/swift_protoc_gen_aspect.bzl

@@ -41,6 +41,7 @@ load(
     "proto_import_path",
     "register_module_mapping_write_action",
 )
+load(":toolchain_utils.bzl", "use_swift_toolchain")
 load(":utils.bzl", "get_compilation_contexts")
 
 # The paths of proto files bundled with the runtime. This is mainly the well
@@ -552,4 +553,5 @@ detail of the `swift_proto_library` rule.
 """,
     fragments = ["cpp"],
     implementation = _swift_protoc_gen_aspect_impl,
+    toolchains = use_swift_toolchain(),
 )

swift/internal/swift_symbol_graph_aspect.bzl

@@ -25,7 +25,7 @@ file in the parent directory.
 load(":attrs.bzl", "swift_toolchain_attrs")
 load(":features.bzl", "configure_features")
 load(":symbol_graph_extracting.bzl", "extract_symbol_graph")
-load(":toolchain_utils.bzl", "get_swift_toolchain")
+load(":toolchain_utils.bzl", "get_swift_toolchain", "use_swift_toolchain")
 load("@bazel_skylib//lib:dicts.bzl", "dicts")
 load(
     "@build_bazel_rules_swift//swift:providers.bzl",
@@ -162,4 +162,5 @@ default value is {default_value}.
         fragments = ["cpp"],
         implementation = aspect_impl,
         provides = [SwiftSymbolGraphInfo],
+        toolchains = use_swift_toolchain(),
     )

swift/internal/toolchain_utils.bzl

@@ -14,7 +14,7 @@
 
 """Helpers used to depend on and access the Swift toolchain."""
 
-SWIFT_TOOLCHAIN_TYPE = "@build_bazel_rules_swift//toolchains:toolchain_type"
+SWIFT_TOOLCHAIN_TYPE = "@build_bazel_rules_swift//toolchains:toolchain-type"
 
 def get_swift_toolchain(ctx, attr = "_toolchain"):
     """Gets the Swift toolchain associated with the rule or aspect.
@@ -41,4 +41,25 @@ def get_swift_toolchain(ctx, attr = "_toolchain"):
 
     fail("To use `swift_common.get_toolchain`, you must declare the " +
          "toolchain in your rule using " +
-         "`toolchains = [swift_common.toolchain_type()]`.")
+         "`toolchains = swift_common.use_toolchain()`.")
+
+def use_swift_toolchain():
+    """Returns a list of toolchain types needed to use the Swift toolchain.
+
+    This function returns a list so that it can be easily composed with other
+    toolchains if necessary. For example, a rule with multiple toolchain
+    dependencies could write:
+
+    ```
+    toolchains = swift_common.use_toolchain() + [other toolchains...]
+    ```
+
+    Returns:
+        A list of toolchain types that should be passed to `rule()` or
+        `aspect()`.
+    """
+
+    # TODO(b/205018581): Intentionally empty for now so that rule definitions
+    # can reference the function while still being a no-op. A future change will
+    # add the toolchain type to this list to enable toolchain resolution.
+    return []

swift/swift_binary.bzl

@@ -21,6 +21,10 @@ load(
     "malloc_linking_context",
     "register_link_binary_action",
 )
+load(
+    "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
+    "use_swift_toolchain",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:utils.bzl",
     "expand_locations",
@@ -133,4 +137,5 @@ please use one of the platform-specific application rules in
     executable = True,
     fragments = ["cpp"],
     implementation = _swift_binary_impl,
+    toolchains = use_swift_toolchain(),
 )

swift/swift_clang_module_aspect.bzl

@@ -52,6 +52,7 @@ load(
 load(
     "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
     "get_swift_toolchain",
+    "use_swift_toolchain",
 )
 load(
     "@build_bazel_rules_swift//swift/internal:utils.bzl",
@@ -782,4 +783,5 @@ it propagates for its targets.
         [apple_common.Objc],
         [CcInfo],
     ],
+    toolchains = use_swift_toolchain(),
 )

swift/swift_common.bzl

@@ -61,6 +61,7 @@ load(
 load(
     "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
     "get_swift_toolchain",
+    "use_swift_toolchain",
 )
 
 # The exported `swift_common` module, which defines the public API for directly
@@ -84,4 +85,5 @@ swift_common = struct(
     library_rule_attrs = swift_library_rule_attrs,
     precompile_clang_module = precompile_clang_module,
     toolchain_attrs = swift_toolchain_attrs,
+    use_toolchain = use_swift_toolchain,
 )

swift/swift_grpc_library.bzl

@@ -31,6 +31,10 @@ load(
     "proto_import_path",
     "register_module_mapping_write_action",
 )
+load(
+    "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
+    "use_swift_toolchain",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:utils.bzl",
     "compact",
@@ -490,4 +494,5 @@ swift_grpc_library(
 """,
     fragments = ["cpp"],
     implementation = _swift_grpc_library_impl,
+    toolchains = use_swift_toolchain(),
 )

swift/swift_import.bzl

@@ -32,6 +32,10 @@ load(
 )
 load(":providers.bzl", "SwiftInfo")
 load(":swift_common.bzl", "swift_common")
+load(
+    "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
+    "use_swift_toolchain",
+)
 
 def _swift_import_impl(ctx):
     archives = ctx.files.archives
@@ -180,4 +184,5 @@ Allows for the use of Swift textual module interfaces and/or precompiled Swift m
 """,
     fragments = ["cpp"],
     implementation = _swift_import_impl,
+    toolchains = use_swift_toolchain(),
 )

swift/swift_library.bzl

@@ -37,6 +37,10 @@ load(
     "@build_bazel_rules_swift//swift/internal:linking.bzl",
     "new_objc_provider",
 )
+load(
+    "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
+    "use_swift_toolchain",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:utils.bzl",
     "compact",
@@ -310,4 +314,5 @@ Compiles and links Swift code into a static library and Swift module.
     outputs = swift_library_output_map,
     implementation = _swift_library_impl,
     fragments = ["cpp"],
+    toolchains = use_swift_toolchain(),
 )

swift/swift_module_alias.bzl

@@ -18,6 +18,10 @@ load(
     "@build_bazel_rules_swift//swift/internal:linking.bzl",
     "new_objc_provider",
 )
+load(
+    "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
+    "use_swift_toolchain",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:utils.bzl",
     "compact",
@@ -185,4 +189,5 @@ symbol is defined; it is not repeated by the alias module.)
 """,
     fragments = ["cpp"],
     implementation = _swift_module_alias_impl,
+    toolchains = use_swift_toolchain(),
 )

swift/swift_test.bzl

@@ -29,6 +29,10 @@ load(
     "@build_bazel_rules_swift//swift/internal:swift_symbol_graph_aspect.bzl",
     "make_swift_symbol_graph_aspect",
 )
+load(
+    "@build_bazel_rules_swift//swift/internal:toolchain_utils.bzl",
+    "use_swift_toolchain",
+)
 load(
     "@build_bazel_rules_swift//swift/internal:utils.bzl",
     "expand_locations",
@@ -407,4 +411,5 @@ to your BUILD file's `package()` declaration instead of the individual targets.
     fragments = ["cpp"],
     test = True,
     implementation = _swift_test_impl,
+    toolchains = use_swift_toolchain(),
 )