Add support for minimum_os_version in Swift rules

proto/swift_proto_utils.bzl

@@ -289,6 +289,7 @@ def compile_swift_protos_for_target(
         swift_infos = get_providers(compiler_deps, SwiftInfo),
         target_name = target_label.name,
         workspace_name = ctx.workspace_name,
+        minimum_os_version = ctx.attr.minimum_os_version,
     )
 
     module_context = compile_result.module_context

swift/internal/attrs.bzl

@@ -183,6 +183,15 @@ the target as `testonly = True`.
                 mandatory = False,
             ),
         } if include_dev_srch_paths_attrib else {},
+        {
+            "minimum_os_version": attr.string(
+                doc = """
+An optional string indicating the minimum OS version supported by the target,
+represented as a dotted version number (for example, "9.0").
+""",
+                mandatory = False,
+            ),
+        },
     )
 
 def swift_config_attrs():

swift/internal/compiling.bzl

@@ -346,6 +346,7 @@ def compile(
         target_name,
         toolchains = None,
         toolchain_type = SWIFT_TOOLCHAIN_TYPE,
+        minimum_os_version = None,
         workspace_name):
     """Compiles a Swift module.
 
@@ -413,6 +414,10 @@ def compile(
         toolchain_type: The toolchain type of the `swift_toolchain` which is
             used for the proper selection of the execution platform inside
             `run_toolchain_action`.
+        minimum_os_version: Overrides the `-target` os version. Either an
+            explicit dotted version number or the string "oldest", which will
+            translate to the oldest supported deployment version for the current
+            SDK.
         workspace_name: The name of the workspace for which the code is being
              compiled, which is used to determine unique file paths for some
              outputs.
@@ -681,6 +686,7 @@ to use swift_common.compile(include_dev_srch_paths = ...) instead.\
         genfiles_dir = feature_configuration._genfiles_dir,
         include_dev_srch_paths = include_dev_srch_paths_value,
         is_swift = True,
+        minimum_os_version = minimum_os_version,
         module_name = module_name,
         original_module_name = original_module_name,
         package_name = package_name,

swift/internal/target_triples.bzl

@@ -249,6 +249,7 @@ target_triples = struct(
     normalize_for_swift = _normalize_for_swift,
     parse = _parse,
     platform_name_for_swift = _platform_name_for_swift,
+    split_os_version = _split_os_version,
     str = _str,
     unversioned_os = _unversioned_os,
 )

swift/swift_binary.bzl

@@ -127,6 +127,7 @@ def _swift_binary_impl(ctx):
             toolchains = toolchains,
             target_name = ctx.label.name,
             workspace_name = ctx.workspace_name,
+            minimum_os_version = ctx.attr.minimum_os_version,
         )
         module_contexts.append(compile_result.module_context)
         compilation_outputs = compile_result.compilation_outputs

swift/swift_compiler_plugin.bzl

@@ -107,6 +107,7 @@ def _swift_compiler_plugin_impl(ctx):
         toolchains = toolchains,
         target_name = ctx.label.name,
         workspace_name = ctx.workspace_name,
+        minimum_os_version = ctx.attr.minimum_os_version,
     )
     module_context = compile_result.module_context
     module_contexts.append(module_context)

swift/swift_library.bzl

@@ -199,6 +199,7 @@ def _swift_library_impl(ctx):
         toolchains = toolchains,
         target_name = ctx.label.name,
         workspace_name = ctx.workspace_name,
+        minimum_os_version = ctx.attr.minimum_os_version,
     )
 
     module_context = compile_result.module_context

swift/swift_test.bzl

@@ -272,6 +272,7 @@ def _do_compile(
         toolchains = toolchains,
         target_name = name,
         workspace_name = workspace_name,
+        minimum_os_version = ctx.attr.minimum_os_version,
     )
 
 def _swift_test_impl(ctx):

swift/toolchains/xcode_swift_toolchain.bzl

@@ -405,6 +405,54 @@ def _make_resource_directory_configurator(developer_dir):
 
     return _resource_directory_configurator
 
+def _make_target_triple_configurator(base_target_triple, sdk_dir):
+    """Creates a configurator that emits the -target and -sdk flags.
+
+    This configurator dynamically adjusts the target triple based on
+    the minimum_os_version prerequisite, if present.
+
+    Args:
+        base_target_triple: The default target triple from the toolchain.
+        sdk_dir: The SDK directory path.
+
+    Returns:
+        A configurator function.
+    """
+
+    def _target_triple_configurator(prerequisites, args):
+        min_os = getattr(prerequisites, "minimum_os_version", None)
+        if min_os:
+            os, _ = target_triples.split_os_version(base_target_triple.os)
+
+            # Handle the "oldest" sentinel value.
+            # HACK: this should be collected from the SDK's Info.plist
+            if min_os == "oldest":
+                if os == "macos":
+                    version = "10.13"
+                elif os == "ios":
+                    version = "12.0"
+                else:
+                    fail(
+                        ("Determining oldest deployment version for '{}' is " +
+                         "unsupported").format(os),
+                    )
+            else:
+                version = min_os
+
+            triple = target_triples.make(
+                cpu = base_target_triple.cpu,
+                vendor = base_target_triple.vendor,
+                os = "{}{}".format(os, version),
+                environment = base_target_triple.environment,
+            )
+        else:
+            triple = base_target_triple
+
+        args.add("-target", target_triples.str(triple))
+        args.add("-sdk", sdk_dir)
+
+    return _target_triple_configurator
+
 def _all_action_configs(
         additional_objc_copts,
         additional_swiftc_copts,
@@ -448,8 +496,10 @@ def _all_action_configs(
                 SWIFT_ACTION_SYNTHESIZE_INTERFACE,
             ],
             configurators = [
-                add_arg("-target", target_triples.str(target_triple)),
-                add_arg("-sdk", apple_toolchain.sdk_dir()),
+                _make_target_triple_configurator(
+                    base_target_triple = target_triple,
+                    sdk_dir = apple_toolchain.sdk_dir(),
+                ),
             ],
         ),
     ]