Allow the Swift driver to be overridden by a command line flag.

swift/BUILD

@@ -25,3 +25,17 @@ bool_setting(
     name = "emit_swiftinterface",
     build_setting_default = False,
 )
+
+# Allows a user to override the default Swift driver during a build, if the
+# toolchain is using the default.
+label_flag(
+    name = "default_swift_executable",
+    build_setting_default = ":empty",
+)
+
+# Empty filegroup used as the default value for `:default_swift_executable`
+# since the `build_setting_default` value is required.
+filegroup(
+    name = "empty",
+    visibility = ["//visibility:private"],
+)

swift/internal/attrs.bzl

@@ -79,3 +79,37 @@ Linux), those dependencies will be **ignored.**
         ],
         **kwargs
     )
+
+def swift_toolchain_driver_attrs():
+    """Returns attributes used to attach custom drivers to toolchains.
+
+    These attributes are useful for compiler development alongside Bazel. The
+    public attribute (`swift_executable`) lets a custom driver be permanently
+    associated with a particular toolchain instance. If not specified, the
+    private default is associated with a command-line option that can be used to
+    provide a custom driver at build time.
+
+    Returns:
+        A dictionary of attributes that should be added to a toolchain rule.
+    """
+    return {
+        "swift_executable": attr.label(
+            allow_single_file = True,
+            cfg = "host",
+            doc = """\
+A replacement Swift driver executable.
+
+If this is empty, the default Swift driver in the toolchain will be used.
+Otherwise, this binary will be used and `--driver-mode` will be passed to ensure
+that it is invoked in the correct mode (i.e., `swift`, `swiftc`,
+`swift-autolink-extract`, etc.).
+""",
+        ),
+        "_default_swift_executable": attr.label(
+            allow_files = True,
+            cfg = "host",
+            default = Label(
+                "@build_bazel_rules_swift//swift:default_swift_executable",
+            ),
+        ),
+    }

swift/internal/swift_toolchain.bzl

@@ -22,13 +22,15 @@ toolchain, see `swift.bzl`.
 load("@bazel_skylib//lib:dicts.bzl", "dicts")
 load("@bazel_skylib//lib:partial.bzl", "partial")
 load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
+load(":attrs.bzl", "swift_toolchain_driver_attrs")
 load(
     ":features.bzl",
     "SWIFT_FEATURE_AUTOLINK_EXTRACT",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
     "features_for_build_modes",
 )
 load(":providers.bzl", "SwiftToolchainInfo")
+load(":utils.bzl", "get_swift_executable_for_toolchain")
 
 def _default_linker_opts(
         cc_toolchain,
@@ -106,8 +108,11 @@ def _swift_toolchain_impl(ctx):
     requested_features.extend(ctx.features)
     requested_features.append(SWIFT_FEATURE_AUTOLINK_EXTRACT)
 
+    # Swift.org toolchains assume everything is just available on the PATH so we
+    # we don't pass any files unless we have a custom driver executable in the
+    # workspace.
     all_files = []
-    swift_executable = ctx.file.swift_executable
+    swift_executable = get_swift_executable_for_toolchain(ctx)
     if swift_executable:
         all_files.append(swift_executable)
 
@@ -117,8 +122,6 @@ def _swift_toolchain_impl(ctx):
     return [
         SwiftToolchainInfo(
             action_environment = {},
-            # Swift.org toolchains assume everything is just available on the
-            # PATH and we don't try to pass the toolchain contents here.
             all_files = depset(all_files),
             cc_toolchain_info = cc_toolchain,
             command_line_copts = ctx.fragments.swift.copts(),
@@ -143,59 +146,49 @@ def _swift_toolchain_impl(ctx):
     ]
 
 swift_toolchain = rule(
-    attrs = dicts.add({
-        "arch": attr.string(
-            doc = """\
+    attrs = dicts.add(
+        swift_toolchain_driver_attrs(),
+        {
+            "arch": attr.string(
+                doc = """\
 The name of the architecture that this toolchain targets.
 
 This name should match the name used in the toolchain's directory layout for
 architecture-specific content, such as "x86_64" in "lib/swift/linux/x86_64".
 """,
-            mandatory = True,
-        ),
-        "os": attr.string(
-            doc = """\
+                mandatory = True,
+            ),
+            "os": attr.string(
+                doc = """\
 The name of the operating system that this toolchain targets.
 
 This name should match the name used in the toolchain's directory layout for
 platform-specific content, such as "linux" in "lib/swift/linux".
 """,
-            mandatory = True,
-        ),
-        "root": attr.string(
-            mandatory = True,
-        ),
-        "swift_executable": attr.label(
-            # TODO(allevato): Use a label-typed build setting to allow this to
-            # have a default that is overridden from the command line.
-            allow_single_file = True,
-            doc = """\
-A replacement Swift driver executable.
-
-If this is empty, the default Swift driver in the toolchain will be used.
-Otherwise, this binary will be used and `--driver-mode` will be passed to ensure
-that it is invoked in the correct mode (i.e., `swift`, `swiftc`,
-`swift-autolink-extract`, etc.).
-""",
-        ),
-        "_cc_toolchain": attr.label(
-            default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
-            doc = """\
+                mandatory = True,
+            ),
+            "root": attr.string(
+                mandatory = True,
+            ),
+            "_cc_toolchain": attr.label(
+                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+                doc = """\
 The C++ toolchain from which other tools needed by the Swift toolchain (such as
 `clang` and `ar`) will be retrieved.
 """,
-        ),
-        "_worker": attr.label(
-            cfg = "host",
-            allow_files = True,
-            default = Label("//tools/worker"),
-            doc = """\
+            ),
+            "_worker": attr.label(
+                cfg = "host",
+                allow_files = True,
+                default = Label("//tools/worker"),
+                doc = """\
 An executable that wraps Swift compiler invocations and also provides support
 for incremental compilation using a persistent mode.
 """,
-            executable = True,
-        ),
-    }),
+                executable = True,
+            ),
+        },
+    ),
     doc = "Represents a Swift compiler toolchain.",
     fragments = ["swift"],
     toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],

swift/internal/utils.bzl

@@ -144,6 +144,44 @@ def expand_locations(ctx, values, targets = []):
     """
     return [ctx.expand_location(value, targets) for value in values]
 
+def get_swift_executable_for_toolchain(ctx):
+    """Returns the Swift driver executable that the toolchain should use.
+
+    Args:
+        ctx: The toolchain's rule context.
+
+    Returns:
+        A `File` representing a custom Swift driver executable that the
+        toolchain should use if provided by the toolchain target or by a command
+        line option, or `None` if the default driver bundled with the toolchain
+        should be used.
+    """
+
+    # If the toolchain target itself specifies a custom driver, use that.
+    swift_executable = getattr(ctx.file, "swift_executable", None)
+
+    # If no custom driver was provided by the target, check the value of the
+    # command-line option and use that if it was provided.
+    if not swift_executable:
+        default_swift_executable_files = getattr(
+            ctx.files,
+            "_default_swift_executable",
+            None,
+        )
+
+        if default_swift_executable_files:
+            if len(default_swift_executable_files) > 1:
+                fail(
+                    "The 'default_swift_executable' option must point to a " +
+                    "single file, but we found {}".format(
+                        str(default_swift_executable_files),
+                    ),
+                )
+
+            swift_executable = default_swift_executable_files[0]
+
+    return swift_executable
+
 def get_output_groups(targets, group_name):
     """Returns files in an output group from each target in a list.
 

swift/internal/xcode_swift_toolchain.bzl

@@ -24,6 +24,7 @@ load("@bazel_skylib//lib:dicts.bzl", "dicts")
 load("@bazel_skylib//lib:partial.bzl", "partial")
 load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
+load(":attrs.bzl", "swift_toolchain_driver_attrs")
 load(
     ":features.bzl",
     "SWIFT_FEATURE_AUTOLINK_EXTRACT",
@@ -37,6 +38,7 @@ load(
     "features_for_build_modes",
 )
 load(":providers.bzl", "SwiftToolchainInfo")
+load(":utils.bzl", "get_swift_executable_for_toolchain")
 
 def _swift_developer_lib_dir(platform_framework_dir):
     """Returns the directory containing extra Swift developer libraries.
@@ -351,7 +353,8 @@ def _xcode_swift_toolchain_impl(ctx):
     #
     # To use a "standard" custom toolchain built using the full Swift build
     # script, use `--define=SWIFT_CUSTOM_TOOLCHAIN=<id>` as shown below.
-    swift_executable = ctx.file.swift_executable
+    swift_executable = get_swift_executable_for_toolchain(ctx)
+
     toolchain_root = ctx.var.get("SWIFT_USE_TOOLCHAIN_ROOT")
     custom_toolchain = ctx.var.get("SWIFT_CUSTOM_TOOLCHAIN")
 
@@ -440,46 +443,36 @@ def _xcode_swift_toolchain_impl(ctx):
     ]
 
 xcode_swift_toolchain = rule(
-    attrs = dicts.add({
-        "swift_executable": attr.label(
-            # TODO(allevato): Use a label-typed build setting to allow this to
-            # have a default that is overridden from the command line.
-            allow_single_file = True,
-            doc = """\
-A replacement Swift driver executable.
-
-If this is empty, the default Swift driver in the toolchain will be used.
-Otherwise, this binary will be used and `--driver-mode` will be passed to ensure
-that it is invoked in the correct mode (i.e., `swift`, `swiftc`,
-`swift-autolink-extract`, etc.).
-""",
-        ),
-        "_cc_toolchain": attr.label(
-            default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
-            doc = """\
+    attrs = dicts.add(
+        swift_toolchain_driver_attrs(),
+        {
+            "_cc_toolchain": attr.label(
+                default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+                doc = """\
 The C++ toolchain from which linking flags and other tools needed by the Swift
 toolchain (such as `clang`) will be retrieved.
 """,
-        ),
-        "_worker": attr.label(
-            cfg = "host",
-            allow_files = True,
-            default = Label(
-                "@build_bazel_rules_swift//tools/worker",
             ),
-            doc = """\
+            "_worker": attr.label(
+                cfg = "host",
+                allow_files = True,
+                default = Label(
+                    "@build_bazel_rules_swift//tools/worker",
+                ),
+                doc = """\
 An executable that wraps Swift compiler invocations and also provides support
 for incremental compilation using a persistent mode.
 """,
-            executable = True,
-        ),
-        "_xcode_config": attr.label(
-            default = configuration_field(
-                name = "xcode_config_label",
-                fragment = "apple",
+                executable = True,
             ),
-        ),
-    }),
+            "_xcode_config": attr.label(
+                default = configuration_field(
+                    name = "xcode_config_label",
+                    fragment = "apple",
+                ),
+            ),
+        },
+    ),
     doc = "Represents a Swift compiler toolchain provided by Xcode.",
     fragments = [
         "apple",