feat(swift): add embedded Swift feature

doc/api.md

@@ -385,7 +385,7 @@ command line parameters file, those actions will be created here.
 | :------------- | :------------- | :------------- |
 | <a id="swift_common.create_linking_context_from_compilation_outputs-actions"></a>actions |  The context's `actions` object.   |  none |
 | <a id="swift_common.create_linking_context_from_compilation_outputs-additional_inputs"></a>additional_inputs |  A `list` of `File`s containing any additional files that are referenced by `user_link_flags` and therefore need to be propagated up to the linker.   |  `[]` |
-| <a id="swift_common.create_linking_context_from_compilation_outputs-alwayslink"></a>alwayslink |  If `False`, any binary that depends on the providers returned by this function will link in all of the library's object files only if there are symbol references. See the discussion on `swift_library` `alwayslink` for why that behavior could result in undesired results.   |  `True` |
+| <a id="swift_common.create_linking_context_from_compilation_outputs-alwayslink"></a>alwayslink |  If `False`, any binary that depends on the providers returned by this function will link in all of the library's object files only if there are symbol references. See the discussion on `swift_library` `alwayslink` for why that behavior could result in undesired results. If `True`, the opposite will happen and this library will be -force_load into the link actions that depend on it. By default (`None`), the value of alwayslink will be set to `True` in the general case, and to `False` if the embedded swift feature is turned on.   |  `None` |
 | <a id="swift_common.create_linking_context_from_compilation_outputs-compilation_outputs"></a>compilation_outputs |  A `CcCompilationOutputs` value containing the object files to link. Typically, this is the second tuple element in the value returned by `compile`.   |  none |
 | <a id="swift_common.create_linking_context_from_compilation_outputs-feature_configuration"></a>feature_configuration |  A feature configuration obtained from `configure_features`.   |  none |
 | <a id="swift_common.create_linking_context_from_compilation_outputs-is_test"></a>is_test |  Deprecated. This argument will be removed in the next major release. Use the `include_dev_srch_paths` attribute instead. Represents if the `testonly` value of the context.   |  `None` |

swift/internal/attrs.bzl

@@ -297,7 +297,6 @@ and emit a `.swiftinterface` file as one of the compilation outputs.
                 mandatory = False,
             ),
             "alwayslink": attr.bool(
-                default = True,
                 doc = """\
 If `False`, any binary that depends (directly or indirectly) on this Swift module
 will only link in all the object files for the files listed in `srcs` when there
@@ -318,7 +317,13 @@ get batched into a single `.o` by the compiler options used.
 Swift Package Manager always passes the individual `.o` files to the linker
 instead of using intermediate static libraries, so it effectively is the same
 as `alwayslink = True`.
+
+Note that by default, this value will default to True. But if the
+swift.enable_embedded feature is on, this value will default to False, as
+the swift features that cause -force_load to be required (such as reflection)
+are not available in that mode.
 """,
+                mandatory = False,
             ),
             "generated_header_name": attr.string(
                 doc = """\

swift/internal/compiling.bzl

@@ -44,6 +44,7 @@ load(
     "SWIFT_FEATURE_EMIT_PRIVATE_SWIFTINTERFACE",
     "SWIFT_FEATURE_EMIT_SWIFTDOC",
     "SWIFT_FEATURE_EMIT_SWIFTINTERFACE",
+    "SWIFT_FEATURE_ENABLE_EMBEDDED",
     "SWIFT_FEATURE_FULL_LTO",
     "SWIFT_FEATURE_HEADERS_ALWAYS_ACTION_INPUTS",
     "SWIFT_FEATURE_INDEX_WHILE_BUILDING",
@@ -1684,6 +1685,10 @@ def _emitted_output_nature(feature_configuration, user_compile_flags):
             feature_configuration = feature_configuration,
             feature_name = SWIFT_FEATURE__WMO_IN_SWIFTCOPTS,
         ) or
+        is_feature_enabled(
+            feature_configuration = feature_configuration,
+            feature_name = SWIFT_FEATURE_ENABLE_EMBEDDED,
+        ) or
         are_all_features_enabled(
             feature_configuration = feature_configuration,
             feature_names = [SWIFT_FEATURE_OPT, SWIFT_FEATURE_OPT_USES_WMO],

swift/internal/feature_names.bzl

@@ -411,3 +411,12 @@ SWIFT_FEATURE_FULL_LTO = "swift.full_lto"
 SWIFT_FEATURE_ENABLE_CPP17_INTEROP = "swift.enable_cpp17_interop"
 SWIFT_FEATURE_ENABLE_CPP20_INTEROP = "swift.enable_cpp20_interop"
 SWIFT_FEATURE_ENABLE_CPP23_INTEROP = "swift.enable_cpp23_interop"
+
+# Enable embedded swift. More details at https://docs.swift.org/embedded/documentation/embedded/waystogetstarted/
+# When enabling that option, a few things will happen:
+# * WMO will be enabled automatically, otherwise the compiler will throw an error as it is a requirement for embedded swift
+# * none of the libraries in the toolchain's usr/lib/swift directory will be linked. Instead, the core swift functionalities
+#   will be provided by the swift embedded module in the usr/lib/swift/embedded toolchain directory, and swiftc will
+#   automatically include it at compile time, and provide it to the linker at link time
+# * alwayslink on swift_library() target will now default to False, as it is not required in embedded mode
+SWIFT_FEATURE_ENABLE_EMBEDDED = "swift.enable_embedded"

swift/internal/linking.bzl

@@ -28,6 +28,7 @@ load(
 load(":developer_dirs.bzl", "developer_dirs_linkopts")
 load(
     ":feature_names.bzl",
+    "SWIFT_FEATURE_ENABLE_EMBEDDED",
     "SWIFT_FEATURE_LLD_GC_WORKAROUND",
     "SWIFT_FEATURE_OBJC_LINK_FLAGS",
 )
@@ -191,7 +192,7 @@ def create_linking_context_from_compilation_outputs(
         *,
         actions,
         additional_inputs = [],
-        alwayslink = True,
+        alwayslink = None,
         compilation_outputs,
         feature_configuration,
         is_test = None,
@@ -221,7 +222,11 @@ def create_linking_context_from_compilation_outputs(
             returned by this function will link in all of the library's object
             files only if there are symbol references. See the discussion on
             `swift_library` `alwayslink` for why that behavior could result
-            in undesired results.
+            in undesired results. If `True`, the opposite will happen and this
+            library will be -force_load into the link actions that depend on it.
+            By default (`None`), the value of alwayslink will be set to `True`
+            in the general case, and to `False` if the embedded swift feature is
+            turned on.
         compilation_outputs: A `CcCompilationOutputs` value containing the
             object files to link. Typically, this is the second tuple element in
             the value returned by `compile`.
@@ -322,6 +327,12 @@ def create_linking_context_from_compilation_outputs(
             ),
         )
 
+    if alwayslink == None:
+        alwayslink = not is_feature_enabled(
+            feature_configuration = feature_configuration,
+            feature_name = SWIFT_FEATURE_ENABLE_EMBEDDED,
+        )
+
     if include_dev_srch_paths != None and is_test != None:
         fail("""\
 Both `include_dev_srch_paths` and `is_test` cannot be specified. Please select \

swift/toolchains/config/compile_config.bzl

@@ -57,6 +57,7 @@ load(
     "SWIFT_FEATURE_ENABLE_CPP17_INTEROP",
     "SWIFT_FEATURE_ENABLE_CPP20_INTEROP",
     "SWIFT_FEATURE_ENABLE_CPP23_INTEROP",
+    "SWIFT_FEATURE_ENABLE_EMBEDDED",
     "SWIFT_FEATURE_ENABLE_LIBRARY_EVOLUTION",
     "SWIFT_FEATURE_ENABLE_SKIP_FUNCTION_BODIES",
     "SWIFT_FEATURE_ENABLE_TESTING",
@@ -386,6 +387,7 @@ def compile_action_configs(
             features = [
                 [SWIFT_FEATURE_OPT, SWIFT_FEATURE_OPT_USES_WMO],
                 [SWIFT_FEATURE__WMO_IN_SWIFTCOPTS],
+                [SWIFT_FEATURE_ENABLE_EMBEDDED],
             ],
         ),
 
@@ -1092,6 +1094,7 @@ def compile_action_configs(
             not_features = [
                 [SWIFT_FEATURE_OPT, SWIFT_FEATURE_OPT_USES_WMO],
                 [SWIFT_FEATURE__WMO_IN_SWIFTCOPTS],
+                [SWIFT_FEATURE_ENABLE_EMBEDDED],
             ],
         ),
 
@@ -1112,6 +1115,7 @@ def compile_action_configs(
             features = [
                 [SWIFT_FEATURE_OPT, SWIFT_FEATURE_OPT_USES_WMO],
                 [SWIFT_FEATURE__WMO_IN_SWIFTCOPTS],
+                [SWIFT_FEATURE_ENABLE_EMBEDDED],
             ],
             not_features = [SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS],
         ),
@@ -1337,6 +1341,17 @@ def compile_action_configs(
                 SWIFT_FEATURE_ENABLE_CPP23_INTEROP,
             ],
         ),
+        ActionConfigInfo(
+            actions = [
+                SWIFT_ACTION_COMPILE,
+            ],
+            configurators = [
+                add_arg("-enable-experimental-feature", "Embedded"),
+            ],
+            features = [
+                SWIFT_FEATURE_ENABLE_EMBEDDED,
+            ],
+        ),
     ]
 
     # NOTE: The positions of these action configs in the list are important,

test/features_tests.bzl

@@ -110,6 +110,14 @@ disable_objc_test = make_action_command_line_test_rule(
     },
 )
 
+embedded_test = make_action_command_line_test_rule(
+    config_settings = {
+        "//command_line_option:features": [
+            "swift.enable_embedded",
+        ],
+    },
+)
+
 def features_test_suite(name, tags = []):
     """Test suite for various features.
 
@@ -324,6 +332,17 @@ def features_test_suite(name, tags = []):
         ],
     )
 
+    embedded_test(
+        name = "{}_embedded_test".format(name),
+        tags = all_tags,
+        expected_argv = [
+            "-enable-experimental-feature",
+            "Embedded",
+        ],
+        mnemonic = "SwiftCompile",
+        target_under_test = "//test/fixtures/basic:second",
+    )
+
     native.test_suite(
         name = name,
         tags = all_tags,