RFC: Add --features=swift.static_stdlib

This PR added the ability to compile Swift binary with stdlib in Linux.

It added `-static-stdlib` to `swiftc` invocation and added
`static-stdlib-args.lnk` to linker flag.

However, there is an issue with Bazel cache:

```
bazel build examples:ddpg --features=swift.static_stdlib
```

It compiles.

Then:

```
bazel build examples:ddpg
```

See error:

```
/usr/bin/ld: cannot find -lDispatchStubs
/usr/bin/ld: cannot find -lCoreFoundation
```

It seems reused the previous autolink extract results.

Disable incremental build when -static-stdlib is presented.

swift/internal/compiling.bzl

@@ -66,6 +66,7 @@ load(
     "SWIFT_FEATURE_USE_OLD_DRIVER",
     "SWIFT_FEATURE_USE_PCH_OUTPUT_DIR",
     "SWIFT_FEATURE_VFSOVERLAY",
+    "SWIFT_FEATURE_STATIC_STDLIB",
     "SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS",
     "SWIFT_FEATURE__WMO_IN_SWIFTCOPTS",
 )
@@ -1037,6 +1038,16 @@ def compile_action_configs(
             ],
             configurators = [_conditional_compilation_flag_configurator],
         ),
+
+        # Enable the built modules to reference static Swift standard libraries.
+        swift_toolchain_config.action_config(
+            actions = [
+                swift_action_names.COMPILE,
+                swift_action_names.DERIVE_FILES,
+            ],
+            configurators = [swift_toolchain_config.add_arg("-static-stdlib")],
+            features = [SWIFT_FEATURE_STATIC_STDLIB],
+        ),
     ]
 
     # NOTE: The positions of these action configs in the list are important,

swift/internal/feature_names.bzl

@@ -291,6 +291,10 @@ SWIFT_FEATURE_ENABLE_SKIP_FUNCTION_BODIES = "swift.skip_function_bodies_for_deri
 # swift.coverage_prefix_map also remap the path in coverage data.
 SWIFT_FEATURE_REMAP_XCODE_PATH = "swift.remap_xcode_path"
 
+# If enabled the built binary will statically link Swift standard libraries.
+# This requires Swift 5.3.1
+SWIFT_FEATURE_STATIC_STDLIB = "swift.static_stdlib"
+
 # A private feature that is set by the toolchain if a flag enabling WMO was
 # passed on the command line using `--swiftcopt`. Users should never manually
 # enable, disable, or query this feature.

swift/internal/swift_toolchain.bzl

@@ -34,6 +34,7 @@ load(
     "SWIFT_FEATURE_USE_AUTOLINK_EXTRACT",
     "SWIFT_FEATURE_USE_MODULE_WRAP",
     "SWIFT_FEATURE_USE_RESPONSE_FILES",
+    "SWIFT_FEATURE_STATIC_STDLIB",
 )
 load(":features.bzl", "features_for_build_modes")
 load(
@@ -202,7 +203,8 @@ def _swift_unix_linkopts_cc_info(
         cpu,
         os,
         toolchain_label,
-        toolchain_root):
+        toolchain_root,
+        static_stdlib):
     """Returns a `CcInfo` containing flags that should be passed to the linker.
 
     The provider returned by this function will be used as an implicit
@@ -216,27 +218,43 @@ def _swift_unix_linkopts_cc_info(
         toolchain_label: The label of the Swift toolchain that will act as the
             owner of the linker input propagating the flags.
         toolchain_root: The toolchain's root directory.
+        static_stdlib: Whether to statically link Swift standard libraries.
 
     Returns:
         A `CcInfo` provider that will provide linker flags to binaries that
         depend on Swift targets.
     """
 
-    # TODO(#8): Support statically linking the Swift runtime.
-    platform_lib_dir = "{toolchain_root}/lib/swift/{os}".format(
-        os = os,
-        toolchain_root = toolchain_root,
-    )
+    if static_stdlib:
+        platform_lib_dir = "{toolchain_root}/lib/swift_static/{os}".format(
+            os = os,
+            toolchain_root = toolchain_root,
+        )
+    else:
+        platform_lib_dir = "{toolchain_root}/lib/swift/{os}".format(
+            os = os,
+            toolchain_root = toolchain_root,
+        )
+
+    linkopts = [
+        "-pie",
+        "-L{}".format(platform_lib_dir),
+        "-Wl,-rpath,{}".format(platform_lib_dir),
+    ]
+
+    # Appending generic linker args from Swift runtime.
+    if static_stdlib:
+        static_stdlib_args = "{platform_lib_dir}/static-stdlib-args.lnk".format(
+            platform_lib_dir = platform_lib_dir,
+        )
+        linkopts.append("@{}".format(static_stdlib_args))
 
     runtime_object_path = "{platform_lib_dir}/{cpu}/swiftrt.o".format(
         cpu = cpu,
         platform_lib_dir = platform_lib_dir,
     )
 
-    linkopts = [
-        "-pie",
-        "-L{}".format(platform_lib_dir),
-        "-Wl,-rpath,{}".format(platform_lib_dir),
+    linkopts += [
         "-lm",
         "-lstdc++",
         "-lrt",
@@ -273,6 +291,7 @@ def _swift_toolchain_impl(ctx):
             ctx.attr.os,
             ctx.label,
             toolchain_root,
+            SWIFT_FEATURE_STATIC_STDLIB in ctx.features,
         )
 
     # Combine build mode features, autoconfigured features, and required

tools/worker/work_processor.cc

@@ -81,6 +81,7 @@ void WorkProcessor::ProcessWorkRequest(
   std::string output_file_map_path;
   std::string emit_module_path;
   bool is_wmo = false;
+  bool is_static_stdlib = false;
   bool is_dump_ast = false;
 
   std::string prev_arg;
@@ -97,6 +98,8 @@ void WorkProcessor::ProcessWorkRequest(
       arg.clear();
     } else if (prev_arg == "-emit-module-path") {
       emit_module_path = arg;
+    } else if (arg == "-static-stdlib") {
+      is_static_stdlib = true;
     } else if (ArgumentEnablesWMO(arg)) {
       is_wmo = true;
     }
@@ -108,7 +111,7 @@ void WorkProcessor::ProcessWorkRequest(
     prev_arg = original_arg;
   }
 
-  bool is_incremental = !is_wmo && !is_dump_ast;
+  bool is_incremental = !is_wmo && !is_dump_ast && !is_static_stdlib;
 
   if (!output_file_map_path.empty()) {
     if (is_incremental) {