[test] Transform Swift Features.def into Lit available features

Take the `Features.def` file used in other parts of the code and create
a file that can be used from the LLVM Lit configuration files to add new
available features that can be checked from the tests with `REQUIRES`
and others.

The file `lit.swift-features.cfg.inc` is preprocessed by Clang and
generates a file with Python syntax that can be loaded from both
`lit.site.cfg.in` files. The preprocessing output is copied into the
different test directories in the build directory, and added it is added
as a dependency of them, so it will be generate when the test run or
when `Features.def` changes.

`EXPERIMENTAL_FEATURES` are only enabled if they are available in
production or the compiler is being built with assertions, while
`UPCOMING_FEATURES` and the rest of the `LANGUAGE_FEATURES` are always
available.

test/CMakeLists.txt

@@ -253,6 +253,16 @@ if(NOT "${COVERAGE_DB}" STREQUAL "")
       COMMENT "Touching covering tests")
 endif()
 
+if(SWIFT_COMPILER_IS_MSVC_LIKE)
+  set(C_PREPROCESSOR_COMMAND
+    "${CMAKE_C_COMPILER}" "/P" "/EP" "/I" "${SWIFT_MAIN_INCLUDE_DIR}" "/TC"
+    "/Fi<<<OUTPUT_FILE>>>")
+else()
+  set(C_PREPROCESSOR_COMMAND
+    "${CMAKE_C_COMPILER}" "-E" "-P" "-I${SWIFT_MAIN_INCLUDE_DIR}" "-x" "c" "-o"
+    "<<<OUTPUT_FILE>>>")
+endif()
+
 foreach(SDK ${SWIFT_SDKS})
   foreach(ARCH ${SWIFT_SDK_${SDK}_ARCHITECTURES})
     # macCatalyst needs to run two sets of tests: one with the normal macosx target triple
@@ -304,6 +314,38 @@ foreach(SDK ${SWIFT_SDKS})
           "${validation_test_bin_dir}/lit.site.cfg"
           "validation-test${VARIANT_SUFFIX}.lit.site.cfg")
 
+      set(test_lit_swift_features_cmd ${C_PREPROCESSOR_COMMAND})
+      list(
+        TRANSFORM test_lit_swift_features_cmd
+        REPLACE "<<<OUTPUT_FILE>>>" "${test_bin_dir}/lit.swift-features.cfg")
+      add_custom_command(
+        OUTPUT "${test_bin_dir}/lit.swift-features.cfg"
+        COMMAND
+          ${test_lit_swift_features_cmd}
+          "${CMAKE_CURRENT_SOURCE_DIR}/lit.swift-features.cfg.inc"
+        DEPENDS
+          "lit.swift-features.cfg.inc"
+          "${SWIFT_MAIN_INCLUDE_DIR}/swift/Basic/Features.def"
+      )
+
+      set(validation_test_lit_swift_features_cmd ${C_PREPROCESSOR_COMMAND})
+      list(
+        TRANSFORM validation_test_lit_swift_features_cmd
+        REPLACE "<<<OUTPUT_FILE>>>" "${validation_test_bin_dir}/lit.swift-features.cfg")
+      add_custom_command(
+        OUTPUT "${validation_test_bin_dir}/lit.swift-features.cfg"
+        COMMAND
+          ${validation_test_lit_swift_features_cmd}
+          "${CMAKE_CURRENT_SOURCE_DIR}/lit.swift-features.cfg.inc"
+        DEPENDS
+          "lit.swift-features.cfg.inc"
+          "${SWIFT_MAIN_INCLUDE_DIR}/swift/Basic/Features.def"
+      )
+      add_custom_target(lit_swift_features_cfg_${VARIANT_SUFFIX}
+        DEPENDS
+          "${test_bin_dir}/lit.swift-features.cfg"
+          "${validation_test_bin_dir}/lit.swift-features.cfg")
+
       set(test_dependencies)
       get_test_dependencies("${SDK}" test_dependencies)
 
@@ -341,6 +383,9 @@ foreach(SDK ${SWIFT_SDKS})
 
       set(validation_test_dependencies)
 
+      list(APPEND test_dependencies lit_swift_features_cfg_${VARIANT_SUFFIX})
+      list(APPEND validation_test_dependencies lit_swift_features_cfg_${VARIANT_SUFFIX})
+
       set(command_upload_stdlib)
       set(command_upload_swift_reflection_test)
       if("${SDK}" STREQUAL "IOS" OR "${SDK}" STREQUAL "TVOS" OR "${SDK}" STREQUAL "WATCHOS" OR "${SDK}" STREQUAL "XROS")

test/lit.site.cfg.in

@@ -182,5 +182,6 @@ if '@SWIFT_BUILD_SWIFT_SYNTAX@' == 'TRUE':
 # Let the main config do the real work.
 if config.test_exec_root is None:
     config.test_exec_root = os.path.dirname(lit.util.abs_path_preserve_drive(__file__))
+lit_config.load_config(config, os.path.join(config.test_exec_root, "lit.swift-features.cfg"))
 lit_config.load_config(
     config, os.path.join(config.swift_src_root, "test", "lit.cfg"))

test/lit.swift-features.cfg.inc

@@ -0,0 +1,33 @@
+/* lit.swift-feature.cfg.inc - Config for Swift features for 'lit' test runner */
+
+/*
+ * This source file is part of the Swift.org open source project
+ *
+ * Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
+ * Licensed under Apache License v2.0 with Runtime Library Exception
+ *
+ * See https://swift.org/LICENSE.txt for license information
+ * See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+ */
+
+/*
+ * This is a configuration file for the 'lit' test runner. It takes the
+ * Features.def for the language and creates 'lit' features associated with
+ * them when the Swift feature is available. It allows using
+ * `REQUIRES: FeatureName` instead of `REQUIRES: asserts`, which tend to be
+ * left behind when the Swift feature becomes non-experimental.
+ *
+ * This file is preprocessed the Clang preprocessor and generates Python files.
+ * C comments are possible, but not Python comments (they look like preprocessor
+ * statements).
+ */
+
+def language_feature(feature_name, enabled):
+    if enabled or "asserts" in config.available_features:
+        config.available_features.add(feature_name)
+
+#define UPCOMING_FEATURE(FeatureName, SENumber, Version) language_feature("swift_feature_" # FeatureName, True)
+#define EXPERIMENTAL_FEATURE(FeatureName, AvailableInProd) language_feature("swift_feature_" # FeatureName, #AvailableInProd == "true")
+#define LANGUAGE_FEATURE(FeatureName, SENumber, Description) language_feature("swift_feature_" # FeatureName, True)
+
+#include <swift/Basic/Features.def>

validation-test/lit.site.cfg.in

@@ -145,4 +145,5 @@ config.freestanding_sdk_name = "@SWIFT_SDK_FREESTANDING_LIB_SUBDIR@"
 
 # Let the main config do the real work.
 config.test_exec_root = os.path.dirname(os.path.realpath(__file__))
+lit_config.load_config(config, os.path.join(config.test_exec_root, "lit.swift-features.cfg"))
 lit_config.load_config(config, "@SWIFT_SOURCE_DIR@/validation-test/lit.cfg")