[cmake] Respect SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER when compiling…

… the stdlib if SWIFT_NATIVE_SWIFT_TOOLS_PATH is not set and CMake_Swift_COMPILER is.

Previously, no matter if SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER was set, we
would use for swiftc SWIFT_NATIVE_SWIFT_TOOLS_PATH/bin/swiftc. This is correct
assuming that the user always passed in that flag. This will no longer always be
true since we are attempting to transition the stdlib slowly to use more
standard cmake. Instead, in that case if SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER
is set and SWIFT_NATIVE_SWIFT_TOOLS_PATH is not set /and/ we have a
CMAKE_Swift_COMPILER, we just use CMAKE_Swift_COMPILER. Hopefully with time we
get rid of SWIFT_NATIVE_SWIFT_TOOLS_PATH.

CMakeLists.txt

@@ -507,6 +507,11 @@ execute_process(COMMAND ${CMAKE_MAKE_PROGRAM} --version
 message(STATUS "CMake Make Program (${CMAKE_MAKE_PROGRAM}) Version: ${_CMAKE_MAKE_PROGRAM_VERSION}")
 message(STATUS "C Compiler (${CMAKE_C_COMPILER}) Version: ${CMAKE_C_COMPILER_VERSION}")
 message(STATUS "C++ Compiler (${CMAKE_CXX_COMPILER}) Version: ${CMAKE_CXX_COMPILER_VERSION}")
+if (CMAKE_Swift_COMPILER)
+  message(STATUS "Swift Compiler (${CMAKE_Swift_COMPILER}) Version: ${CMAKE_Swift_COMPILER_VERSION}")
+else()
+  message(STATUS "Swift Compiler (None).")
+endif()
 if(SWIFT_PATH_TO_CMARK_BUILD)
   execute_process(COMMAND ${SWIFT_PATH_TO_CMARK_BUILD}/src/cmark --version
     OUTPUT_VARIABLE _CMARK_VERSION
@@ -523,6 +528,13 @@ else()
   set(SWIFT_PREBUILT_CLANG TRUE)
 endif()
 
+# Also mark if we have a prebuilt swift before we do anything.
+if("${SWIFT_NATIVE_SWIFT_TOOLS_PATH}" STREQUAL "")
+  set(SWIFT_PREBUILT_SWIFT FALSE)
+else()
+  set(SWIFT_PREBUILT_SWIFT TRUE)
+endif()
+
 include(SwiftSharedCMakeConfig)
 
 # NOTE: We include this before SwiftComponents as it relies on some LLVM CMake

stdlib/cmake/modules/SwiftSource.cmake

@@ -630,7 +630,17 @@ function(_compile_swift_files
   if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
     set(HOST_EXECUTABLE_SUFFIX .exe)
   endif()
-  set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc${HOST_EXECUTABLE_SUFFIX}")
+  if(SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER)
+    if(SWIFT_PREBUILT_SWIFT)
+      set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc${HOST_EXECUTABLE_SUFFIX}")
+    elseif(CMAKE_Swift_COMPILER)
+      set(swift_compiler_tool "${CMAKE_Swift_COMPILER}")
+    else()
+      message(ERROR "Must pass in prebuilt tools using SWIFT_NATIVE_SWIFT_TOOLS_PATH or set CMAKE_Swift_COMPILER")
+    endif()
+  else()
+    set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc${HOST_EXECUTABLE_SUFFIX}")
+  endif()
 
   set(swift_compiler_tool_dep)
   if(SWIFT_INCLUDE_TOOLS)