[Backtracing] Fix Windows build.

While I was doing this, it turns out Saleem was fixing things to avoid
having to patch the Windows include directories, which is awesome but
necessitates an extra change to the backtracing stuff to make the build
not fail on Windows.

rdar://105409147

Author: al45tair

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -2567,6 +2567,11 @@ function(_add_swift_target_executable_single name)
 
   string(MAKE_C_IDENTIFIER "${name}" module_name)
 
+  if(SWIFTEXE_SINGLE_SDK STREQUAL WINDOWS)
+    list(APPEND SWIFTEXE_SINGLE_COMPILE_FLAGS
+      -vfsoverlay;"${SWIFT_WINDOWS_VFS_OVERLAY}")
+  endif()
+
   handle_swift_sources(
       dependency_target
       unused_module_dependency_target
@@ -2602,7 +2607,6 @@ function(_add_swift_target_executable_single name)
       ${SWIFTEXE_SINGLE_ARCHITECTURE}_INCLUDE)
     target_include_directories(${name} SYSTEM PRIVATE
       ${${SWIFTEXE_SINGLE_ARCHITECTURE}_INCLUDE})
-
     if(NOT ${CMAKE_C_COMPILER_ID} STREQUAL MSVC)
       # MSVC doesn't support -Xclang. We don't need to manually specify
       # the dependent libraries as `cl` does so.
@@ -2612,6 +2616,7 @@ function(_add_swift_target_executable_single name)
         "SHELL:-Xclang --dependent-lib=msvcrt$<$<CONFIG:Debug>:d>")
     endif()
   endif()
+
   target_compile_options(${name} PRIVATE
     ${c_compile_flags})
   target_link_directories(${name} PRIVATE

stdlib/public/Backtracing/Context.swift

@@ -268,7 +268,12 @@ extension arm_gprs {
   }
   #endif
 
-  #if arch(x86_64)
+  #if os(Windows)
+  struct NotYetImplemented: Error {}
+  public static func withCurrentContext<T>(fn: (X86_64Context) throws -> T) throws -> T {
+    throw NotYetImplemented()
+  }
+  #elseif arch(x86_64)
   @_silgen_name("_swift_get_cpu_context")
   static func _swift_get_cpu_context() -> X86_64Context
 
@@ -418,7 +423,12 @@ extension arm_gprs {
 
   public static var registerCount: Int { return 50 }
 
-  #if arch(i386)
+  #if os(Windows)
+  struct NotYetImplemented: Error {}
+  public static func withCurrentContext<T>(fn: (I386Context) throws -> T) throws -> T {
+    throw NotYetImplemented()
+  }
+  #elseif arch(i386)
   @_silgen_name("_swift_get_cpu_context")
   static func _swift_get_cpu_context() -> I386Context
 
@@ -614,7 +624,12 @@ extension arm_gprs {
   }
 #endif
 
-  #if arch(arm64)
+  #if os(Windows)
+  struct NotYetImplemented: Error {}
+  public static func withCurrentContext<T>(fn: (ARM64Context) throws -> T) throws -> T {
+    throw NotYetImplemented()
+  }
+  #elseif arch(arm64)
   @_silgen_name("_swift_get_cpu_context")
   static func _swift_get_cpu_context() -> ARM64Context
 
@@ -748,14 +763,19 @@ extension arm_gprs {
 
   public static var registerCount: Int { return 16 }
 
-#if arch(arm)
+  #if os(Windows)
+  struct NotYetImplemented: Error {}
+  public static func withCurrentContext<T>(fn: (ARMContext) throws -> T) throws -> T {
+    throw NotYetImplemented()
+  }
+  #elseif arch(arm)
   @_silgen_name("_swift_get_cpu_context")
   static func _swift_get_cpu_context() -> ARMContext
 
   public static func withCurrentContext<T>(fn: (ARMContext) throws -> T) rethrows -> T {
     return try fn(_swift_get_cpu_context())
   }
-#endif
+  #endif
 
   private func isValid(_ register: Register) -> Bool {
     if register.rawValue < 16 {

stdlib/public/Backtracing/get-cpu-context.asm

@@ -1,2 +1,4 @@
 ;;; Placeholder for Windows assembly code
 ;;; This uses different syntax from everywhere else :-(
+
+        end

stdlib/public/libexec/swift-backtrace/Utils.swift

@@ -51,6 +51,8 @@ internal func parseUInt64<S: StringProtocol>(_ s: S) -> UInt64? {
   }
 }
 
+#if os(macOS)
+
 struct PosixError: Error {
   var errno: Int32
 
@@ -137,3 +139,4 @@ internal func spawn(_ path: String, args: [String]) throws {
   }
 }
 
+#endif // os(macOS)