[Backtracing] Disable assembly language on ARM Windows.

On ARM Windows, we need MARMASM to assemble the context capture code,
but CMake doesn't support MARMASM until version 3.26, which isn't yet
released.

(Confusingly it appears to have partial but non-working support in the
present versions.)

rdar://106284325

Author: al45tair

CMakeLists.txt

@@ -49,16 +49,24 @@ enable_language(CXX)
 # On Windows, use MASM or MARMASM
 set(SWIFT_ASM_DIALECT ASM)
 set(SWIFT_ASM_EXT S)
+set(SWIFT_ASM_AVAILABLE YES)
 if(CMAKE_SYSTEM_NAME STREQUAL Windows)
   if(CMAKE_SYSTEM_PROCESSOR MATCHES "ARM64")
-    set(SWIFT_ASM_DIALECT ASM_MARMASM)
+    if(CMAKE_VERSION VERSION_LESS "3.26")
+      message(warning "We can't build assembly language for ARM64 until CMake 3.26")
+      set(SWIFT_ASM_AVAILABLE NO)
+    else()
+      set(SWIFT_ASM_DIALECT ASM_MARMASM)
+    endif()
   else()
     set(SWIFT_ASM_DIALECT ASM_MASM)
   endif()
   set(SWIFT_ASM_EXT asm)
 endif()
 
-enable_language(${SWIFT_ASM_DIALECT})
+if(SWIFT_ASM_AVAILABLE)
+  enable_language(${SWIFT_ASM_DIALECT})
+endif()
 
 # Use C++14.
 set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")

stdlib/public/Backtracing/CMakeLists.txt

@@ -28,6 +28,15 @@ set(BACKTRACING_SOURCES
   get-cpu-context.${SWIFT_ASM_EXT}
 )
 
+set(BACKTRACING_COMPILE_FLAGS)
+
+if(SWIFT_ASM_AVAILABLE)
+  list(APPEND BACKTRACING_SOURCES get-cpu-context.${SWIFT_ASM_EXT})
+  list(APPEND BACKTRACING_COMPILE_FLAGS "-DSWIFT_ASM_AVAILABLE")
+else()
+  message(warning "Assembly language not available on this platform; backtracing will fail.")
+endif()
+
 set(LLVM_OPTIONAL_SOURCES
   get-cpu-context.S
   get-cpu-context.asm
@@ -48,6 +57,7 @@ add_swift_target_library(swift_Backtracing ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
 
   SWIFT_COMPILE_FLAGS
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+    ${BACKTRACING_COMPILE_FLAGS}
     -parse-stdlib
 
   LINK_FLAGS

stdlib/public/Backtracing/Context.swift

@@ -268,10 +268,10 @@ extension arm_gprs {
   }
   #endif
 
-  #if os(Windows)
-  struct NotYetImplemented: Error {}
+  #if os(Windows) || !SWIFT_ASM_AVAILABLE
+  struct NotImplemented: Error {}
   public static func withCurrentContext<T>(fn: (X86_64Context) throws -> T) throws -> T {
-    throw NotYetImplemented()
+    throw NotImplemented()
   }
   #elseif arch(x86_64)
   @_silgen_name("_swift_get_cpu_context")
@@ -423,10 +423,10 @@ extension arm_gprs {
 
   public static var registerCount: Int { return 50 }
 
-  #if os(Windows)
-  struct NotYetImplemented: Error {}
+  #if os(Windows) || !SWIFT_ASM_AVAILABLE
+  struct NotImplemented: Error {}
   public static func withCurrentContext<T>(fn: (I386Context) throws -> T) throws -> T {
-    throw NotYetImplemented()
+    throw NotImplemented()
   }
   #elseif arch(i386)
   @_silgen_name("_swift_get_cpu_context")
@@ -624,10 +624,10 @@ extension arm_gprs {
   }
 #endif
 
-  #if os(Windows)
-  struct NotYetImplemented: Error {}
+  #if os(Windows) || !SWIFT_ASM_AVAILABLE
+  struct NotImplemented: Error {}
   public static func withCurrentContext<T>(fn: (ARM64Context) throws -> T) throws -> T {
-    throw NotYetImplemented()
+    throw NotImplemented()
   }
   #elseif arch(arm64) || arch(arm64_32)
   @_silgen_name("_swift_get_cpu_context")
@@ -763,10 +763,10 @@ extension arm_gprs {
 
   public static var registerCount: Int { return 16 }
 
-  #if os(Windows)
-  struct NotYetImplemented: Error {}
+  #if os(Windows) || !SWIFT_ASM_AVAILABLE
+  struct NotImplemented: Error {}
   public static func withCurrentContext<T>(fn: (ARMContext) throws -> T) throws -> T {
-    throw NotYetImplemented()
+    throw NotImplemented()
   }
   #elseif arch(arm)
   @_silgen_name("_swift_get_cpu_context")