CMAKE_TOOLCHAIN_FILE: Fix Windows saved input toolchain file syntax error

The value of _CMAKE_INPUT_TOOLCHAIN_FILE can contain backslashes when
Windows path separators are used. This commit ensures regression tests
check such cases and saves the value with a bracket argument to avoid
causing a syntax error due to the backslashes.

Fixes: #28024

Modules/CMakeDetermineSystem.cmake

@@ -227,7 +227,7 @@ if(CMAKE_BINARY_DIR)
   set(INCLUDE_CMAKE_TOOLCHAIN_FILE_IF_REQUIRED)
   if(CMAKE_TOOLCHAIN_FILE)
     string(CONCAT INCLUDE_CMAKE_TOOLCHAIN_FILE_IF_REQUIRED
-      "set(_CMAKE_INPUT_TOOLCHAIN_FILE \"${_CMAKE_INPUT_TOOLCHAIN_FILE}\")\n"
+      "set(_CMAKE_INPUT_TOOLCHAIN_FILE [==[${_CMAKE_INPUT_TOOLCHAIN_FILE}]==])\n"
       "set(_CMAKE_SYSTEM_TOOLCHAIN_FILE \"${CMAKE_TOOLCHAIN_FILE}\")\n"
       "include(\"\${_CMAKE_SYSTEM_TOOLCHAIN_FILE}\")"
     )

Tests/RunCMake/CompilerChange/RunCMakeTest.cmake

@@ -48,6 +48,15 @@ endif()
 # Set up "toolchain" files
 file(WRITE "${RunCMake_BINARY_DIR}/foo.cmake" "set(toolchain_var foo)\n")
 file(WRITE "${RunCMake_BINARY_DIR}/bar.cmake" "set(toolchain_var bar)\n")
+if(UNIX)
+  # Can't use file(TO_NATIVE_PATH) because it escapes spaces and cmake_path is
+  # 3.20
+  set(foo_TOOLCHAIN_FILE "${RunCMake_BINARY_DIR}/foo.cmake")
+  set(bar_TOOLCHAIN_FILE "${RunCMake_BINARY_DIR}/bar.cmake")
+else()
+  set(foo_TOOLCHAIN_FILE "${RunCMake_BINARY_DIR}\\foo.cmake")
+  set(bar_TOOLCHAIN_FILE "${RunCMake_BINARY_DIR}\\bar.cmake")
+endif()
 
 # New toolchain path comes from file
 block()
@@ -70,11 +79,11 @@ endblock()
 block()
   set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/ToolchainFromCmdline-build)
   run_cmake_with_options(ToolchainFromCmdline-step1
-    "-DCMAKE_TOOLCHAIN_FILE=${RunCMake_BINARY_DIR}/foo.cmake"
+    "-DCMAKE_TOOLCHAIN_FILE=${foo_TOOLCHAIN_FILE}"
   )
   set(RunCMake_TEST_NO_CLEAN 1)
   run_cmake_with_options(ToolchainFromCmdline-step2
-    "-DCMAKE_TOOLCHAIN_FILE=${RunCMake_BINARY_DIR}/bar.cmake"
+    "-DCMAKE_TOOLCHAIN_FILE=${bar_TOOLCHAIN_FILE}"
   )
 endblock()
 
@@ -85,7 +94,7 @@ block()
     ${RunCMake_SOURCE_DIR}/ToolchainFromCmdlineOnce-stdout.txt
   )
   run_cmake_with_options(ToolchainFromCmdlineOnce-step1
-    "-DCMAKE_TOOLCHAIN_FILE=${RunCMake_BINARY_DIR}/foo.cmake"
+    "-DCMAKE_TOOLCHAIN_FILE=${foo_TOOLCHAIN_FILE}"
   )
   set(RunCMake_TEST_NO_CLEAN 1)
   run_cmake(ToolchainFromCmdlineOnce-step2)

Tests/RunCMake/CompilerChange/ToolchainFromCmdline-step2-stderr.txt

@@ -2,4 +2,4 @@ You have changed variables that require your cache to be deleted.
 Configure will be re-run and you may have to reset some variables.
 The following variables have changed:
 CMAKE_TOOLCHAIN_FILE= [^
-]*/Tests/RunCMake/CompilerChange/bar\.cmake
+]*[\/]Tests[\/]RunCMake[\/]CompilerChange[\/]bar\.cmake