VS: Add option for per-target PlatformToolset

Add a `VS_PLATFORM_TOOLSET` target property to set `PlatformToolset` in
the `.vcxproj` file for specific targets.  Document that this is safe
only when the named toolset uses the same underlying compiler as the
primary toolset.

Fixes: #17429

3 files changed, 43 insertions, 0 deletions

diff --git a/Tests/RunCMake/VS10Project/RunCMakeTest.cmake b/Tests/RunCMake/VS10Project/RunCMakeTest.cmake
index 5ccca01..06ccaae 100644
--- a/Tests/RunCMake/VS10Project/RunCMakeTest.cmake
+++ b/Tests/RunCMake/VS10Project/RunCMakeTest.cmake
@@ -33,6 +33,7 @@ run_cmake(VsPrecompileHeadersReuseFromCompilePDBName)
 run_cmake(VsDeployEnabled)
 run_cmake(VsSettings)
 run_cmake(VsSourceSettingsTool)
+run_cmake(VsPlatformToolset)
 
 run_cmake(VsWinRTByDefault)
 
diff --git a/Tests/RunCMake/VS10Project/VsPlatformToolset-check.cmake b/Tests/RunCMake/VS10Project/VsPlatformToolset-check.cmake
new file mode 100644
index 0000000..416220b
--- /dev/null
+++ b/Tests/RunCMake/VS10Project/VsPlatformToolset-check.cmake
@@ -0,0 +1,36 @@
+macro(ReadPlatformToolset tgt outvar)
+  set(vcProjectFile "${RunCMake_TEST_BINARY_DIR}/${tgt}.vcxproj")
+  if(NOT EXISTS "${vcProjectFile}")
+    set(RunCMake_TEST_FAILED "Project file ${tgt}.vcxproj does not exist.")
+    return()
+  endif()
+
+  set(HAVE_PlatformToolset 0)
+
+  file(STRINGS "${vcProjectFile}" lines)
+  foreach(line IN LISTS lines)
+    if(line MATCHES "^ *<PlatformToolset>([^<>]+)</PlatformToolset>")
+      set(${outvar} "${CMAKE_MATCH_1}")
+      set(HAVE_PlatformToolset 1)
+      break()
+    endif()
+  endforeach()
+
+  if(NOT HAVE_PlatformToolset)
+    set(RunCMake_TEST_FAILED "Project file ${tgt}.vcxproj does not have a <PlatformToolset> field.")
+    return()
+  endif()
+endmacro()
+
+ReadPlatformToolset(NormalPlatformToolset NORMAL_TOOLSET)
+ReadPlatformToolset(OverridenPlatformToolset OVERRIDEN_TOOLSET)
+
+if (NOT "${OVERRIDEN_TOOLSET}" STREQUAL "MyCustomToolset")
+  set(RunCMake_TEST_FAILED "Failed to override the platform toolset")
+  return()
+endif()
+
+if ("${NORMAL_TOOLSET}" STREQUAL "MyCustomToolset")
+  set(RunCMake_TEST_FAILED "Main toolset was overriden (it shouldn't)")
+  return()
+endif()
diff --git a/Tests/RunCMake/VS10Project/VsPlatformToolset.cmake b/Tests/RunCMake/VS10Project/VsPlatformToolset.cmake
new file mode 100644
index 0000000..dce9717
--- /dev/null
+++ b/Tests/RunCMake/VS10Project/VsPlatformToolset.cmake
@@ -0,0 +1,6 @@
+enable_language(CXX)
+
+add_library(NormalPlatformToolset foo.cpp)
+add_library(OverridenPlatformToolset foo.cpp)
+set_target_properties(OverridenPlatformToolset
+                      PROPERTIES VS_PLATFORM_TOOLSET MyCustomToolset)