cmake: fix MSVC PCH support

I'm not sure if the previous implementation ever worked.

Externals/imgui/CMakeLists.txt

@@ -14,6 +14,5 @@ target_include_directories(imgui PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}")
 
 target_link_libraries(imgui
 PRIVATE
-  common
   fmt::fmt
 )

Source/CMakeLists.txt

@@ -72,9 +72,8 @@ if (MSVC)
   add_compile_options(/external:W0)
   add_compile_options(/external:templates-)
 
+  # Compile PCH
   add_subdirectory(PCH)
-  add_definitions(/I${PCH_DIRECTORY})
-  add_definitions(/Yu${PCH_PATH})
 
   # Don't include timestamps in binaries
   add_link_options(/Brepro)

Source/Core/Common/CMakeLists.txt

@@ -308,3 +308,14 @@ if(UNIX)
 elseif(WIN32)
   target_link_libraries(common PRIVATE "-INCLUDE:enableCompatPatches")
 endif()
+
+if(MSVC)
+  # Add precompiled header
+  # it will propergate down to everything that depends on common
+  target_link_libraries(common PUBLIC pch)
+
+  # We need to disable PCH for this one file, because it's C and our PCH is C++
+  set_source_files_properties(
+    ${CMAKE_CURRENT_SOURCE_DIR}/ImageC.c
+    PROPERTIES COMPILE_FLAGS "/Y- /I${CMAKE_SOURCE_DIR}/Source/PCH/nopch")
+endif()

Source/PCH/CMakeLists.txt

@@ -1,6 +1,24 @@
+# The PCH that dolphin uses for MSVC is non-standard;
+# Instead of having one PCH per module, dolphin has one PCH shared between all modules.
+# So we need to implement PCH manually, rather than using the PCH support built into cmake
+
 add_library(pch pch.h pch.cpp)
-set(PCH_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-set(PCH_NAME ${PCH.pch})
-target_compile_options(pch PUBLIC /Ycpch.h /Fp${PCH_DIRECTORY}/${PCH_NAME})
+
+# pch.cpp should be compiled with the /Yc command, which creates the precompiled header
+target_compile_options(pch PRIVATE /Yc"pch.h" /Fo$<TARGET_FILE_DIR:pch>)
+
+# /Fp sets the location of the PCH. By forcing it to a fixed location, all modules
+# will share this one PCH
+target_compile_options(pch PUBLIC /Fp$<TARGET_FILE_DIR:pch>)
+
+# Sharing a PCH breaks pdb files. So we use the /Z7 option to inline the pdb into
+# the binary. That also requires us to disable minimal rebuilds.
+target_compile_options(pch PUBLIC /Z7 /Gm-)
+
+# targets which include pch need these compile options
+# /Yu - Use precompiled header named "pch.h"
+# /FI - Force include "pch.h" at top of every source file
+target_compile_options(pch INTERFACE /Yu"pch.h" /FI"pch.h")
+
 # fmt/format.h is included in the PCH
 target_link_libraries(pch PUBLIC fmt::fmt)

Source/PCH/nopch/pch.h

@@ -0,0 +1,4 @@
+// dummy include to help with disabling pch for a single file
+// cmake doesn't provide a clean way to disable MSVC's force include option
+
+// So we can just point it at an empty file instead.