When attempting to build from vcbuild.bat (which has to be modified to detect VS2015 by looking for the VS140COMNTOOLS variable), there are 40 symbols that end up not being defined in the final link step:

_T_Library, _T_Routine, _PD_Struct, _T_Struct, _T_Callback, _Call_Routine, _Init_Serial_Scheme, _dtoa, _STRTOD, _Cmp_Struct, _MT_Struct, _MT_Routine, _Free_Routine, _reb_i32_add_overflow, _reb_u32_add_overflow, _reb_i64_add_overflow, _CT_Routine, _CT_Library, _CT_Callback, _CT_Struct, _Struct_To_Block, _reb_qsort, _reb_i64_sub_overflow, _reb_i64_mul_overflow, _Init_Core_Ext, _OS_Close_Window, _OS_Destroy_Graphics, _OS_Image_To_Cursor, _OS_Show_Soft_Keyboard, _OS_Get_Metrics, _OS_Init_Windows, _OS_Update_Windows, _OS_Open_Windows, _OS_Init_Graphics, _OS_Load_Cursor, _OS_Destroy_Cursor, _OS_Set_Cursor, _Dev_Serial, _WinMain

When attempting to build from the r3.sln, VS2015 has the following errors:

Severity Code Description Project File Line
Error C1083 Cannot open source file: 'C:\Users\Vincent\Documents\dev\rebol\src\libffi\src\x86\win32.S': No such file or directory libffi C:\Users\Vincent\Documents\dev\rebol\make\msvc\libffi\c1 1
Error A2088 END directive required at end of fiplai libffi C:\Users\Vincent\Documents\dev\rebol\make\msvc\libffi\Win32\Release\win32_plain.asm 1
Error C1083 Cannot open include file: 'ffi.h': No such file or directory r3-view C:\Users\Vincent\Documents\dev\rebol\src\core\t-routine.c 33