[cmake][NFC] Check for incorrect usage of LLVM components in LINK_LIBS

[stephenneuendorffer]

Summary

Using LLVM components in LINK_LIBS means that the mechanisms for
replacing component dependencies with libLLVM.so break. Try to catch
this incorrect usage up front, instead of waiting until later when we
get difficult to understand runtime errors from incorrectly linked
libraries.

[mehdi_amini]

mlir/cmake/modules/AddMLIR.cmake

113

Is the kind of things like LINK_COMPONENTS documented somewhere? I always end-up having to reverse engineer all this from the implementation, but I'll forget about the detail by the next time I have to do something significant there

[stephenneuendorffer]

mlir/cmake/modules/AddMLIR.cmake

113

https://mlir.llvm.org/docs/Tutorials/CreatingADialect/#cmake-best-practices-1
https://llvm.org/docs/Projects.html

The LLVM global documentation is woefully limited, but I haven't gotten motivated to wade into it.

mlir/cmake/modules/AddMLIR.cmake

if(NOT ARG_EXCLUDE_FROM_LIBMLIR)
       set_property(GLOBAL APPEND PROPERTY MLIR_STATIC_LIBS ${name})
       set_property(GLOBAL APPEND PROPERTY MLIR_LLVM_LINK_COMPONENTS ${ARG_LINK_COMPONENTS})
       set_property(GLOBAL APPEND PROPERTY MLIR_LLVM_LINK_COMPONENTS ${LLVM_LINK_COMPONENTS})
     endif()
   endif()
 
   # MLIR libraries uniformly depend on LLVMSupport.  Just specify it once here.
   list(APPEND ARG_LINK_COMPONENTS Support)
+
+  # LINK_COMPONENTS is necessary to allow libLLVM.so to be properly
+  # substituted for individual library dependencies if LLVM_LINK_LLVM_DYLIB
+  # Perhaps this should be in llvm_add_library instead?  However, it fails
+  # on libclang-cpp.so
+  get_property(llvm_component_libs GLOBAL PROPERTY LLVM_COMPONENT_LIBS)
+  foreach(lib ${ARG_LINK_LIBS})
+    if(${lib} IN_LIST llvm_component_libs)
+      message(SEND_ERROR "${name} specifies LINK_LIBS ${lib}, but LINK_LIBS cannot be used for LLVM libraries.  Please use LINK_COMPONENTS instead.")
+    endif()
+  endforeach()
+
   list(APPEND ARG_DEPENDS mlir-generic-headers)
   llvm_add_library(${name} ${LIBTYPE} ${ARG_UNPARSED_ARGUMENTS} ${srcs} DEPENDS ${ARG_DEPENDS} LINK_COMPONENTS ${ARG_LINK_COMPONENTS} LINK_LIBS ${ARG_LINK_LIBS})
 
   if(TARGET ${name})
     target_link_libraries(${name} INTERFACE ${LLVM_COMMON_LIBS})
 
     if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
       set(export_to_mlirtargets)