Comment # 7 on bug 1091551 from Michal Srb

I have now added the Conflicts to devel:tools:compiler/llvm4 and submitted it
to Leap 42.3:
https://build.opensuse.org/request/show/603029

The libLLVM4 package can be installed in parallel with any of the llvm 3.8
packages, so it will be possible to install rust and Firefox together with the
original llvm installation.

Most of the remaining packages, such as llvm4, clang4, llvm4-devel, etc now
have conflicts with the appropriate llvm 3.8 packages, so they can not be
installed at the same time and there won't be any file conflicts.

I think you will have to change the BuildDependency in rust from llvm-devel to
llvm4-devel, because llvm-devel is from the original llvm 3.8.