We've been focused on first class Windows support since day one. The only major issue for LLVM itself that I'm aware of is the lack of PDB debug info, which is less of a problem for Rust because the system debuggers don't debug Rust in the first place. Most of the clang problems for Windows that I'm aware of relate to all the MSVC extensions in windows.h, which is not a problem for Rust as it doesn't use system header files.