[clang] [llvm] [ARM] Emit an error when the hard-float ABI is enabled but can't be used. (PR #111334)

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workingjubilee wrote:

> I agree but was having trouble putting it into words. I don't have a reference I can put my hands on, but we have generally considered the llvm error messages to be a poor substitute to those produced by clang and have often gone the other way, converting llvm errors to clang errors.

I am not exactly sure how LLVM developers are imagining that frontend developers determine that they should diagnose a situation and provide a friendlier error? The primary motivation we have is that we notice that LLVM vomits up a nasty stacktrace in some situation, and then we figure it out and cover up the sewer hole that people could otherwise fallthrough to.

I suppose this also works if LLVM developers use their mystical powers to instantly transmit an awareness of all the situations every frontend developer should be diagnosing and erroring on. Was there a psychic mind meld I missed? Because I had to spend an entire night and day going over all the edge cases for the interrupt ABIs that we "support" in Rust, instead of merely possessing mystical knowledge about which cases the frontend should catch and error on.

https://github.com/llvm/llvm-project/pull/111334