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Specialize `try_destructure_mir_constant` for its sole user (pretty printing) We can't remove the query, as we need to invoke it from rustc_middle, but can only implement it in mir interpretation/const eval. r? `@RalfJung` for a first round. While we could move all the logic into pretty printing, that would end up duplicating a bit of code with const eval, which doesn't seem great either.