Auto merge of #94131 - Mark-Simulacrum:fmt-string, r=oli-obk
Always format to internal String in FmtPrinter This avoids monomorphizing for different parameters, decreasing generic code instantiated downstream from rustc_middle -- locally seeing 7% unoptimized LLVM IR line wins on rustc_borrowck, for example. We likely can't/shouldn't get rid of the Result-ness on most functions, though some further cleanup avoiding fmt::Error where we now know it won't occur may be possible, though somewhat painful -- fmt::Write is a pretty annoying API to work with in practice when you're trying to use it infallibly.
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