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Previously, the `Outputable` instance for `HsArg` was being used to pretty-print each `HsArgPar` in a list of `HsArg`s individually, which simply doesn't work. In lieu of the `Outputable` instance, we now use a dedicated `pprHsArgsApp` function to print a list of `HsArg`s as a single unit. I have also added documentation to the `Outputable` instance for `HsArg` to more clearly signpost that it is only suitable for debug pretty-printing. Fixes #19737.
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