Can NamedFieldPuns be added to `GHC.LanguageExtensions.Types.Extension`?

[Alfredo Di Napoli]

Dear all,

As some of you might know, for the past few months I have been working on
changing GHC's diagnostic messages from plain SDocs to richer Haskell types.

As part of this work, I have added a mechanism to embed hints into
diagnostics, defined in `GHC.Types.Hint` in `HEAD`. One of the main
workhorse of this `GhcHint` type is the `SuggestExtension
LangExt.Extension` constructor, which embeds the extension to enable to use
a particular feature. The `LangExt.Extension` type comes from
`GHC.LanguageExtensions.Types`, and up until now there has always been a
1:1 mapping between the language pragma for the extension and the type
itself.

Today I was working on turning this error into a proper Haskell type:

badPun :: Located RdrName -> TcRnMessage
badPun fld = TcRnUnknownMessage $ mkPlainError noHints $
  vcat [text "Illegal use of punning for field" <+> quotes (ppr fld),
        text "Use NamedFieldPuns to permit this"]

I was ready to yield a `SuggestExtension LangExt.NamedFieldPuns` when I
discovered that there is no `NamedFieldPuns` constructor. Rather, there is
a `RecordPuns` , which refer to a deprecated flag, and we simply map
`NamedFieldPuns` back to it in `GHC.Driver.Session`:

...
  depFlagSpec' "RecordPuns"                   LangExt.RecordPuns
    (deprecatedForExtension "NamedFieldPuns"),
...
  flagSpec "NamedFieldPuns"                   LangExt.RecordPuns,
...

This is problematic for the `GhcHint` type, because now if I was to yield
`SuggestExtension LangExt.RecordPuns` to the user, I could still
pretty-print the suggestion to turn `RecordPuns` into `NamedFieldPuns`, but
this means that IDEs or third-party library would have access to the
"raw" Haskell datatype, and at that point they will be stuck with a
suggestion to enable a deprecated extension! (or best case scenario they
will have to transform the suggestion into something more sensible, which
partially defeats the point of this refactoring work I have been doing).

I am not sure this behaviour is unique for just `NamedFieldPuns`, but my
question is:

1. What prevents us from adding `NamedFieldPuns` as a proper constructor
for the `Extension` type and in principle remove `RecordPuns`? Backward
compatibility I assume?


Many thanks,

Alfredo
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