Move record disambiguation to the renamer

Merged sheaf requested to merge sheaf/ghc:T21443 into master

This patch moves the field-based logic for disambiguating record updates to the renamer. The type-directed logic, scheduled for removal, remains in the typechecker.

To do this properly (and fix the myriad of bugs surrounding the treatment of duplicate record fields), we took the following main steps:

  1. Create GREInfo, a renamer-level equivalent to TyThing which stores information pertinent to the renamer. This allows us to uniformly treat imported and local Names in the renamer, as described in Note [GREInfo].

  2. Remove GreName. Instead of a GlobalRdrElt storing GreNames, which distinguished between normal names and field names, we now store simple Names in GlobalRdrElt, along with the new GREInfo information which allows us to recover the FieldLabel for record fields.

  3. Add namespacing for record fields, within the OccNames themselves. This allows us to remove the mangling of duplicate field selectors.

    This change ensures we don't print mangled names to the user in error messages, and allows us to handle duplicate record fields in Template Haskell.

  4. Move record disambiguation to the renamer, and operate on the level of data constructors instead, to handle #21443 (closed).

    The error message text for ambiguuous record updates has also been changed to reflect that type-directed disambiguation is on the way out.

(3) means that OccEnv is now a bit more complex: we first key on the textual name, which gives an inner map keyed on NameSpace:

OccEnv a ~ FastStringEnv (UniqFM NameSpace a)

Note that this change, along with (2), both increase the memory residency of GlobalRdrEnv = OccEnv [GlobalRdrElt], which causes a few tests to regress somewhat in compile-time allocation.

Even though (3) simplified a lot of code (in particular the treatment of field selectors within Template Haskell and in error messages), it came with one important wrinkle: in the situation of

  -- M.hs-boot
  module M where { data A; foo :: A -> Int }
  -- M.hs
  module M where { data A = MkA { foo :: Int } }

we have that M.hs-boot exports a variable foo, which is supposed to match with the record field foo that M exports. To solve this issue, we add a new impedance-matching binding to M

  foo{var} = foo{fld}

This mimics the logic that existed already for impedance-binding DFunIds, but getting it right was a bit tricky. See Note [Record field impedance matching] in GHC.Tc.Module.

Edited by sheaf

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