A bunch of changes related to eta reduction

This is a large collection of changes all relating to eta
reduction, originally triggered by #18993, but there followed
a long saga.

Specifics:

* Move state-hack stuff from GHC.Types.Id (where it never belonged)
  to GHC.Core.Opt.Arity (which seems much more appropriate).

* Add a crucial mkCast in the Cast case of
  GHC.Core.Opt.Arity.eta_expand; helps with T18223

* Add clarifying notes about eta-reducing to PAPs.
  See Note [Do not eta reduce PAPs]

* I moved tryEtaReduce from GHC.Core.Utils to GHC.Core.Opt.Arity,
  where it properly belongs.  See Note [Eta reduce PAPs]

* In GHC.Core.Opt.Simplify.Utils.tryEtaExpandRhs, pull out the code for
  when eta-expansion is wanted, to make wantEtaExpansion, and all that
  same function in GHC.Core.Opt.Simplify.simplStableUnfolding.  It was
  previously inconsistent, but it's doing the same thing.

* I did a substantial refactor of ArityType; see Note [ArityType].
  This allowed me to do away with the somewhat mysterious takeOneShots;
  more generally it allows arityType to describe the function, leaving
  its clients to decide how to use that information.

  I made ArityType abstract, so that clients have to use functions
  to access it.

* Make GHC.Core.Opt.Simplify.Utils.rebuildLam (was stupidly called
  mkLam before) aware of the floats that the simplifier builds up, so
  that it can still do eta-reduction even if there are some floats.
  (Previously that would not happen.)  That means passing the floats
  to rebuildLam, and an extra check when eta-reducting (etaFloatOk).

* In GHC.Core.Opt.Simplify.Utils.tryEtaExpandRhs, make use of call-info
  in the idDemandInfo of the binder, as well as the CallArity info. The
  occurrence analyser did this but we were failing to take advantage here.

  In the end I moved the heavy lifting to GHC.Core.Opt.Arity.findRhsArity;
  see Note [Combining arityType with demand info], and functions
  idDemandOneShots and combineWithDemandOneShots.

  (These changes partly drove my refactoring of ArityType.)

* In GHC.Core.Opt.Arity.findRhsArity
  * I'm now taking account of the demand on the binder to give
    extra one-shot info.  E.g. if the fn is always called with two
    args, we can give better one-shot info on the binders
    than if we just look at the RHS.

  * Don't do any fixpointing in the non-recursive
    case -- simple short cut.

  * Trim arity inside the loop. See Note [Trim arity inside the loop]

* Make SimpleOpt respect the eta-reduction flag
  (Some associated refactoring here.)

* I made the CallCtxt which the Simplifier uses distinguish between
  recursive and non-recursive right-hand sides.
     data CallCtxt = ... | RhsCtxt RecFlag | ...
  It affects two things:
     - We call an RHS context interesting only if it is non-recursive
       see Note [RHS of lets] in GHC.Core.Unfold
     - We only eta-reduce non-recursive RHS, rather than eta-reducing
       every lambda. I'm not sure about the "non-recursive" bit; ToDo.

* Remove eta-reduction in GHC.CoreToStg.Prep, a welcome simplification.
  See Note [No eta reduction needed in rhsToBody] in GHC.CoreToStg.Prep.

Other incidental changes

* Fix a fairly long-standing outright bug in the ApplyToVal case of
  GHC.Core.Opt.Simplify.mkDupableContWithDmds. I was failing to take the
  tail of 'dmds' in the recursive call, which meant the demands were All
  Wrong.  I have no idea why this has not caused problems before now.

* Delete dead function GHC.Core.Opt.Simplify.Utils.contIsRhsOrArg

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/GHC/StgToCmm/Bind.hs b/compiler/GHC/StgToCmm/Bind.hs
index 7d89b71309..6b6a802490 100644
--- a/compiler/GHC/StgToCmm/Bind.hs
+++ b/compiler/GHC/StgToCmm/Bind.hs
@@ -18,6 +18,7 @@ import GHC.Prelude hiding ((<*>))
 import GHC.Driver.Session
 
 import GHC.Core          ( AltCon(..) )
+import GHC.Core.Opt.Arity( isOneShotBndr )
 import GHC.Runtime.Heap.Layout
 import GHC.Unit.Module