Use the eager unifier in the constraint solver

This patch continues the refactoring of the constraint solver
described in #23070.

The Big Deal in this patch is to call the regular, eager unifier from the
constraint solver, when we want to create new equalities. This
replaces the existing, unifyWanted which amounted to
yet-another-unifier, so it reduces duplication of a rather subtle
piece of technology. See

  * Note [The eager unifier] in GHC.Tc.Utils.Unify
  * GHC.Tc.Solver.Monad.wrapUnifierTcS

I did lots of other refactoring along the way

* I simplified the treatment of right hand sides that contain CoercionHoles.
  Now, a constraint that contains a hetero-kind CoercionHole is non-canonical,
  and cannot be used for rewriting or unification alike.  This required me
  to add the ch_hertero_kind flag to CoercionHole, with consequent knock-on
  effects. See wrinkle (2) of `Note [Equalities with incompatible kinds]` in
  GHC.Tc.Solver.Equality.

* I refactored the StopOrContinue type to add StartAgain, so that after a
  fundep improvement (for example) we can simply start the pipeline again.

* I got rid of the unpleasant (and inefficient) rewriterSetFromType/Co functions.
  With Richard I concluded that they are never needed.

* I discovered Wrinkle (W1) in Note [Wanteds rewrite Wanteds] in
  GHC.Tc.Types.Constraint, and therefore now prioritise non-rewritten equalities.

Quite a few error messages change, I think always for the better.

Compiler runtime stays about the same, with one outlier: a 17% improvement in T17836

Metric Decrease:
    T17836
    T18223

1 files changed, 13 insertions, 0 deletions

diff --git a/compiler/GHC/Tc/Solver.hs b/compiler/GHC/Tc/Solver.hs
index eaa62e44ea..9c371af463 100644
--- a/compiler/GHC/Tc/Solver.hs
+++ b/compiler/GHC/Tc/Solver.hs
@@ -3088,6 +3088,18 @@ neededEvVars implic@(Implic { ic_given = givens
 
 -------------------------------------------------
 simplifyDelayedErrors :: Bag DelayedError -> TcS (Bag DelayedError)
+-- Simplify any delayed errors: e.g. type and term holes
+-- NB: At this point we have finished with all the simple
+--     constraints; they are in wc_simple, not in the inert set.
+--     So those Wanteds will not rewrite these delayed errors.
+--     That's probably no bad thing.
+--
+--     However if we have [W] alpha ~ Maybe a, [W] alpha ~ Int
+--     and _ : alpha, then we'll /unify/ alpha with the first of
+--     the Wanteds we get, and thereby report (_ : Maybe a) or
+--     (_ : Int) unpredictably, depending on which we happen to see
+--     first.  Doesn't matter much; there is a type error anyhow.
+--     T17139 is a case in point.
 simplifyDelayedErrors = mapBagM simpl_err
   where
     simpl_err :: DelayedError -> TcS DelayedError
@@ -3104,6 +3116,7 @@ simplifyDelayedErrors = mapBagM simpl_err
      -- code, because we have all the givens already set up
     simpl_hole h@(Hole { hole_ty = ty, hole_loc = loc })
       = do { ty' <- rewriteType loc ty
+           ; traceTcS "simpl_hole" (ppr ty $$ ppr ty')
            ; return (h { hole_ty = ty' }) }
 
 {- Note [Delete dead Given evidence bindings]