Allow unbound Refl binders in a RULE
Trac #13410 was failing because we had a RULE with a binder (c :: t~t) and the /occurrences/ of c on the LHS were being optimised to Refl, leaving a binder that would not be filled in by matching the LHS of the rule. I flirted with trying to ensure that occurrences (c :: t~t) are not optimised to Relf, but that turned out to be fragile; it was being done, for good reasons, in multiple places, including - TyCoRep.substCoVarBndr - Simplify.simplCast - Corecion.mkCoVarCo So I fixed it in one place by making Rules.matchN deal happily with an unbound binder (c :: t~t). Quite easy. See "Coercion variables" in Note [Unbound RULE binders] in Rules. In addition, I needed to make CoreLint be happy with an bound RULE binder that is a Relf coercion variable In debugging this, I was perplexed that occurrences of a variable (c :: t~t) mysteriously turned into Refl. I found out how it was happening, and decided to move it: * In TyCoRep.substCoVarBndr, do not substitute Refl for a binder (c :: t~t). * In mkCoVarCo do not optimise (c :: t~t) to Refl. Instead, we do this optimisation in optCoercion (specifically opt_co4) where, surprisingly, the optimisation was /not/ being done. This has no effect on what programs compile; it just moves a relatively-expensive optimisation to optCoercion, where it seems more properly to belong. It's actually not clear to me which is really "better", but this way round is less surprising. One small simplifying refactoring * Eliminate TyCoRep.substCoVarBndrCallback, which was only called locally.
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