Improve warnings for rules that might not fire
Two main things here * Previously we only warned about the "head" function of the rule, but actually the warning applies to any free variable on the LHS. * We now warn not only when one of these free vars can inline, but also if it has an active RULE (c.f. Trac #10528) See Note [Rules and inlining/other rules] in Desugar This actually shows up quite a few warnings in the libraries, notably in Control.Arrow, where it correctly points out that rules like "compose/arr" forall f g . (arr f) . (arr g) = arr (f . g) might never fire, because the rule for 'arr' (dictionary selection) might fire first. I'm not really sure what to do here; there is some discussion in Trac #10595. A minor change is adding BasicTypes.pprRuleName to pretty-print RuleName.
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