TemplateHaskell: RULE TH:lift renders output of lift dependent on optimisation level

This is RULE TH:lift:

liftString :: Quote m => String -> m Exp
-- Used in GHC.Tc.Gen.Expr to short-circuit the lifting for strings
liftString s = return (LitE (StringL s))


-- TH has a special form for literal strings,
-- which we should take advantage of.
-- NB: the lhs of the rule has no args, so that
--     the rule will apply to a 'lift' all on its own
--     which happens to be the way the type checker
--     creates it.
{-# RULES "TH:liftString" lift = liftString #-}

I think we should try to remove it in favor of an overlapping instance for Lift [Char].

One reason is the fact that it only seems to trigger in the auto-specialised definition of lift @[Char].

The main reason is that optimisation may cause this RULE to apply more often, whenever an a in lift @[a] gets substituted to Char, hence lift @[Char]. This means we get either LitE (StringL []) or... ConE (List... []), and the former "potentially more often" than the latter with -O2. I'm convinced that optimisation level should not matter for the syntax produced by Lift, and the overlapping instance achieves this.