DataCon wrappers get in the way of rules

This is a spin-off of #12618##12689:

Simon writes there:

Actually this is already a problem today. It's just rendered more prominent now that even (:) has a wrapper. Consider

data T = MkT {-# UNPACK #-} !Int

{-# RULES

"fT" f MkT = True
"gT" forall x. g (MkT x) = x
  #-}

f :: (Int -> T) -> Bool
{-# NOINLINE f #-}
f x = True

g :: T -> Int
{-# NOINLINE g #-}
g (MkT x) = x+1

yields

Foo.hs:9:1: warning: [-Winline-rule-shadowing]
    Rule "fT" may never fire because 'Foo.$WMkT' might inline first
    Probable fix: add an INLINE[n] or NOINLINE[n] pragma for 'Foo.$WMkT'

Foo.hs:10:1: warning: [-Winline-rule-shadowing]
    Rule "gT" may never fire because 'Foo.$WMkT' might inline first
    Probable fix: add an INLINE[n] or NOINLINE[n] pragma for 'Foo.$WMkT'

What to do? If we are to match these rules, we really must delay inlining the wrapper for MkT (after inlining we get a mess of unboxing etc). So either we must allow you to add a NOINLINE pragma to MkT; or we must add one automatically (e.g. NOINLINE [1]).

Delaying all consructor-wrapper inlining to phase 1 is potentially quite drastic, because case-of-known-constructor wouldn't happen until the wrappers are inlined. Maybe that's ok; I'm not sure. Worth trying I think.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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