Support Linear FFI functions
## Summary
The desugaring of FFI-defined functions drops linearity information.
## Steps to reproduce
The following module
```
{-# LANGUAGE LinearTypes #-}
module II where
foreign import ccall "exp" c_exp :: Double #-> Double
```
fails linear lint:
```
*** Core Lint errors : in result of Desugar (before optimization) ***
II.hs:4:1: warning:
Linearity failure in lambda: ds_du1
'Many ⊈ 'One
In the RHS of c_exp :: Double #-> Double
In the body of lambda with binder ds_du1 :: Double
Substitution: [TCvSubst
In scope: InScope {}
Type env: []
Co env: []]
II.hs:4:1: warning:
Linearity failure in lambda: ds_du1
'Many ⊈ 'One
In the RHS of c_exp :: Double #-> Double
In the unfolding of c_exp :: Double #-> Double
In the body of lambda with binder ds_du1 :: Double
Substitution: [TCvSubst
In scope: InScope {}
Type env: []
Co env: []]
*** Offending Program ***
Rec {
$wccall_du7
:: Double# -> State# RealWorld -> (# State# RealWorld, Double# #)
[LclId]
$wccall_du7
= \ (ds_du3 :: Double#) ->
{__pkg_ccall_GC main Double#
-> State# RealWorld -> (# State# RealWorld, Double# #)}_du6
ds_du3
c_exp :: Double #-> Double
[LclIdX,
Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
WorkFree=True, Expandable=True,
Guidance=ALWAYS_IF(arity=1,unsat_ok=False,boring_ok=False)
Tmpl= \ (ds_du1 [Occ=Once!] :: Double) ->
case ds_du1 of { D# ds_du3 [Occ=Once] ->
case $wccall_du7 ds_du3 realWorld# of
{ (# _ [Occ=Dead], ds_du4 [Occ=Once] #) ->
D# ds_du4
}
}}]
c_exp
= (\ (ds_du1 :: Double) ->
case ds_du1 of ds_du2 { D# ds_du3 ->
case $wccall_du7 ds_du3 realWorld# of wild_00
{ (# ds_du5, ds_du4 #) ->
D# ds_du4
}
})
`cast` (<Double #-> Double>_R
:: (Double #-> Double) ~R# (Double #-> Double))
$trModule :: Module
[LclIdX]
$trModule = Module (TrNameS "main"#) (TrNameS "II"#)
end Rec }
*** End of Offense ***
```
## Expected behavior
Linear FFI functions are perfectly reasonable a thing to have. So we should desugar them properly. This was actually spotted in review: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/852/diffs?diff_id=9699#cf5b233b6ec5c0a6951d1018fee507d2d2d71391_274_274 . It's a matter of pushing the information until this spot.
## Environment
* GHC version used: current master
issue