Shadowing in toIface* output

[ÉRDI Gergő]

Hi,

I'm trying to save (Prep'd) Core bindings right next to the serialized 
`ModIface` (so basically `put_`ing them into the same bytestream, after the 
`ModIface`), and that's exactly what the functions in `GHC.CoreToIface` 
seem to be for, so I expected it to Just Work. However, I noticed that I 
very frequently get problems with shadowing. For example, Core that looks 
like `\v{u1} v{u2} -> v{u1}` would get translated to `\v v -> v`, which is 
disastrous since these locally bound `Var`s are represented as just their 
`getOccFS` (i.e. the `FastString` `"v"`).

But this can't be right: if `toIfaceExpr` &c. would fail this blatently, 
then the unfoldings couldn't be saved & restored, which is something GHC 
itself does as part of normal `.hi` file handling. So clearly I must be 
doing something wrong.

So I guess my question could be, what could be causing `toIfaceExpr` (a 
pure function!) to behave this way for my Cores? But then, if I look at 
the implementation of `toIface*`, I can see that it really doesn't do 
anything smarter than just storing `getOccFS` in the interface (no 
uniques in sight)-- so maybe my *real* question is, what is GHC itself 
doing so that it doesn't have this same problem?

Thanks,
 	Gergo