simonpj:
| RULES are always parsed (no flags or language extensions needed). They | also go into the .hi files (unless you use the obscure option to change | that), so they are exported for all client modules.
The latter isn't true, and I think that's what Henning is objecting to.
Currently, without -O GHC puts the absolute minimum in interface files to get the clients to compile: just the exports and their types. For example, if you have f x = x GHC will not put that unfolding in the interface file, tiny though it is.
Currently without -O GHC therefore does *not* put RULES in the interface file. I thought that was consistent, since they are to do with optimisation.
If, however, there's a consensus that RULES should be persisted even without -O, that'd be easy to arrange. For example, I think that deprecations are persisted unconditionally.
So the next build with GHC 6.10 branch we should no longer see 'forall' parse failures from hackage libraries that use RULES? -- Don