Adding traces causes too much recompilation due to specialisation

Calling the family of pprTrace functions sometimes leads to specialisation, which leads to orphan rules being created which then lead to all parent modules being recompiled. During development this can significantly increase the iteration time if you are building stage2 with optimisations.

One example of an orphan rule after adding one call to pprTraceIt in GHC.Driver.Env.

1288 "SPEC/GHC.Driver.Env $fOutputableGenWithIsBoot1 @ModuleName" [orphan] [2] forall (w['Many] :: GHC.Uti     ls.Outputable.Outputable                                                        
1289                                                                                                 GHC.U     nit.Module.Name.ModuleName).                                                    
1290   GHC.Unit.Types.$fOutputableGenWithIsBoot1 @GHC.Unit.Module.Name.ModuleName    
1291                                             w                                   
1292   = $dOutputable_$s$fOutputableGenWithIsBoot1 

Two possible solutions

• Performance shouldn't matter too much for tracing so I think a simple solution is just mark GHC.Driver.Ppr with -O0.
• Add SPECIALIZABLE pragmas to all Outputable instances?
• Add a flag which stops orphan/auto rules being exported, this stops the orphan module hash being updated and all dependent modules being recompiled. There is already some attempt to do this, in the case there are no orphans or finsts, but in the case of GHC there are always orphans and finsts. (See Note [Orphans and auto-generated rules])