Treatment of unknown pragmas

[Eric Seidel]

> > This sounds exactly like the existing ANN pragma, which is what I've wanted LiquidHaskell to move towards for a long time. What is wrong with using the ANN pragma?
> 
> Significant compilation performance penalty and extra recompilation.
> ANN pragmas is what HLint currently uses.

The extra recompilation is annoying for HLint, true, since you probably don't care about your annotations being visible from other modules, whereas LiquidHaskell does. 

But I'm surprised by the compilation performance penalty. I would have expected ANN to be fairly cheap. That seems worthy of a bug report, regardless of the current discussion about unknown pragmas.