I wanted to suggest a feature similar to inout: conditional purity. That is, sometimes a function is pure iff the delegates passed to it are pure, and as of right now, I don't think there's any way to document this other than by overloading the function as a template (which results in lots of code duplication).
The idea would be to make this a bit more general than just a particular fix for "pure", though, since if in the future, new qualifiers arise (I guess this might also work for @trusted and such), then the solution should work for those too.
