Signaling suboptimal implementations

[Paolo Redaelli]

How can we signal sub-optimal implementations?

I'm reviewing the string cluster in order to remove the bug in deep_twin 
and I discovered that ROPE.infix "<" is currently O(n^2) while almost 
anyone will expect it to be O(n).

To achieve O(n) performance I would use iterators. But "new_iterator" 
allocates a new reference object. It is not wise to allocate subobjects 
in such basic features.

I wish to signal this, ideally during compilation.

I'm putting a note in the feature description but I'm wondering what's 
the best way to manage this issues.