add a comment as to the purpose of tainted regions

Author: nikomatsakis

src/librustc/infer/region_inference/mod.rs

@@ -276,6 +276,11 @@ pub struct RegionSnapshot {
     skolemization_count: u32,
 }
 
+/// When working with skolemized regions, we often wish to find all of
+/// the regions that are either reachable from a skolemized region, or
+/// which can reach a skolemized region, or both. We call such regions
+/// *tained* regions.  This struct allows you to decide what set of
+/// tainted regions you want.
 #[derive(Debug)]
 pub struct TaintDirections {
     incoming: bool,