improve comment on `LiveVar`

diff --git a/src/librustc_mir/borrow_check/nll/type_check/liveness/liveness_map.rs b/src/librustc_mir/borrow_check/nll/type_check/liveness/liveness_map.rs
index d3064d7165fb141c7ea0f9738bfd27924b37e043..15affbbc27a20b9468195ef68913de3dee843c1c 100644 (file)
--- a/src/librustc_mir/borrow_check/nll/type_check/liveness/liveness_map.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/liveness/liveness_map.rs
@@ -93,5 +93,10 @@ impl NllLivenessMap {
     }
 }
 
-/// Index given to each local variable whose type contains a region.
-newtype_index!(LiveVar);
+/// Index given to each local variable for which we need to
+/// compute liveness information. For many locals, we are able to
+/// skip liveness information: for example, those variables whose
+/// types contain no regions.
+newtype_index!(
+    LiveVar
+);