Update compiler/rustc_typeck/src/check/region.rs

Author: nikomatsakis

compiler/rustc_typeck/src/check/region.rs

@@ -797,6 +797,11 @@ impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
 
 /// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
 /// in the case of closures, this will be redirected to the enclosing function.
+///
+/// Performance: This is a query rather than a simple function to enable
+/// re-use in incremental scenarios. We may sometimes need to rerun the
+/// type checker even when the HIR hasn't changed, and in those cases
+/// we can avoid reconstructing the region scope tree. 
 pub fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
     let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
     if typeck_root_def_id != def_id {