new borrow checker (mass squash)

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 80df8fb91a5..a42f640a175 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -22,6 +22,12 @@ use opt_vec::OptVec;
 // children (potentially passing in different contexts to each), call
 // visit::visit_* to apply the default traversal algorithm (again, it can
 // override the context), or prevent deeper traversal by doing nothing.
+//
+// Note: it is an important invariant that the default visitor walks the body
+// of a function in "execution order" (more concretely, reverse post-order
+// with respect to the CFG implied by the AST), meaning that if AST node A may
+// execute before AST node B, then A is visited first.  The borrow checker in
+// particular relies on this property.
 
 // Our typesystem doesn't do circular types, so the visitor record can not
 // hold functions that take visitors. A vt enum is used to break the cycle.