message: Add Visit::visit_children
  Summary: Useful in general.
diff: @@ -36,6 +36,16 @@ pub(crate) enum VisitMut<'a, P: AstPayload> {
  Expr(&'a mut AstExprP<P>),
  }
  +impl<'a, P: AstPayload> Visit<'a, P> {
  + #[allow(dead_code)]
  + pub(crate) fn visit_children(&self, mut f: impl FnMut(Visit<'a, P>)) {
  + match self {
  + Self::Stmt(x) => x.visit_children(f),
  + Self::Expr(x) => x.visit_expr(|x| f(Visit::Expr(x))),
  + }
  + }
  +}
  +
  impl<P: AstPayload> StmtP<P> {
  pub(crate) fn visit_children<'a>(&'a self, mut f: impl FnMut(Visit<'a, P>)) {
  match self {
  