Visitor pattern example doesn't compile.

[DzikaMandarynka]

Trying to compile Visitor pattern example on the current rust version returns an following error

error[E0432]: unresolved import `ast`
  --> src/main.rs:21:9
   |
21 |     use ast::*;
   |         ^^^
   |
   = note: `use` statements changed in Rust 2018; read more at <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html>
help: a similar path exists
   |
21 |     use crate::ast::*;
   |         +++++++

warning: unused variable: `n`
  --> src/main.rs:36:30
   |
36 |     fn visit_name(&mut self, n: &Name) -> i64 {
   |                              ^ help: if this is intentional, prefix it with an underscore: `_n`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

For more information about this error, try `rustc --explain E0432`.

It also produces a warning about a unused variable