Questions about the "Discussion" section of the Visitor pattern

[emme1444]

Regarding the visitor pattern found under Chapter 3.1.6, Section "Discussion".

It is common for the visit_* methods to return void (as opposed to in the example). In that case it is possible to factor out the
traversal code and share it between algorithms (and also to provide noop default methods). In Rust, the common way to do this
is to provide walk_* functions for each datum. For example,

I have a hard time understanding this section. I have a few questions:

• What is meant by "each datum" here? I'm guessing an AST node is a datum, but just to clearify.
• There is no self parameter in the example walk_expr function following the paragraph. Where would I put this function? At first I thought one would put it on "each datum", but then there is no self so yeah... it can't be a method. It could be an associated function, that might make sense I guess. Also, it does get an Expr as an argument, as a substitution for self i guess. But again just to clearify.
• The first parameter visitor is typed as &mut Visitor, however because Visitor is a trait I get a compiler error saying I should use the dyn keyword. This confuses me. Maybe it's just a typo?

Thanks!