Hi, I came accros the same questioning and I achieved to modify the AST.

Basically when you did parse the file, you can take the AST as a mutable node and then work on it.
Here's an example with modifying the AST assuming the top node is a module:

fn main() -> Result<(), std::io::Error> {
    // Some stuff to load the file into ruby_file_content
    let parser = Parser::new(ruby_file_content, options);
    let result = parser.do_parse();
    let mut node = result.ast.expect("Failed to read AST from ParserResult");
    mutate_module(&mut node);
    // Some stuff over node
    return Ok(());
 }
 
 
fn mutate_module(node: &mut Box<Node>) {
    match node.as_mut() {
        Node::Module(module) => {
            if let Some(body) = module.body.as_mut() {
                match body.as_mut() {
                    Node::Begin(begin) => push_string_to_begin(begin),
                    Node::Class(_) => {
                        let mut new_body = make_begin(body.as_ref().clone());
                        push_string_to_begin(&mut new_body);
                        module.body = Some(Box::new(Node::Begin(new_body)));
                    }
                    _ => {}
                }
            }
        }
        _ => {}
    }
}

fn push_string_to_begin(begin: &mut Begin) {
    begin.statements.push(Node::Str(Str {
        value: Bytes::new("test".as_bytes().to_vec()),
        begin_l: Some(Loc { begin: 0, end: 1 }),
        end_l: Some(Loc { begin: 0, end: 1 }),
        expression_l: Loc { begin: 0, end: 1 },
    }))
}

fn make_begin(node: Node) -> Begin {
    return Begin {
        statements: vec![node],
        begin_l: None,
        end_l: None,
        expression_l: Loc { begin: 0, end: 1 },
    };
}

The function mutate_module will modify the module in the following way:

If the module is holding a begin statement (more than just holding a class): append the string "test" to the begin statement.
If the module is holding a class statement: transform that statement to a begin statement and append the string "test" to the begin statement.

Result when outputting the AST to a ruby file (see: https://github.com/NuriYuri/rust_ruby_code_writer/blob/main/src/main.rs ):

module Yuki
  class Tilemap
    class MapData
      # [...] some redacted code
    end
  end
  "test"
 end

Note: I'm still learning rust, I have no experience aside few tests I did ^^

Is there a way to modify AST and generate source code? #75

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