如何获取语法::: ast :: Ident的绝对名称？

我正在尝试从以下Rust代码中提取函数名称。

// example.rs

pub mod hello {
    pub mod world {
        pub fn greetings() {
            println!("Hello, world!")
        }
    }
}

这是尝试从中提取函数名称的代码example.rs。

//runner.rs

/*
 * This program will only compile with nightly Rust
 *
 * To compile
 * rustc runner.rs
 *
 * To run
 * LD_LIBRARY_PATH=$(rustc --print sysroot)/lib ./runner
*/

#![feature(rustc_private)]
extern crate syntax;

use syntax::visit::{ self, Visitor, FnKind };
use syntax::ast::{ FnDecl, Block, NodeId, Mac };
use syntax::codemap::{ Span };
use syntax::{ parse, ast };
use std::path::Path;

struct MyVisitor;

impl<'x> Visitor<'x> for MyVisitor {
    fn visit_fn<'v>(&mut self, fk: FnKind<'v>, fd: &'v FnDecl, b: &'v Block, s: Span, _: NodeId) {
        let name;
        match fk {
            visit::FnKind::Method(_ident, ref _method_sig, _option) => {
                name = (*_ident.name.as_str()).to_string();
            }
            visit::FnKind::ItemFn(_ident, ref _generics, _unsafety, _constness, _abi, _visibility) => {
                name = (*_ident.name.as_str()).to_string();
            }
            visit::FnKind::Closure => {
                name = "".to_string();
            }
        };
        println!("{}", name);
        visit::walk_fn(self, fk, fd, b, s);
    }

    fn visit_mac<'v>(&mut self, _mac: &'v Mac) {
        // do nothing
        // just overriding here because parent panics as
        // panic!("visit_mac disabled by default");
    }
}

fn build_crate(path: &std::path::Path) -> ast::Crate {
    let sess = parse::ParseSess::new();
    let filemap = sess.codemap().load_file(path).unwrap();
    let cfg = ast::CrateConfig::new();
    let reader = parse::lexer::StringReader::new(&sess.span_diagnostic, filemap);
    let mut parser = parse::parser::Parser::new(&sess, cfg, Box::new(reader));
    return parser.parse_crate_mod().unwrap();
}

fn main() {
    let krate = build_crate(Path::new("./example.rs"));
    let mut visitor = MyVisitor {};
    visit::walk_crate(&mut visitor, &krate);
}

问题是它打印greetings为输出，但是我要使用完全限定的名称，即hello::world::greetings。我怎么做？