Decorator

Cargo.toml

[package]
name = "decorator"
version = "0.0.1"

[lib]
name = "decorator_ext"
path = "lib.rs"
plugin = true
crate-type = ["dylib"]

[[bin]]
name = "decorator"
path = "main.rs"

lib.rs

#![allow(unstable)]
#![feature(box_syntax, plugin_registrar, quote)]

extern crate fmt_macros;
extern crate rustc;
extern crate syntax;

use rustc::plugin::Registry;
use syntax::ast;
use syntax::codemap::{DUMMY_SP, Span};
use syntax::ext::base;
use syntax::parse::token;
use syntax::ptr::P;

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_syntax_extension(
        token::intern("string"),
        base::Decorator(box expand_string)
    );
}

fn expand_string(cx: &mut base::ExtCtxt, _: Span, meta_item: &ast::MetaItem,
    item: &ast::Item, mut push: Box<FnMut(P<ast::Item>)>)
{
    let target = item.ident;
    let format = match meta_item.node {
        ast::MetaNameValue(_, ast::Lit {node: ast::LitStr(ref x, _), ..}) => x,
        _ => return
    };
    let format = format.get();
    let p = fmt_macros::Parser::new(format);
    let needed_members = p
        .filter_map(|x| match x {
            fmt_macros::Piece::NextArgument(fmt_macros::Argument {
                position: fmt_macros::Position::ArgumentNamed(name), ..
            }) => {
                let id = cx.ident_of(name);
                Some(quote_tokens!(cx, $id = self.$id))
            },
            _ => None
        })
        .collect::<Vec<_>>()
        .connect(&ast::TtToken(DUMMY_SP, token::Comma));
    push(quote_item!(cx,
        impl ::std::fmt::String for $target {
            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                write!(f, $format, $needed_members)
            }
        }
    ).unwrap());
}

main.rs

#![feature(plugin)]

#[plugin]
#[no_link]
extern crate decorator_ext;

#[string="{name}: Lv {level}"]
struct Pokemon {
    name: String,
    level: u8
}

fn main() {
    println!("{}", Pokemon {
        name: "ムクホーク".to_string(),
        level: 100
    });
}