lib.rs

#![feature(proc_macro)]

extern crate proc_macro;

use proc_macro::*;

#[proc_macro]
pub fn foo(input: TokenStream) -> TokenStream {
    let mut body = Vec::<TokenTree>::new();

    // input = `let a = 2;`
    body.extend(input.into_iter());

    // manually add `let b = a;`
    body.push(Term::new("let", Span::call_site()).into());
    body.push(Term::new("b", Span::call_site()).into());
    body.push(Op::new('=', Spacing::Alone).into());
    body.push(Term::new("a", Span::call_site()).into());
    body.push(Op::new(';', Spacing::Alone).into());

    // parse and add `let c = b;`
    body.extend("let c = b;".parse::<TokenStream>().unwrap().into_iter());

    let mut ret = Vec::<TokenTree>::new();
    ret.push(Term::new("fn", Span::call_site()).into());
    ret.push(Term::new("main", Span::call_site()).into());
    ret.push(Group::new(Delimiter::Parenthesis, TokenStream::empty()).into());
    ret.push(Group::new(Delimiter::Brace, body.into_iter().collect()).into());
    ret.into_iter().collect()
}

main.rs

#![feature(proc_macro)]

extern crate foo;


foo::foo! {
    let a = 3;
}