src/test/run-pass-fulldeps/auxiliary/cond_plugin.rs

// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// no-prefer-dynamic

#![crate_type = "proc-macro"]
#![feature(proc_macro, proc_macro_non_items)]

extern crate proc_macro;

use proc_macro::*;

#[proc_macro]
pub fn cond(input: TokenStream) -> TokenStream {
    let mut conds = Vec::new();
    let mut input = input.into_iter().peekable();
    while let Some(tree) = input.next() {
        let cond = match tree {
            TokenTree::Group(tt) => tt.stream(),
            _ => panic!("Invalid input"),
        };
        let mut cond_trees = cond.clone().into_iter();
        let test = cond_trees.next().expect("Unexpected empty condition in `cond!`");
        let rhs = cond_trees.collect::<TokenStream>();
        if rhs.is_empty() {
            panic!("Invalid macro usage in cond: {}", cond);
        }
        let is_else = match test {
            TokenTree::Term(word) => word.as_str() == "else",
            _ => false,
        };
        conds.push(if is_else || input.peek().is_none() {
            quote!({ $rhs })
        } else {
            quote!(if $test { $rhs } else)
        });
    }

    conds.into_iter().flat_map(|x| x.into_iter()).collect()
}