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#![crate_type = "proc-macro"]
#![recursion_limit = "256"]
#![doc(html_root_url = "https://docs.rs/const_fn/0.1.1")]
extern crate proc_macro;
extern crate proc_macro2;
extern crate quote;
extern crate syn;
use proc_macro::TokenStream;
use proc_macro2::TokenStream as TokenStream2;
use quote::{quote, ToTokens};
use syn::{parse_quote, ItemFn};
#[proc_macro_attribute]
pub fn const_fn(args: TokenStream, function: TokenStream) -> TokenStream {
assert!(!args.is_empty(), "`#[const_fn]` requires an argument");
let mut function: ItemFn =
syn::parse(function).expect("`#[const_fn]` can only be used on functions");
let mut const_function = function.clone();
match &function.constness {
None => const_function.constness = Some(Default::default()),
Some(_) => function.constness = None,
}
let args = TokenStream2::from(args);
function.attrs.push(parse_quote!(#[cfg(not(#args))]));
const_function.attrs.push(parse_quote!(#[cfg(#args)]));
let mut function = function.into_token_stream();
function.extend(const_function.into_token_stream());
TokenStream::from(function)
}