src/constfn.rs

use crate::error::{Error, Result};
use proc_macro::{Ident, Span, TokenStream, TokenTree};
use std::iter;

#[derive(PartialOrd, PartialEq)]
enum Qualifiers {
    None,
    Async,
    Unsafe,
    Extern,
    Abi,
}

impl Qualifiers {
    fn from_ident(ident: &Ident) -> Self {
        match ident.to_string().as_str() {
            "async" => Qualifiers::Async,
            "unsafe" => Qualifiers::Unsafe,
            "extern" => Qualifiers::Extern,
            _ => Qualifiers::None,
        }
    }
}

pub(crate) fn insert_const(input: TokenStream, const_span: Span) -> Result<TokenStream> {
    let ref mut input = crate::iter::new(input);
    let mut out = TokenStream::new();
    let mut qualifiers = Qualifiers::None;
    let mut pending = Vec::new();

    while let Some(token) = input.next() {
        match token {
            TokenTree::Ident(ref ident) if ident.to_string() == "fn" => {
                let const_ident = Ident::new("const", const_span);
                out.extend(iter::once(TokenTree::Ident(const_ident)));
                out.extend(pending);
                out.extend(iter::once(token));
                out.extend(input);
                return Ok(out);
            }
            TokenTree::Ident(ref ident) if Qualifiers::from_ident(ident) > qualifiers => {
                qualifiers = Qualifiers::from_ident(ident);
                pending.push(token);
            }
            TokenTree::Literal(_) if qualifiers == Qualifiers::Extern => {
                qualifiers = Qualifiers::Abi;
                pending.push(token);
            }
            _ => {
                qualifiers = Qualifiers::None;
                out.extend(pending.drain(..));
                out.extend(iter::once(token));
            }
        }
    }

    Err(Error::new(const_span, "only allowed on a fn item"))
}