rustversion/
constfn.rs

1use crate::error::{Error, Result};
2use proc_macro::{Ident, Span, TokenStream, TokenTree};
3use std::iter;
4
5#[derive(PartialOrd, PartialEq)]
6enum Qualifiers {
7    None,
8    Async,
9    Unsafe,
10    Extern,
11    Abi,
12}
13
14impl Qualifiers {
15    fn from_ident(ident: &Ident) -> Self {
16        match ident.to_string().as_str() {
17            "async" => Qualifiers::Async,
18            "unsafe" => Qualifiers::Unsafe,
19            "extern" => Qualifiers::Extern,
20            _ => Qualifiers::None,
21        }
22    }
23}
24
25pub(crate) fn insert_const(input: TokenStream, const_span: Span) -> Result<TokenStream> {
26    let ref mut input = crate::iter::new(input);
27    let mut out = TokenStream::new();
28    let mut qualifiers = Qualifiers::None;
29    let mut pending = Vec::new();
30
31    while let Some(token) = input.next() {
32        match token {
33            TokenTree::Ident(ref ident) if ident.to_string() == "fn" => {
34                let const_ident = Ident::new("const", const_span);
35                out.extend(iter::once(TokenTree::Ident(const_ident)));
36                out.extend(pending);
37                out.extend(iter::once(token));
38                out.extend(input);
39                return Ok(out);
40            }
41            TokenTree::Ident(ref ident) if Qualifiers::from_ident(ident) > qualifiers => {
42                qualifiers = Qualifiers::from_ident(ident);
43                pending.push(token);
44            }
45            TokenTree::Literal(_) if qualifiers == Qualifiers::Extern => {
46                qualifiers = Qualifiers::Abi;
47                pending.push(token);
48            }
49            _ => {
50                qualifiers = Qualifiers::None;
51                out.extend(pending.drain(..));
52                out.extend(iter::once(token));
53            }
54        }
55    }
56
57    Err(Error::new(const_span, "only allowed on a fn item"))
58}
rustversion/constfn.rs

rustversion/
constfn.rs
