--- /dev/null
+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"))
+}