Drop dependency on syn/full feature

Cargo.toml

@@ -16,7 +16,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "1.0.1", features = ["full"] }
+syn = "1.0.1"
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]

src/constfn.rs

@@ -0,0 +1,65 @@
+use proc_macro::{Delimiter, Ident, TokenStream, TokenTree};
+use std::iter;
+use syn::{Error, Result, Token};
+
+#[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_token: Token![const]) -> Result<TokenStream> {
+ let mut out = TokenStream::new();
+ let mut stack = vec![input.into_iter()];
+ let mut qualifiers = Qualifiers::None;
+ let mut pending = Vec::new();
+
+ 'outer: while let Some(iter) = stack.last_mut() {
+ while let Some(token) = iter.next() {
+ match token {
+ TokenTree::Group(ref group) if group.delimiter() == Delimiter::None => {
+ stack.push(group.stream().into_iter());
+ continue 'outer;
+ }
+ TokenTree::Ident(ref ident) if ident.to_string() == "fn" => {
+ let const_ident = Ident::new("const", const_token.span.unwrap());
+ out.extend(iter::once(TokenTree::Ident(const_ident)));
+ out.extend(pending);
+ out.extend(iter::once(token));
+ out.extend(stack.into_iter().rev().flatten());
+ 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));
+ }
+ }
+ }
+ stack.pop();
+ }
+
+ Err(Error::new(const_token.span, "only allowed on a fn item"))
+}

src/lib.rs

@@ -149,6 +149,7 @@ extern crate proc_macro;
 
 mod attr;
 mod bound;
+mod constfn;
 mod date;
 mod expr;
 mod time;
@@ -160,7 +161,7 @@ use crate::version::Version;
 use proc_macro::TokenStream;
 use proc_macro2::{Ident, Span, TokenStream as TokenStream2};
 use quote::quote;
-use syn::{parse_macro_input, ItemFn, Result};
+use syn::{parse_macro_input, Result};
 
 const RUSTVERSION: Version = include!(concat!(env!("OUT_DIR"), "/version.rs"));
 
@@ -245,11 +246,7 @@ fn try_attr(args: attr::Args, input: TokenStream) -> Result<TokenStream> {
  }
 
  match args.then {
- Then::Const(const_token) => {
- let mut input: ItemFn = syn::parse(input)?;
- input.sig.constness = Some(const_token);
- Ok(TokenStream::from(quote!(#input)))
- }
+ Then::Const(const_token) => constfn::insert_const(input, const_token),
  Then::Attribute(then) => {
  let input = TokenStream2::from(input);
  Ok(TokenStream::from(quote! {