0 < -1 becomes 0 <- 1 inside macro_rules when proc-macro2 has feature nightly

[CodeSandwich]

When proc-macro2 has set feature nightly, parsing inside macro_rules becomes buggy. 0 < -1, which is comparison gets glued into 0 <- 1, which is assignment.

Reproduction

Tested using rustc 1.27.0-nightly (acd3871 2018-05-10).

Macro Cargo.toml

[package]
name = "macros"
version = "0.1.0"
[lib]
proc-macro = true
[dependencies]
syn = { version = "0.13.10", features = ["full"] }
quote = "0.5.2"
proc-macro2 = { version = "0.3.8" }

Macro lib.rs

#![feature(proc_macro)]
extern crate proc_macro;
extern crate proc_macro2;
extern crate syn;
extern crate quote;
use proc_macro::TokenStream;
use quote::ToTokens;
#[proc_macro_attribute]
pub fn nothing(_: TokenStream, token_stream: TokenStream) -> TokenStream {
    let tokens = syn::parse::<syn::Item>(token_stream)
        .unwrap()
        .into_tokens()
        .into();
    println!("{}", tokens);
    tokens
}

Macro user lib.rs

#![feature(proc_macro)]
#![allow(unused_macros)]
extern crate macros;
use macros::nothing;
#[nothing]
macro_rules! mr {
    () => { 0 < -1 }
}

Output

macro_rules ! mr ( (  ) => { 0 < - 1 } ) ;

Macro Cargo.toml modification

proc-macro2 = { version = "0.3.8", features = ["nightly"] }

Output after modification

macro_rules ! mr ( (  ) => { 0 <- 1 } ) ;

[dtolnay]

Thanks! This is a compiler bug -- I filed rust-lang/rust#50700 to follow up.

[CodeSandwich]

Thank you very much :)