Panic in proc_macro::TokenStream::from_str

[dtolnay]

TokenStream::from_str is supposed to return Result<proc_macro::TokenStream, proc_macro::LexError>. Currently it appears to print errors to stderr and then panic, rather than returning errors.

Cargo.toml

[package]
name = "repro"
version = "0.0.0"
edition = "2018"

[lib]
proc-macro = true

src/lib.rs

extern crate proc_macro;

use proc_macro::TokenStream;
use std::str::FromStr;

#[proc_macro_derive(ParseTokenStream)]
pub fn derive(_input: TokenStream) -> TokenStream {
    let _ = TokenStream::from_str("\\s");
    TokenStream::new()
}

src/main.rs

#[derive(repro::ParseTokenStream)]
struct S;

fn main() {}

Output of cargo check:

$ cargo check

error: unknown start of token: \
 --> src/main.rs:1:10
  |
1 | #[derive(repro::ParseTokenStream)]
  |          ^^^^^^^^^^^^^^^^^^^^^^^

error: proc-macro derive panicked
 --> src/main.rs:1:10
  |
1 | #[derive(repro::ParseTokenStream)]
  |          ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

Mentioning @eddyb, @petrochenkov, @alexcrichton.
Mentioning @canndrew who reported this in dtolnay/proc-macro2#168.

[dtolnay]

Maybe this will involve adding an emit method to LexError to format the error in compiler style to stderr.

[petrochenkov]

The impl even has a dedicated comment for this

NOTE: some errors may cause panics instead of returning LexError. We reserve the right to change these errors into LexErrors later.

rustdoc doesn't display it though.
In practice LexError is never returned :)