Don't use parse_group for a visibility.

[nnethercote]

There is no need to explicitly handle an empty group, because the standard group ignoring code has the same effect.

[nnethercote]

I'm not 100% certain this PR is valid, but it passes tests, as run with cargo test --release --all-features. Having said that, I do get the "This is not a nightly compiler so not all tests were able to run" warning because I couldn't work out how to get things working with a nightly compiler. (cargo +nightly test --release --all-features gave the same warning.)

If the PR isn't valid, it would be good to have a test that fails when this change is applied.

[dtolnay]

Here is an example of the kind of thing that this breaks.

# Cargo.toml

[package]
name = "repro"
edition = "2021"

[lib]
proc-macro = true

[dependencies]
syn = { version = "2", features = ["extra-traits"] }

// src/main.rs

macro_rules! m {
    ($vis:vis $ty:ty) => {
        #[repro::repro]
        struct Wrapper($vis $ty);
    };
}

m!(str);

fn main() {}

// src/lib.rs

use proc_macro::TokenStream;
use syn::parse::{ParseStream, Result};
use syn::{parenthesized, parse_macro_input, Ident, Token, Type, Visibility};

#[proc_macro_attribute]
pub fn repro(_args: TokenStream, input: TokenStream) -> TokenStream {
    parse_macro_input!(input with parse);
    TokenStream::new()
}

fn parse(input: ParseStream) -> Result<()> {
    let _: Token![struct] = input.parse()?;
    let _: Ident = input.parse()?;

    let paren;
    parenthesized!(paren in input);
    let vis: Visibility = paren.parse()?;
    let ty: Type = paren.parse()?;

    let _: Token![;] = input.parse()?;

    println!("{:#?}", vis);
    println!("{:#?}", ty);
    Ok(())
}

[dtolnay]

I couldn't work out how to get things working with a nightly compiler

Nightly was broken by this LLVM linkage issue: rust-lang/rust#121889, 7aef1ed

[dtolnay]

I have added a test in #1601.

Thank you!