By the grace of Lord dtolnay

.gitignore

/target

Cargo.lock

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 3

[[package]]
name = "macros"
version = "0.1.0"
dependencies = [
 "proc-macro2",
 "quote",
 "syn",
 "venial",
]

[[package]]
name = "proc-macro2"
version = "1.0.46"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "94e2ef8dbfc347b10c094890f778ee2e36ca9bb4262e86dc99cd217e35f3470b"
dependencies = [
 "unicode-ident",
]

[[package]]
name = "quote"
version = "1.0.21"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "bbe448f377a7d6961e30f5955f9b8d106c3f5e449d493ee1b125c1d43c2b5179"
dependencies = [
 "proc-macro2",
]

[[package]]
name = "syn"
version = "1.0.101"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "e90cde112c4b9690b8cbe810cba9ddd8bc1d7472e2cae317b69e9438c1cba7d2"
dependencies = [
 "proc-macro2",
 "quote",
 "unicode-ident",
]

[[package]]
name = "unicode-ident"
version = "1.0.4"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "dcc811dc4066ac62f84f11307873c4850cb653bfa9b1719cee2bd2204a4bc5dd"

[[package]]
name = "venial"
version = "0.4.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "f3d723d4b0ebf523971bc63776095798272df2873910f16d53bb680aa2f4d609"
dependencies = [
 "proc-macro2",
 "quote",
]

Cargo.toml

[package]
name = "macros"
version = "0.1.0"
edition = "2021"

[lib]
proc-macro = true
path = "macros.rs"

[[bin]]
name = "repro"
path = "repro.rs"

[dependencies]
proc-macro2 = "1.0.46"
quote = "1.0.21"
syn = "1.0.101"
venial = "0.4.0"

macros.rs

use proc_macro::TokenStream;

#[proc_macro]
pub fn repro(item: TokenStream) -> TokenStream {
    eprintln!("{item:#?}");
    venial::parse_declaration(item.into()).expect("Won't even get here");
    Default::default()
}

#[proc_macro]
pub fn non_repro_syn(input: TokenStream) -> TokenStream {
    syn::parse_macro_input!(input as syn::DeriveInput);
    Default::default()
}

#[proc_macro]
pub fn non_repro_normalize(item: TokenStream) -> TokenStream {
    let item = normalize(item.into());
    eprintln!("{item:#?}");
    venial::parse_declaration(item).expect("Won't even get here");
    Default::default()
}

fn normalize(ts: proc_macro2::TokenStream) -> proc_macro2::TokenStream {
    ts.into_iter()
        .flat_map(|tt| match tt {
            proc_macro2::TokenTree::Group(g) if g.delimiter() == proc_macro2::Delimiter::None => {
                normalize(g.stream())
            }
            proc_macro2::TokenTree::Group(group) => {
                let inner = normalize(group.stream());
                let mut ngroup = proc_macro2::Group::new(group.delimiter(), inner);
                ngroup.set_span(group.span());
                [proc_macro2::TokenTree::Group(ngroup)]
                    .into_iter()
                    .collect()
            }
            x => [x].into_iter().collect(),
        })
        .collect()
}

repro.rs

macro_rules! repro {
    ($visibility:vis) => {
        macros::repro! {
            $visibility struct Struct {
                $visibility member: Type,
            }
        }
    };
}

repro!(pub);

fn main() {
    println!("Hello, world!");
}