Parsing a `syn::ItemFn` fails with "unexpected token" on nightlies newer than 2020-05-24

[rrbutani]

Caused by rust-lang/rust#72388; this has more details.

Here's a (kind of, but not really) minified test case:

macro_rules! import_macro {
    ($(($rust:ident, $js:ident, $i:ident))*) => ($(
        #[wasm_bindgen]
        pub fn $rust(a: &[$i]) -> Vec<$i> {
            $js(a, Some(a), None)
        }
    )*)
}

import_macro! {
    (import_rust_i8, import_js_i8, i8)
}

#[wasm_bindgen] tries to parse the function as a syn::ItemFn which fails on the function's block (specifically on the ( after $js). As mentioned in the linked issue, as best as I can tell, this happens because trailer_expr (in expr.rs) returns if it sees a group which causes Block::parse_within to only see $js rather than the whole expression. Block::parse_within then expects a semicolon after $js (since it's not the last thing in the block) which it doesn't see and so it errors.

As mentioned in the linked issue's thread, removing the token::Group check in trailer_expr seems to fix this problem but I don't know if this will cause other problems; I'm not very familiar with syn's internals.

I noticed #832 altered that exact check; I tested again with 1.0.26 and I still get the same error.

Also, slightly unrelated, but would it be correct for the error here to be "expected semicolon"?

[dtolnay]

Fixed in 1.0.27.

[rrbutani]

Thank you!