Literal from macro expansion is not accepted in typescript_custom_section since rust 1.47.0

[Frizi]

Describe the Bug

Since the update to rust 1.47.0, typescript_custom_section doesn't work with literal captures expanded by macro.
In previous versions of rust, everything worked as expected.

The reported error is

Expected a string literal to be used with #[wasm_bindgen(typescript_custom_section)]

Reproduction

macro_rules! generate_ts {
    ($lit:literal) => {
        #[wasm_bindgen(typescript_custom_section)]
        const _: &str = $lit;
    };
}

generate_ts!("TEST");

The snippet above represent the code that used to work, but doesn't anymore. I've printed the actual AST that syn::ItemConst::macro_parse received, and got this:

Group(
  	ExprGroup {
		attrs: [],
		group_token: Group,
		expr: Lit(
			ExprLit {
				attrs: [],
				lit: Str(LitStr { token: "TEST" })
			}
		)
	}
)

Which means, now rust expands the literal (and possibly other) captures as an expression group with one item, and this should probably be handled in various places in the parser.

Current workaround

I managed to fix it in my specific case by using tt capture instead of literal, but this is obviously more general than i would like.

[alexcrichton]

Hm can you confirm what version of Rust you're using? 1.17 is quite old, and I'd recommend using something more in the 1.40 range

[Frizi]

@alexcrichton oh sorry, that was totally a typo on my side, the version is 1.48.0, I have confirmed it not working since 1.47.0, and working on 1.46.0.

[alexcrichton]

Aha makes sense! I'll try to investigate this this week.