Figure out where topo::nested is losing spans

[anp]

I'm pretty sure that the worst error messages I'm seeing now are from #[topo::nested] functions (removing the annotation improves the error message).

[anp]

This occurs with literally any logic in the proc macro, including just a parse/quote pair:

#[proc_macro_attribute]
pub fn nested(args: TokenStream, input: TokenStream) -> TokenStream {
    let mut input_fn: ItemFn = syn::parse(input).unwrap();
    // other stuff normally goes here
    input_fn.into_token_stream().into()
}

[anp]

Based on dtolnay/async-trait#91 (comment) this is caused by a compiler bug (rust-lang/rust#43081)

Should be resolved by rust-lang/rust#72306.