lib.rs

extern crate proc_macro;

use quote::quote;
use proc_macro2::{TokenTree, TokenStream};

#[proc_macro]
pub fn why(tokens: proc_macro::TokenStream) -> proc_macro::TokenStream {
    use quote::ToTokens;
    let a = quote! { 1 + 2 };
    // delimit a to maintain correct precedence behaviour
    let group = proc_macro2::Group::new(
        proc_macro2::Delimiter::None, a.into_token_stream()
    );
    let a_delimited = proc_macro2::TokenTree::Group(group);

    let b = quote! {3 * #a_delimited };
    dbg!(b.into())
}