add escaping of pound(#) with double-pound(##) for nested quote!

[siavash-hamedani]

let trait_name = ...
quote! {
    #[proc_macro_derive(#trait_name)]
    ...
    let derivee = ...
    ...
    quote!{
        impl #trait_name for ## derivee {
            ...
        }
    }
}

[dtolnay]

Thanks for the PR!

I would prefer not to accept this because it complicates how users need to think about # inside of quote. Notice that currently the # that appear in ordinary Rust syntax do not require escaping in quote -- for example notice the first # in your PR description: #[proc_macro_derive(...)]. As soon as we provide an explicit escape sequence, too-savvy users would start writing this as ##[proc_macro_derive(...)] and damage readability.

Using quote to generate quote invocations is an uncommon case that I am not interested in catering to. It is already supported in a way that is maybe even more readable, by writing:

struct Interp(&'static str);

impl ToTokens for Interp {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        tokens.append(Punct::new('#', Spacing::Alone));
        tokens.append(Ident::new(self.0, Span::call_site()));
    }
}

let a = Interp("a");
let q = quote! { quote!(#a) };
assert_eq!("quote ! ( # a )", q.to_string());

As a secondary concern, this change would break the meaning of existing working code.

let v = quote!(X);
let q = quote!(# #v);
assert_eq!("# X", q.to_string()); // before

// after this PR it would be "# v"