Remove dependency on syn & quote

[jhpratt]

Given that this crate has relatively simple behavior, it should be feasible to remove the few proc macro dependencies. Although they are widely used, it's not desirable for every downstream crate to have to rely on them.

I am personally looking for a way to have const fn be enabled automagically for end users of the time crate. With the stabilization of const if/match/loop, a very large portion of the methods are const-capable. The only way to do this without needlessly duplicating every method is with a proc macro — which I presume is why this crate exists. Time is relatively lightweight as it is, and I'd like to keep it that way.

With regard to how this is possible, I am admittedly not experienced at proc macros. However, I believe it would be possible to just search for the first instance of const before the fn keyword, which would always(?) be the proper token. This could then be omitted as necessary for the duplication of the function definition.

After a quick look at this crate's code, it looks like the omission of the quote dependency should be trivial, at the least.

[taiki-e]

We certainly need only the information on the const (or fn) and the token that before it. So, we should be able to remove the dependency on syn. (#17 uses syn::parse::Perse, but it should actually be able to replace it with code that parses proc_macro2::TokenStream directly.)

quote is necessary to convert syn or own syntax type into tokens, so I'm not sure if we can remove the dependency on quote, but, as far as I know, quote is very light dependency compared to syn. So I think the benefit of removing quote dependency is not very big.
(FYI: here is the current compile time of #17 (with cargo build --release -Z timings):

[jhpratt]

Yeah, quote definitely isn't as big, but given how light this crate is anyways I personally feel it's best to make it even lighter. On my laptop I actually have a proof of concept where I modified the code to work without any dependencies whatsoever. That's obviously not including #17.

Of course, it's your crate, so the decision is ultimately yours to make. This is just my thought as someone who is intested in cutting down build times as much as possible.

[taiki-e]

Published 0.4.0 which fixes this issue. (dependency on proc-macro2 has also been removed.)