add syn_full feature to serde_derive

[dimpolo]

This allows users of serde_derive to transitively enable the "syn/full" feature.
While this fixes a problem I'm having I haven't put much thought into its general desirability.

Background

I'd like code like this to compile:

#[derive(serde::Deserialize)]
struct Foo<T: strum::EnumCount> {
    foo: heapless::Vec<u8, T::COUNT>,
}

However Rust currently requires us to write it like so (link)

#![feature(generic_const_exprs)]

#[derive(serde::Deserialize)]
struct Foo<T: strum::EnumCount>
where
    [(); { T::COUNT }]:,
{
    foo: heapless::Vec<u8, { T::COUNT }>,
}

The weird where clause requires the "syn/full" feature

[oli-obk]

As a work around you can make your crate depend on syn while enabling that feature (without actually using it. As long as it's in Cargo.toml, everything will work out)

[dimpolo]

As a work around you can make your crate depend on syn while enabling that feature (without actually using it. As long as it's in Cargo.toml, everything will work out)

That's something I tried. Unfortunately using this method I still get the unsupported expression; enable syn's features=["full"] error.

It seems as though features aren't unified for proc macros?

(Might be related: rust-lang/cargo#7833)

[NobodyXu]

@dimpolo I think if you put syn in build-dependencies then it should get unified with proc-macro deps.

[dimpolo]

@dimpolo I think if you put syn in build-dependencies then it should get unified with proc-macro deps.

Yes! That works! Thanks for the help! I'll close this PR