Cargo features need to be additive

[dtolnay]

Filing here for lack of a better place to file it. This affects all of serde_codegen, serde_codegen_internals, aster and quasi.

Cargo features are supposed to be additive. The with-syntex features across the ecosystem are not additive because they remove the libsyntax-based methods. This results in breakage like adding a dependency on serde_derive can break existing code that uses aster without syntex.

We need to move to additive features like:

[features]
default = ["with-syntex"]
with-syntex = ["syntex_syntax"]
with-libsyntax = []

where you can build with both syntex and libsyntax simultaneously if necessary.

[dtolnay]

No longer urgent because serde_derive does not use syntex anymore, so the problem has not gotten worse.

[dtolnay]

We may be done here by just dropping Syntex. The std feature may not strictly follow this but that should be fine, you won't be using std Serde with no-std serde_json or vice versa. @oli-obk just check whether there is anything to fix here for 0.9.0 and let's file separate tickets in syntex/quasi/aster as necessary to follow up later.

[oli-obk]

Indeed I can confirm that the only feature breaking from the additiveness is std. We can't make it additive, because collections requires nightly.