Recommendation for feature flag name that enables 'serde' and also enables 'serde' in dependencies

[frewsxcv]

The implicit feature inferred by Cargo for the optional dependency is called serde, not use-serde or with-serde, so we like for explicit features to behave the same way.

https://rust-lang-nursery.github.io/api-guidelines/naming.html#feature-names-are-free-of-placeholder-words-c-feature

What is the recommended naming convention for a serde feature flag that also enables serde feature flags of dependencies? As an example:

[features]
use-serde = ["serde", "geo-types/serde"]

• https://github.com/georust/geo/blob/b57ab9ffac65e0ce45e549c45acc4ef6408fd310/geo/Cargo.toml#L29

[dtolnay]

Ideally it would still be "serde". This requires the namespaced-features feature of Cargo: rust-lang/cargo#5565 (comment).

Hmm, on stable Cargo it may be possible to work around it with a shim crate that re-exports from serde:

[features]
serde = ["serde1", "geo-types/serde"]

[dependencies]
geo-types = "..."
serde1 = { version = "1.0", optional = true }

I don't currently have a suggestion for a name other than "serde". 😀

[BurntSushi]

I've seen serde1 used somewhere as the feature name, although I don't remember where.

[KodrAus]

@BurntSushi rand is one example that uses serde1.

[tikue]

@dtolnay as I found out the hard way, crates that use namespaced-features can't currently be published.

[grovesNL]

If anyone else is looking for a way to just expose the feature as serde in this case (avoiding having to name the feature something like serde1 or use-serde), the approach in https://github.com/RustCrypto/RSA/pull/41/files seems to be a good workaround and seems to be used in a few crates now.

[CAD97]

Rather than use #[serde(serde_crate)], consider an extern crate serde_crate as serde; at the top of the library. This reexports the library back as just ::serde.

[mainrs]

@CAD97 in the library that exposes a feature called serde? I've added #[cfg(feature = "serde")] extern crate serde_crate as serde; but I still get the compilation error:

can't find crate for `serde`
 --> zors_lib\src\visitable\mod.rs:9:38
  |
9 | #[cfg_attr(feature = "serde", derive(Deserialize))]
  |                                      ^^^^^^^^^^^ can't find crate

[CAD97]

... I could have sworn that I did this before and it worked. (It must have been for a project that was not using the derive, or with a macro that wasn't actually serde's.)

But yes, now that I think about it, serde_derive does extern crate serde (and in edition2015 mode), so wouldn't work with extern crate re-renaming. (If serde_derive were to change to using edition2018, it would probably be better to emit use ::serde, which unambiguously means an extern crate in edition2018 mode.)

And this gets worse on nightly, because serde is in the sysroot 😞

D:\repos\cad97\playground>cargo +beta check --quiet
error[E0463]: can't find crate for `serde`
 --> src\lib.rs:5:21
  |
5 | #[derive(Serialize, Deserialize)]
  |                     ^^^^^^^^^^^ can't find crate
  |
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: could not compile `playground`.

D:\repos\cad97\playground>cargo +nightly check --quiet
error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
 --> src\lib.rs:5:10
  |
5 | #[derive(Serialize)]
  |          ^^^^^^^^^
  |
  = note: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
  = help: add `#![feature(rustc_private)]` to the crate attributes to enable
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: could not compile `playground`.

[bminixhofer]

For anyone else finding this, there is a solution for the #[derive(Serialize)] issue here: serde-rs/serde#1938 (comment)

#[serde(crate = "serde_by_some_other_name")] should work.