Servo forks serde with [patch]

[DustinByfuglien]

I'm trying a build simple servo app with cargo build (rustc 1.24.0-nightly (3bee2b44c 2017-12-16)):

extern crate servo;

fn main() {
    println!("Servo version: {}", servo::config::servo_version());
}

Cargo.toml includes:

[dependencies]
servo = { git = "https://github.com/servo/servo" }

But building is terminating with exit code 101:

error: Package serde_derive v1.0.23 does not have these features: deserialize_from

Windows 7x64.

Help please.

[fabricedesre]

Servo uses a serde fork, the current version in Cargo.lock is https://github.com/gankro/serde?branch=deserialize_from_enums3#fc611736

Does that match yours? Note how in https://github.com/paulrouget/servo-embedding-example they recommend to copy Servo's Cargo.lock in your project to avoid these kind of issues.

[DustinByfuglien]

Thank you for the link.
Now I understand wat was wrong.

[SimonSapin]

Servo has this section in its root Cargo.toml, which you probably need to copy in yours:

[replace]
"serde:1.0.23" = { git = "https://github.com/gankro/serde", branch = "deserialize_from_enums3" }
"serde_derive:1.0.23" = { git = "https://github.com/gankro/serde", branch = "deserialize_from_enums3", feature="deserialize_from" }

---

This also seems to make using [patch] impossible. @gankro, is there a plan to upstream these serde features, or somehow stop using [replace]?

https://github.com/rust-lang/cargo/blob/0.23.0/src/cargo/util/toml/mod.rs#L743-L745

        if self.patch.is_some() && self.replace.is_some() {
            bail!("cannot specify both [replace] and [patch]");
        }

[SimonSapin]

#19590 replaces [replace] with [patch] which fixes some problems, but this is still not ideal. When [patch]’ing another library which depends on serde = "1.0", cargo insisted on upgrading that dependency to 1.0.24 from crates.io instead of 1.0.23 from the forked git repo, even after explicitly downgrading it: cargo update -p serde:1.0.24 --precise 1.0.23 would succeed, but then the next build would "upgrade" again.

This may be a Cargo bug, but [patch] / [replace] is still something that should not be in master IMO.

[dtolnay]

I will be publishing a Serde version with most of the changes from the fork in the next few days. As I understand it, at that point Servo will be able to drop the [patch] of serde but wants to keep using a fork of serde_derive -- so we need to upstream the rest of the changes somehow or figure out a long-term solution for the fork.

[Gankra]

With the serde work upstreamed servo could remove both patches, as the serde_derive fork “only” introduces a transparent optimization. Performance of gecko is more critical (in the short term), so we would continue to apply the derive patch there.

[SimonSapin]

More breakage cause by this: #19698. Though I think part of the reason is that the fork’s version number is behind crates.io. Doing either of:

• Keep the fork up to date whenever upstream publishes a new version
• Or change the fork’s version number from 1.0.23 to something like 1.0.1023

might help. What do think @gankro ?

[Gankra]

You can remove the fork in the next webrender update, it’s been upstreamed. Webrender still forks serde_derive but it’s for a transparent optimization, failing to include it just means you’re a bit slower.

[jdm]

The fork was indeed removed in #19614.