Allow serde 0.8 and remove use of compiler plugins

[nox]

This change is

[nox]

The serde feature is actually used outside of Servo, and it would break at least feeds-to-pocket.

[nox]

This would also break wolfram-alpha-rs and xkcd-rs.

@FraGag and @indiv0: Would you mind having to relax the serde bound in your own crates at the same time as we merge this here, so that we avoid a 2.0.0 bump in rust-url?

[FraGag]

Well this is annoying. If I relax the requirement on serde in feeds-to-pocket but stay on url 1.1.1, then feeds-to-pocket picks up serde 0.8.x while url stays on serde 0.7.x, which causes build errors. On the other hand, if I don't relax the requirement on serde in feeds-to-picket but update url to your branch, then it's the other way around: feeds-to-pocket stays on 0.7.x while url picks up 0.8.x, which also causes build errors. If I update both at once, then it's fine. :\ If rust-lang/cargo#2064 was fixed, then this wouldn't be an issue.

feeds-to-pocket is an executable, not a library, so the package comes with its Cargo.lock, and unless a user runs cargo update, in theory the crate should still build, because my Cargo.lock currently references url 1.1.1. Still, I'll publish an update with relaxed requirements after url 1.2.0 is published.

[nox]

@FraGag Given feeds-to-pocket is an executable, there is no need for relaxed requirements there, even if you update rust-url after this is published, it won't make it use serde 0.8 as long as you don't also update serde AFAIK.

[SimonSapin]

Even though publishing this change can break projects that don’t have a Cargo.lock file because Cargo over-optimitic in using the latest possible version of serde, as far as I understand it is possible then to downgrade with something like cargo update -p serde --precise 0.7.15.

As such, I believe this does not qualify as a breaking API change, and so 1.2.0 rather than 2.0.0 is fine. (Or even 1.1.2, but let’s not split hairs about this, Cargo treats them the same.)

@bors-servo r+

[bors-servo]

📌 Commit 9f2f582 has been approved by SimonSapin

[bors-servo]

⌛ Testing commit 9f2f582 with merge 51d17bd...

[bors-servo]

☀️ Test successful - travis

[FraGag]

Just as a heads up (in particular to @nox): I just reinstalled feeds-to-pocket v0.1.1 locally with cargo install and now it fails to build. Apparently, crates uploaded to crates.io never contain Cargo.lock, even if it's not in the project's .gitignore. As a result, Cargo had to recalculate the dependencies and selected Serde 0.7 for feeds-to-pocket and Serde 0.8 for rust-url, which causes build errors because url::Url implements Serde 0.8's serde::Serialize trait, whereas feeds-to-pocket needs it to implement Serde 0.7's serde::Serialize trait.

So this did end up being a breaking change, but I agree with @SimonSapin that this kind of change should not be treated as a breaking change. Rather, Cargo could and should be improved such that situations like this do not cause breakage in the future (see e.g. rust-lang/cargo#2064, which I also mentioned above).