Deprecate RustcEncodable and RustcDecodable.

[m-ou-se]

We can't remove the RustcEncodable and RustcDecodable derive macros from the prelude, but we can deprecate them.

[rust-highfive]

r? @kennytm

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[m-ou-se]

r? @petrochenkov

[m-ou-se]

I suppose this technically requires an FCP.

As far as I understand, these macros are ancient left-overs from builtin support for rustc-serialize. We don't even show these in the nightly docs anymore. Let's mark them as deprecated.

@rfcbot merge

[rfcbot]

Team member @m-ou-se has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @KodrAus
• @dtolnay
• @joshtriplett
• @m-ou-se
• @sfackler

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Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[jyn514]

See also https://internals.rust-lang.org/t/what-is-rustc-decodable-encodable-and-why-is-it-stable/13744.

[BurntSushi]

Wow. I'm surprised these haven't been deprecated yet.

These were indeed ancient leftovers from the pre-serde era of automatic type driven serialization. IIRC, their names were specifically prefixed with Rustc because we knew that they would eventually be replaced by something better. (And indeed, they were, with many thanks to @erickt and @dtolnay!)

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[eddyb]

Wow. I'm surprised these haven't been deprecated yet.

Likely only due to technical limitations, I seem to recall @petrochenkov making it so all of the builtin macros have a presence in the standard library (even w/o any body), at some point in the last couple years.

[m-ou-se]

Seems a bit overkill to wait for a 10 day FCP (and miss the release cycle) for a simple long-overdue deprecation. Marking the FCP as finished.

[eddyb]

Just to be sure, can we have a test that shows a deprecation warning? I'm worried the current system will only warn on a reexport (or at least an implicit import), not on any use of the derives.

[m-ou-se]

warning: use of deprecated macro `RustcEncodable`: rustc-serialize is deprecated and no longer supported
 --> src/main.rs:1:10
  |
1 | #[derive(RustcEncodable)]
  |          ^^^^^^^^^^^^^^
  |
  = note: `#[warn(deprecated)]` on by default

[eddyb]

Thanks! Feels a bit strange that there were no preexisting tests that changed output, but maybe that's because such a test would need to get the library from crates.io (or at least some mock version of it)?

(compiler/rustc_serialize no longer has a compatible interface to the old derives being deprecated here, it was changed for the custom derives in compiler/rustc_macros/src/serialize.rs, and should probably be renamed anyway)

[petrochenkov]

@bors r+

[bors]

📌 Commit 924e522 has been approved by petrochenkov