Soft-destabilize RustcEncodable & RustcDecodable, remove from prelude in next edition

[jhpratt]

cc rust-lang/libs-team#272

Any use of RustcEncodable and RustcDecodable now triggers a deny-by-default lint. The derives have been removed from the 2024 prelude. I specifically chose not to document this in the module-level documentation, as the presence in existing preludes is not documented (which I presume is intentional).

This does not implement the proposed change for rustfix, which I will be looking into shortly.

With regard to the items in the preludes being stable, this should not be an issue because #15702 has been resolved.

r? libs-api

[RalfJung]

Would it be worth making this warning show up in cargo's future-compat reports? Not all future-compat warnings show up there, unfortunately. It needs to have reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow. Maybe soft_unstable should get that flag?

[jhpratt]

I think that would be worthwhile, but in a separate PR as it's only tangentially related to this.

[ehuss]

@Amanieu Do you know what the status of this PR is? Has the libs team discussed this change?

We would like to conclude the set of edition changes, and we'd like to know if this will be included.

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp merge

In 2015–2021 edition code, derive(RustcEncodable) and derive(RustcDecodable) will fail with a deny-by-default lint which can be suppressed with #![allow(soft_unstable)]. The deny-by-default does not break dependencies pulled from crates.io and built by Cargo, because Cargo uses --cap-lints=allow for those. A future incompat warning would be shown in that case instead, with cargo report future-incompatibilities to examine the situation.

In 2024 edition code, derive(RustcEncodable) and derive(RustcDecodable) fail as these derive macros are no longer in the 2024 edition prelude. You can import them e.g. with use core::prelude::rust_2021::RustcEncodable; to get the soft unstable lint behavior.

[rfcbot]

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

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

No concerns currently listed.

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!

See this document for info about what commands tagged team members can give me.

[jhpratt]

^^ migration code will be done separately if approved.

[bors]

☔ The latest upstream changes (presumably #121295) made this pull request unmergeable. Please resolve the merge conflicts.

[matthiaskrgr]

@bors r-
https://github.com/rust-lang-ci/rust/actions/runs/8405121132/job/23017470854#step:27:12036

[jhpratt]

Should I allow both the soft_unstable and deprecated lints? The error isn't particularly surprising, as this is the exact situation the soft destabilization is intended to exclude.

[dtolnay]

The failure is in https://github.com/rust-lang/cargo/blob/77506e57392d94450bb3ed52cf75e3263bbb2792/tests/testsuite/check.rs#L222-L285 which is a 7 year old test for rust-lang/cargo#3419, which is from 2 days after the initial implementation of cargo check landed in nightly Cargo in rust-lang/cargo#3296.

I would recommend just deleting the test from Cargo. The implementation of cargo check has matured enough since then and I don't see anything useful in that test.

[jhpratt]

The test in question has been removed. After the next sync with the rust-lang/cargo repo, this PR should be good to go.

[ehuss]

Cargo should be updated via #123040.

@bors r+

[bors]

📌 Commit fbf21c5 has been approved by ehuss

It is now in the queue for this repository.

[bors]

⌛ Testing commit fbf21c5 with merge 0dcc130...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing 0dcc130 to master...

[rust-timer]

Finished benchmarking commit (0dcc130): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	1

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.7%	[-3.9%, -0.6%]	12
Improvements ✅ (secondary)	-3.1%	[-4.4%, -2.1%]	32
All ❌✅ (primary)	-1.7%	[-3.9%, -0.6%]	12

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 669.488s -> 669.238s (-0.04%)
Artifact size: 315.76 MiB -> 315.79 MiB (0.01%)