Turn order dependent trait objects future incompat warning into a hard error

[oli-obk]

fixes #56484

r? @ghost

will FCP when we have a crater result

[oli-obk]

@bors try

[bors]

⌛ Trying commit 0ae0780 with merge 9d63f82...

[bors]

☀️ Try build successful - checks-actions
Build commit: 9d63f82 (9d63f825c0d409b0e1743a2c84b33f86965e5929)

[oli-obk]

@craterbot check

[craterbot]

👌 Experiment pr-136968 created and queued.
🤖 Automatically detected try build 9d63f82
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-136968 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-136968 is completed!
📊 506 regressed and 4 fixed (582562 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[oli-obk]

🤔 hmm all the failures are because of crates with lockfiles for traitobject 0.1.0, even though traitobject 0.1.1 resolves the issues.

[oli-obk]

cc @rust-lang/lang

We've had a future incompatibility warning for years for code like

trait Trait {}
impl Trait for dyn Send + Sync {}
impl Trait for dyn Sync + Send {}

We already hard error if the trait has methods, but due to some widespread breakage, we decided not to make it a hard error. All (yes all) of the breakage is due to crates depending (mostly transitively) on the traitobject. @reem recently gave me crates.io ownership and I published a patch update, so everyone can just cargo update -p traitobject without causing any other problems.

There are only 10 crates on crates.io that break from this PR, and all of them also were due to lockfiles. All but

• https://crates.io/crates/gnostr-cat/0.0.40 (cli tool, git version already doesn't contain traitobject anymore at all)
• https://crates.io/crates/simple-http-server/0.6.10 (cli tool, PR for bumping traitobject dep open)

haven't seen updates in 4 years and are usually depending on an old version of hyper that still had traitobject in its dependency graph at all.

We should turn this FCW into a hard error and thus be able to rip out all the code that was necessary to maintain it

@rfcbot merge

[rfcbot]

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

• @BoxyUwU
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

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.

[oli-obk]

r? types