Convert proc_macro_back_compat lint to an unconditional error.

[nnethercote]

We still check for the rental/allsorts-rental crates. But now if they are detected we just emit a fatal error, instead of emitting a warning and providing alternative behaviour.

The original "hack" implementing alternative behaviour was added in #73345.

The lint was added in #83127.

The tracking issue is #83125.

The direct motivation for the change is that providing the alternative behaviour is interfering with #125174 and follow-on work.

r? @estebank

[nnethercote]

I'm not 100% certain this is the right way to do this, but it seems like a reasonable first attempt.

cc @petrochenkov @Aaron1011

[bors]

☔ The latest upstream changes (presumably #125410) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

@bors try

Gonna do a crater run as a sanity check that we haven't missed something, but otherwise I'm ok with landing this change as is. We provide actionable user feedback while removing the hack and without blocking your other work. This is the best possible resolution, beyond a more advanced/generalized mechanism to handle popular dependency versions with known issues.

[bors]

🔒 Merge conflict

This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository, you can resolve the conflict following these steps:

1. git checkout rental-hard-error (switch to your branch)
2. git fetch upstream master (retrieve the latest master)
3. git rebase upstream/master -p (rebase on top of it)
4. Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
5. git push self rental-hard-error --force-with-lease (update this PR)

You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub. It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Error message

Removing tests/ui/proc-macro/pretty-print-hack-show.remapped.stdout
Removing tests/ui/proc-macro/pretty-print-hack-show.local.stdout
Auto-merging compiler/rustc_lint_defs/src/lib.rs
Auto-merging compiler/rustc_lint_defs/src/builtin.rs
Auto-merging compiler/rustc_lint/src/lints.rs
Auto-merging compiler/rustc_lint/src/context/diagnostics.rs
CONFLICT (content): Merge conflict in compiler/rustc_lint/src/context/diagnostics.rs
Auto-merging compiler/rustc_expand/src/mbe/transcribe.rs
Automatic merge failed; fix conflicts and then commit the result.

[nnethercote]

I rebased.

@bors try

[bors]

⌛ Trying commit cf0c2c7 with merge c57a122...

[bors]

☀️ Try build successful - checks-actions
Build commit: c57a122 (c57a122e6bc7d3e243f3b2a000b6916cb51ef149)

[nnethercote]

@craterbot check

[craterbot]

👌 Experiment pr-125596 created and queued.
🤖 Automatically detected try build c57a122
🔍 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

[Noratrieb]

this looks very translatable now

[craterbot]

🚧 Experiment pr-125596 is now running

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

[craterbot]

🎉 Experiment pr-125596 is completed!
📊 100 regressed and 1 fixed (466636 total)
📰 Open the full report.

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

[nnethercote]

Crater results are very much as expected.

The vast majority of the failures have the expected message:

error: using an old version of rental
|
= note: older versions of the rental crate no longer compile; please update to rental v0.5.6, or switch to one of the rental alternative

A few had "no space left on device" (just running out of disk space):

• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/knirb.rust-tutorial/log.txt
• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/robinlimit.rust_learning_basic_level/log.txt
• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/to-json.testchamber/log.txt
• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/yoelbassin.juliText/log.txt
• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/robinlimit.rust_learning_basic_level/log.txt

One had "error: failed to download aho-corasick v1.1.3" (clearly a network error)

• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/reg/moon_pdk_api-0.0.6/log.txt

Two had "collect2: fatal error: ld terminated with signal 7 [Bus error], core dumped" (linker crash?!)

• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/zainokta.log-parser/log.txt
• https://crater-reports.s3.amazonaws.com/pr-125596/try%23c57a122e6bc7d3e243f3b2a000b6916cb51ef149/gh/znmead.guessing_game/log.txt

Other notes

• Most of the rental failures are in personal repos, presumably for long-abandoned personal projects.
• Only 11 are are in published crates. They are all at least 3 years old.

This all looks fine to me. @estebank, what do you think?

[estebank]

@bors r+

[bors]

📌 Commit cf0c2c7 has been approved by estebank

It is now in the queue for this repository.

[nnethercote]

@bors rollup