resolve: Future proof derive helper attributes

[petrochenkov]

Derive helpers no longer require going through recovery mode (fixes#53481).
They also report an error if they are ambiguous with any other macro in scope, so we can resolve the question about their exact priority sometime later (cc #52226).

[rust-highfive]

r? @oli-obk

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[petrochenkov]

r? @alexcrichton

[petrochenkov]

@bors p=1
This needs to be merged before beta, which is in a few days.

[bors]

☔ The latest upstream changes (presumably #53778) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 8c49e4a41f9915d38a5802c3f1976f4d42ff51ae has been approved by alexcrichton

[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 derhelp(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 derhelp --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

Auto-merging src/librustc_resolve/macros.rs
CONFLICT (content): Merge conflict in src/librustc_resolve/macros.rs
Auto-merging src/librustc_resolve/lib.rs
CONFLICT (content): Merge conflict in src/librustc_resolve/lib.rs
Automatic merge failed; fix conflicts and then commit the result.

[bors]

☔ The latest upstream changes (presumably #53913) made this pull request unmergeable. Please resolve the merge conflicts.

[petrochenkov]

@bors r=alexcrichton

[bors]

📌 Commit 2b3e98f has been approved by alexcrichton

[bors]

⌛ Testing commit 2b3e98f with merge 994cdd9...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 994cdd9 to master...