Unsupport #[derive(Trait)] sugar for #[derive_Trait] legacy plugin attributes

[petrochenkov]

This is a long deprecated unstable feature that doesn't mesh well with regular resolution/expansion.

How to fix broken code:

• The recommended way is to migrate to stable procedural macros - derives or attributes (https://doc.rust-lang.org/nightly/book/first-edition/procedural-macros.html).
• If you are using rocket 0.3.* in particular, the recommended solution is to migrate to rocket 0.4.*.
• If that's not possible right now for some reason, you can keep code working with a simple mechanical replacement #[derive(Legacy)] -> #[derive_Legacy].

Closes #29644

r? @ghost

[petrochenkov]

r? @alexcrichton

[bors]

☔ The latest upstream changes (presumably #53461) made this pull request unmergeable. Please resolve the merge conflicts.

[petrochenkov]

cc @SergioBenitez who reported about the feature still being used in Rocker in July.

That said, #[derive(Legacy)] -> #[derive_Legacy] is not a huge regression in usability (at least for nightly), even if the feature is still used.

[SergioBenitez]

We've migrated all of Rocket's derives to procedural macros. That being said, that migration is not yet part of an official release, so this would be a breaking change for almost all of Rocket's users.

If it's possible to delay this PR until Rocket sees a new release, which should happen in a matter of weeks as there is only a single blocking feature remaining, that would be much appreciated.

[petrochenkov]

@SergioBenitez
Yeah, I think this can wait for a few weeks.
Is there any issue that I can subscribe to for tracking Rocket's release status?

[SergioBenitez]

@petrochenkov The 0.4 milestone tracks the missing features. Converting all of Rocket's code generation to proc-macros is tracked in rwf2/Rocket#693.

[Centril]

@SergioBenitez what's the status now?

[SergioBenitez]

@Centril We are no longer using this in Rocket as of the the 0.4 release candidate. The GA of 0.4 has not yet been released; that should happen on the 14th. Ideally this wouldn't be merged until some time after the GA is available to give users a chance to migrate.

[nagisa]

Rocket 0.4 is still not GA (is currently on -rc2, which has been published yesterday).

[petrochenkov]

Yay.
@bors r=eddyb,alexcrichton

[bors]

📌 Commit 8ab115c has been approved by eddyb,alexcrichton

[bors]

⌛ Testing commit 8ab115c with merge a2fb99b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb,alexcrichton
Pushing a2fb99b to master...