Make PROC_MACRO_DERIVE_RESOLUTION_FALLBACK a hard error

[Aaron1011]

r? @ghost

[Aaron1011]

@bors try

[bors]

:hourglass: Trying commit 191f8a971b06684f802fcc33a2398bd15df8a3ab with merge 96e388c3ccda74650413d435528d8e0ed2872a21...

[bors]

:sunny: Try build successful - checks-actions Build commit: 96e388c3ccda74650413d435528d8e0ed2872a21 (96e388c3ccda74650413d435528d8e0ed2872a21)

[Aaron1011]

@craterbot check

[craterbot]

:ok_hand: Experiment pr-84022 created and queued. :robot: Automatically detected try build 96e388c3ccda74650413d435528d8e0ed2872a21 :warning: Try build based on commit 2e495d2e845cf27740e3665f718acfd3aa17253e, but latest commit is 191f8a971b06684f802fcc33a2398bd15df8a3ab. Did you forget to make a new try build? :mag: You can check out the queue and this experiment's details.

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

[craterbot]

:construction: Experiment pr-84022 is now running

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

[craterbot]

:tada: Experiment pr-84022 is completed! :bar_chart: 90 regressed and 8 fixed (155930 total) :newspaper: Open the full report.

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

[Aaron1011]

Unfortunately, one of the regressions is in the unmaintained crate https://github.com/MaikKlein/enumflags

[Aaron1011]

cc @petrochenkov - I'm not sure how you'd like to move forward with this. We could try to move people off https://github.com/MaikKlein/enumflags before we remove the hack, but that would still result in the crate become unusable (the last update was in 2019, and the repository has been archived).

[petrochenkov]

I'm not sure how you'd like to move forward with this.

Leave it as is for a couple more years? There's no pressure to remove deprecation lints urgently. It can also be turned into an error on 2021 edition only in the meantime.

[crlf0710]

@Aaron1011 Do you want to turn this into an error on 2021 and later editions only? Or do you prefer this be closed?

[Aaron1011]

@crlf0710 I think that would be reasonable. While this could theoretically break some 2018 edition code in the presence of a sufficiently weird proc-macro (e.g. a proc-macro which marks the input tokens with a 2021 edition span), I think it would be good to make progress on this issue.

[JohnCSimon]

triage: @Aaron1011 - what is the status of this PR? Thanks.

[Aaron1011]

@bors try

[bors]

:hourglass: Trying commit 36066293017525460ed9946570f9d819805c7ab5 with merge 5fe169892b3912c8118f59c03c27c622bbade2b5...

[bors]

:sunny: Try build successful - checks-actions Build commit: 5fe169892b3912c8118f59c03c27c622bbade2b5 (5fe169892b3912c8118f59c03c27c622bbade2b5)

[Aaron1011]

@craterbot check

[craterbot]

:ok_hand: Experiment pr-84022 created and queued. :robot: Automatically detected try build 5fe169892b3912c8118f59c03c27c622bbade2b5 :mag: You can check out the queue and this experiment's details.

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

[craterbot]

:construction: Experiment pr-84022 is now running

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

[craterbot]

:tada: Experiment pr-84022 is completed! :bar_chart: 69 regressed and 9 fixed (244852 total) :newspaper: Open the full report.

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

[Aaron1011]

cc @rust-lang/compiler

This PR removes a backwards-compatibility hack that was added four years ago in https://github.com/rust-lang/rust/pull/51952. Currently, we accept code like this:

// proc-macro crate
#[proc_macro_derive(AnswerFn)]
pub fn derive_answer_fn(_item: TokenStream) -> TokenStream {
	quote ! { mod generated_mod { struct Generated(TypeFromParent); }
}

// other crate
struct TypeFromParent;

#[derive(MyDeriveMacro)]
struct SomeOtherType;

Normally, this code would fail to compile, since TypeFromParent has not been imported into generate_mod. However, we currently detect this kind of usage in derive-macros only, and let it compile with a warning. (Note - this is about path resolution, not hygiene).

Now that https://crates.io/crates/enumflags-derive/0.4.2 has been released, all affected crates should have an upgrade path.

This has been part of cargo report-future-incompat since feature the future-incompat report feature was stabilized (https://github.com/rust-lang/rust/pull/89523), and has been deny-by-default since https://github.com/rust-lang/rust/pull/88041. There have been zero issues opened on rust-lang/rust mentioning either of these decisions.

This PR removes all of the backwards-compatibility code (along with the lint), simplifying rustc_resolve in the process. Any crates broken by this change are almost certainly unmaintained, and have an upgrade path if they use a dependency that's tested by Crater.

[lcnr]

@rfcbot fcp merge

[rfcbot]

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

• [x] @Aaron1011
• [x] @cjgillot
• [x] @davidtwco
• [x] @eddyb
• [x] @estebank
• [x] @lcnr
• [x] @matthewjasper
• [x] @michaelwoerister
• [ ] @nagisa
• [ ] @nikomatsakis
• [x] @oli-obk
• [x] @petrochenkov
• [x] @pnkfelix
• [x] @wesleywiser

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.

[Aaron1011]

cc @nagisa @estebank @nikomatsakis @pnkfelix for review and checkboxes

[rfcbot]

:bell: This is now entering its final comment period, as per the review above. :bell:

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[Aaron1011]

r? @petrochenkov

[petrochenkov]

@bors r+

[bors]

:pushpin: Commit 7d82cadd97acc66993b69304c5a1a04ef7d1fa36 has been approved by petrochenkov

It is now in the queue for this repository.

[pnkfelix]

this probably should have been marked as rollup=never