migrate rustc_macros to syn 2.0

[fee1-dead]

WIP at this point since I need to work on migrating the code that heavily uses NestedMeta for parsing. Perhaps a full refactor would be nice..

[rustbot]

r? @Nilstrieb

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[bors]

☔ The latest upstream changes (presumably #109668) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

rustc_macros::diagnostics was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @TaKO8Ki

These commits modify the Cargo.lock file. Random changes to Cargo.lock can be introduced when switching branches and rebasing PRs.
This was probably unintentional and should be reverted before this PR is merged.

If this was intentional then you can ignore this comment.

[fee1-dead]

@rustbot ready

[Noratrieb]

Thanks a lot! A few small improvements.

I see a lot of bespoke meta handling which makes the code a lot more complex. Maybe it would be a good idea to investigate using something like darling in the future?

[Noratrieb]

The error messages got a little worse but that seems fine to me.

[Noratrieb]

(for a potential future PR, not for now) the else here could maybe be an error

[fee1-dead]

Maybe it would be a good idea to investigate using something like darling in the future?

It might be good, but currently we don't have darling in our Cargo.lock, and I don't know the procedure for adding a library like that.

[Noratrieb]

I think the usual procedure is just adding it and then double checking with someone from the compiler team that it really makes sense or something like that. Might je worth quickly asking on zulip before doing the implementation work.

[Noratrieb]

@bors r+

[bors]

📌 Commit d764c2d has been approved by Nilstrieb

It is now in the queue for this repository.

[bors]

⌛ Testing commit d764c2d with merge 32ea4bb...

[bors]

☀️ Test successful - checks-actions
Approved by: Nilstrieb
Pushing 32ea4bb to master...

[rust-timer]

Finished benchmarking commit (32ea4bb): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.5%, 2.3%]	11
Regressions ❌ (secondary)	1.7%	[1.5%, 1.9%]	2
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-3.0%	[-3.6%, -2.7%]	4
All ❌✅ (primary)	1.7%	[-0.5%, 2.3%]	12