macros: diagnostic derive on enums

[davidtwco]

Part of #100717.

Extends #[derive(Diagnostic)] to work on enums too where each variant acts like a distinct diagnostic - being able to represent diagnostics this way can be quite a bit simpler for some parts of the compiler.

r? @compiler-errors
cc @Xiretza

[rustbot]

rustc_macros::diagnostics was changed

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

[davidtwco]

I only added this test for the enums, in addition to the existing case that needed changed and is a failing test - most of the logic is the same so I don't think it is super necessary to test everything again with enums but I can if we want.

[compiler-errors]

Looks great!

[compiler-errors]

@bors r+

[bors]

📌 Commit f20c882 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit f20c882 with merge de0b511...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing de0b511 to master...

[rust-timer]

Finished benchmarking commit (de0b511): 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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.1%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Footnotes

1. the arithmetic mean of the percent change ↩

2. number of relevant changes ↩