Improve error message for E0081

[Bryysen]

Closes #97319

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @lcnr (or someone else) soon.

Please see the contribution instructions for more information.

[Bryysen]

Note that this was done by using the (existing) Display implementation for the Discr type, instead of just printing the val field directly. Additionally, we check if the duplicate discriminant was explicitly set, if not we notify that it is being incremented from the previous discriminant. That note can definitely be improved, but maybe in a different PR.

[compiler-errors]

You probably need to re-bless your test stderr. Did you run ./x.py test src/test/ui --bless?

[Bryysen]

Hmm I could have sworn i blessed the tests... strange

[compiler-errors]

Do you need to git add src/test/ui? Tests still aren't blessed.

[Bryysen]

I'm on it. Ended up pulling from origin somehow and now i have to recompile everything locally to make the changes. I have the dream combo of weak laptop + slow connection so everything takes ages 😞

[Bryysen]

Moving the note to its own span seems kinda noisy:

error[E0081]: discriminant value `-1` assigned more than once
 --> tst.rs:4:5
  |
2 |     First = -1,
  |             -- first assignment of `-1`
3 |     Second = -2,
4 |     Last,
  |     ^^^^
  |
note: because `Last` has no explicit discriminant, it is being assigned the increment of the previous discriminant
 --> tst.rs:4:5
  |
4 |     Last,
  |     ^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0081`.

Could also just remove it all-together
Edit:
Or make it point to the variant incremented from, Second in this case
Edit2: Gonna remove it for now, improving that part can be done properly in another PR

[compiler-errors]

I actually kinda miss that "assigned value that was incremented from the previous value" error... could you add that back? Maybe pointing at the previous value, instead of the current one?

Something like

enum Foo {
  A = 1, // first assignment of `0` here
  B = 0, //~ assigned value for `C` was incremented from the previous value
  C,

[compiler-errors]

@Bryysen, let's go with #97456 (comment).

r? @compiler-errors

[compiler-errors]

two tiny nitpicks, then we're good to go!

[compiler-errors]

Wonderful, thanks for dealing with the back-and-forth of my comments. I will approve as soon as CI is green.

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit f671bc9 has been approved by compiler-errors

[bors]

⌛ Testing commit f671bc9 with merge abc7681...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing abc7681 to master...

[rust-timer]

Finished benchmarking commit (abc7681): comparison url.

Instruction count

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

Max RSS (memory usage)

Results

• Primary benchmarks: 😿 relevant regression found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	2.3%	2.3%	1
Regressions 😿 (secondary)	2.7%	2.7%	1
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-3.0%	-3.5%	2
All 😿🎉 (primary)	2.3%	2.3%	1

Cycles

Results

• Primary benchmarks: 😿 relevant regressions found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	3.5%	3.8%	2
Regressions 😿 (secondary)	3.1%	3.6%	2
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-3.8%	-3.8%	1
All 😿🎉 (primary)	3.5%	3.8%	2

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²