Add note on non-exhaustiveness when matching on str and nested non-exhaustive enums

[sebastiantoh]

Fixes #105479

r? @Nadrieril

[rustbot]

Some changes might have occurred in exhaustiveness checking

cc @Nadrieril

[compiler-errors]

Instead of adding an additional note, I wonder if it's easier to just specialize the help message "ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown" below to be a bit more context specific, explaining something a bit more directly about how strings aren't exhaustive.

[sebastiantoh]

Do you mean something like the following?

help: as `str` cannot be matched exhaustively, ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown 

I think it would make the help message too lengthy. My understanding is that notes can be used to provide further explanation or context on what caused the error instead of cluttering the help message with too much context.

But I don't have a strong opinion on this, so I'll be happy to make the change if needed. If we do make the change, do you think it would be necessary to make the change for pointer-sized ints as well for consistency??

[Nadrieril]

I would also prefer we didn't add a note, but I don't know how we'd phrase it. The tricky thing is that the non exhaustive type may be deep in a pattern somewhere (e.g. Result<Option<&str>, ...>), which prevents us from saying e.g. "ensure all possible strings are being handled".

[Nadrieril]

Oh oops forgot about this one before merging. I think it's ok like this, if one of you has a better idea we can make another PR. My secret plan is to suggest actual missing strings, something like patterns Some(""), Some("a"), Some("b") and many others not covered but I expect that will take some work.

[Nadrieril]

Thanks again @sebastiantoh! The logic looks sound. r=me once the phrasing nitpicks have been addressed

[sebastiantoh]

Thanks for the review! I've addressed the phrasing comments

@bors r=Nadrieril

[Nadrieril]

hm maybe you need rights to do this

@bors r+

[bors]

📌 Commit d87b87d has been approved by Nadrieril

It is now in the queue for this repository.

[bors]

⌛ Testing commit d87b87d with merge 21305f4...

[bors]

☀️ Test successful - checks-actions
Approved by: Nadrieril
Pushing 21305f4 to master...

[rust-timer]

Finished benchmarking commit (21305f4): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.1%	[0.8%, 1.4%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	1.1%	[0.8%, 1.4%]	6

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)	-	-	0
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-3.4%	[-4.5%, -1.3%]	3
All ❌✅ (primary)	-1.4%	[-1.4%, -1.4%]	1

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)	3.8%	[1.7%, 6.1%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.8%	[1.7%, 6.1%]	5

Binary size

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

Bootstrap: 629.273s -> 630.367s (0.17%)
Artifact size: 316.72 MiB -> 316.68 MiB (-0.01%)

[pnkfelix]

seems like continued spurious noise on bitmaps-3.1.0

@rustbot label: +perf-regression-triaged