Emit explanatory note for move errors in packed struct derives

[gurry]

Derive expansions for packed structs with non-Copy fields cause move errors because they prefer copying over borrowing since borrowing the fields of a packed struct can result in unaligned access.

This underlying cause of the errors, however, is not apparent to the user. This PR adds a diagnostic note to make it clear to the user (the new note is on the second last line):

tests/ui/derives/deriving-with-repr-packed-move-errors.rs:13:16
   |
12 | #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Default)]
   |          ----- in this derive macro expansion
13 | struct StructA(String);
   |                ^^^^^^ move occurs because `self.0` has type `String`, which does not implement the `Copy` trait
   |
   = note: `#[derive(Debug)]` triggers a move because taking references to the fields of a packed struct is undefined behaviour
   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

Fixes #117406

Partially addresses #110777

[rustbot]

r? @oli-obk

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[gurry]

@compiler-errors In your opinion, should this PR also close #110777? It is pretty much the same issue as #117406 but I was slightly hesitant to close it given the direction of the discussion in it.

[compiler-errors]

@gurry does this PR add the note to #110777 as well? If so, then I consider that to be sufficient.

[gurry]

@gurry does this PR add the note to #110777 as well? If so, then I consider that to be sufficient.

Yeah, the two issues are reporting the same problem and therefore the additional note emitted through this PR will work for #110777 as well.

I'll leave #110777 open for now as per your suggestion.

Please review.

[compiler-errors]

@bors r+

[bors]

📌 Commit 4b3ece4 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4b3ece4 with merge 7b97a5c...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 7b97a5c to master...

[rust-timer]

Finished benchmarking commit (7b97a5c): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

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)	-	-	0
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	0.9%	[0.5%, 1.4%]	4
Regressions ❌ (secondary)	4.8%	[4.6%, 5.0%]	2
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	1
All ❌✅ (primary)	0.6%	[-0.6%, 1.4%]	5

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)	0.6%	[0.4%, 0.9%]	4
Regressions ❌ (secondary)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.4%, 0.9%]	4

Binary size

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)	0.0%	[0.0%, 0.0%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 662.837s -> 662.627s (-0.03%)
Artifact size: 308.90 MiB -> 308.96 MiB (0.02%)