Launch a non-unwinding panic for misaligned pointer deref

[saethlin]

This panic already never unwinds, but that's only because it always hits the unwind guard that's created by our UnwindAction::Terminate. Hitting the unwind guard generates a huge double-panic backtrace. Now we generate a normal-looking panic message when this check is hit.

r? @thomcc

[nbdd0121]

We should also make this function #[rustc_nounwind] and document that this function MUST NOT unwind.

I wouldn't worry about non-local dependency that you mentioned at #112403 (comment), since this is a lang item and it's expected that lang item may need special guarantees. A #[rustc_nounwind] just near the lang item attribute with a comment should be clear enough to prevent mistakes.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[bors]

☔ The latest upstream changes (presumably #112681) made this pull request unmergeable. Please resolve the merge conflicts.

[thomcc]

This looks fine to me.

@bors r+

[bors]

📌 Commit 7a2490e has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7a2490e with merge 0c2c243...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 0c2c243 to master...

[rust-timer]

Finished benchmarking commit (0c2c243): 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)	2.2%	[1.2%, 3.2%]	3
Regressions ❌ (secondary)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	2.2%	[1.2%, 3.2%]	3

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

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
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	30
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	30

Bootstrap: 655.45s -> 656.801s (0.21%)

[RalfJung]

Would be good to mention panic_misaligned_pointer_dereference here so one can grep for what that symbol is. (I was not aware/forgot that different AssertKind dispatch to completely different code on assertion failure.)