2026-01-26 release/6.3 snapshot on Linux miscompiles RigidArray tests

[lorentey]

As of 2026-01-26, the latest release/6.3 snapshot on Linux (dated 2026-01-26) seems to be affected by a miscompilation issue that results in bogus traps.

https://github.com/apple/swift-collections/actions/runs/21369256404/job/61509217413?pr=560

Other platforms seem not to be affected. The 6.3 snapshot on Windows dates from the same day and it works fine.

I'm chalking this up as a compiler issue for now. It may turn critical if we get a new snapshot and it still reproduces on it.

Test Case 'RigidArrayTests.test_insert_copying_Span' started at 2026-01-26 18:32:50.959
Swift/UnsafeBufferPointer.swift:872: Fatal error: buffer cannot contain every element from source.

*** Signal 4: Backtracing from 0x7fcaafdc9bdf... done ***

*** Program crashed: Illegal instruction at 0x00007fcaafdc9bdf ***

Platform: x86_64 Linux (Ubuntu 22.04.5 LTS)

Thread 0 "swift-collectio" crashed:

  0               0x00007fcaafdc9bdf closure #1 in closure #1 in _assertionFailure(_:_:file:line:flags:) + 623 in libswiftCore.so
  1 [ra]          0x00007fcaaff54f88 _assertionFailure(_:_:file:line:flags:) + 327 in libswiftCore.so
  2 [ra] [system] 0x0000561983324d50 closure #1 in UnsafeMutableBufferPointer.initialize<A>(fromContentsOf:) + 191 in swift-collectionsPackageTests.xctest at //<compiler-generated>
  3 [ra] [thunk]  0x0000561983324e21 partial apply for closure #1 in UnsafeMutableBufferPointer.initialize<A>(fromContentsOf:) + 48 in swift-collectionsPackageTests.xctest at //<compiler-generated>
  4 [ra]          0x00007fcaafeeb6b3 protocol witness for Sequence.withContiguousStorageIfAvailable<A>(_:) in conformance UnsafeMutableBufferPointer<A> + 34 in libswiftCore.so
  5 [ra]          0x00007fcaafeeb689 protocol witness for Sequence._copyContents(initializing:) in conformance UnsafeMutableBufferPointer<A> + 72 in libswiftCore.so
  6 [ra] [thunk]  0x00007fcaaff37d69 protocol witness for Sequence.withContiguousStorageIfAvailable<A>(_:) in conformance UnsafeBufferPointer<A> + 8 in libswiftCore.so
  7 [ra]          0x00007fcab00b2b08 dispatch thunk of Sequence.withContiguousStorageIfAvailable<A>(_:) + 7 in libswiftCore.so
  8 [ra]          0x0000561983326334 UnsafeMutableBufferPointer.initializeAll<A>(fromContentsOf:) + 403 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/InternalCollectionsUtilities/UnsafeMutableBufferPointer+Extras.swift:223:18
  9 [ra]          0x0000561983097a7b closure #1 in closure #1 in RigidArray.insert(copying:at:) + 154 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/BasicContainers/RigidArray+Insertions.swift:282:16
 10 [ra] [thunk]  0x00005619830984b8 partial apply for closure #1 in closure #1 in RigidArray.insert(copying:at:) + 39 in swift-collectionsPackageTests.xctest at //<compiler-generated>
 11 [ra] [system] 0x000056198309476b OutputSpan<>.withUnsafeMutableBufferPointer<A, B>(_:) + 1146 in swift-collectionsPackageTests.xctest at //<compiler-generated>
 12 [ra]          0x00005619830979bc closure #1 in RigidArray.insert(copying:at:) + 155 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/BasicContainers/RigidArray+Insertions.swift:281:14
 13 [ra]          0x0000561983098cc4 RigidArray<>.insert<A>(count:at:initializingWith:) + 659 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/BasicContainers/RigidArray+Insertions.swift:92:12
 14 [ra]          0x0000561983098dfd RigidArray.insert(copying:at:) + 204 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/BasicContainers/RigidArray+Insertions.swift:280:10
 15 [ra]          0x0000561983097b10 closure #1 in RigidArray.insert(copying:at:) + 111 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/BasicContainers/RigidArray+Insertions.swift:339:19
 16 [ra] [thunk]  0x00005619830983df partial apply for closure #1 in RigidArray.insert(copying:at:) + 30 in swift-collectionsPackageTests.xctest at //<compiler-generated>
 17 [ra] [system] 0x0000561983095251 Span<>.withUnsafeBufferPointer<A, B>(_:) + 1056 in swift-collectionsPackageTests.xctest at //<compiler-generated>
 18 [ra]          0x0000561983098f03 RigidArray.insert(copying:at:) + 146 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Sources/BasicContainers/RigidArray+Insertions.swift:338:24
 19 [ra]          0x00005619830b5c18 closure #1 in closure #1 in closure #1 in RigidArrayTests.test_insert_copying_Span() + 1399 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/BasicContainersTests/RigidArrayTests.swift:720:13
 20 [ra] [thunk]  0x00005619830b8dbb partial apply for closure #1 in closure #1 in closure #1 in RigidArrayTests.test_insert_copying_Span() + 26 in swift-collectionsPackageTests.xctest at //<compiler-generated>
 21 [ra]          0x00005619834b4e44 withLifetimeTracking<A, B>(file:line:_:) + 227 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/_CollectionsTestSupport/Utilities/LifetimeTracker.swift:65:14
error: Exited with unexpected signal code 4
 22 [ra]          0x00005619830b5681 closure #1 in closure #1 in RigidArrayTests.test_insert_copying_Span() + 128 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/BasicContainersTests/RigidArrayTests.swift:708:9
 23 [ra] [thunk]  0x00005619830b8d24 partial apply for closure #1 in closure #1 in RigidArrayTests.test_insert_copying_Span() + 19 in swift-collectionsPackageTests.xctest at //<compiler-generated>
 24 [ra]          0x00005619834aa9a4 withEvery<A>(_:in:file:line:run:) + 1363 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/_CollectionsTestSupport/AssertionContexts/Combinatorics.swift:55:9
 25 [ra]          0x00005619830b55a8 closure #1 in RigidArrayTests.test_insert_copying_Span() + 359 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/BasicContainersTests/RigidArrayTests.swift:707:7
 26 [ra]          0x00005619830a4a49 withSomeArrayLayouts<A, B>(_:ofCapacities:file:line:run:) + 2120 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/BasicContainersTests/ArrayLayout.swift:63:11
 27 [ra]          0x00005619830bd77b RigidArrayTests.test_insert_copying_Span() + 250 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/Tests/BasicContainersTests/RigidArrayTests.swift:706:5
 28 [ra] [system] 0x0000561983605c73 implicit closure #30 in implicit closure #29 in variable initialization expression of static RigidArrayTests.__allTests__RigidArrayTests + 34 in swift-collectionsPackageTests.xctest at /__w/swift-collections/swift-collections/.build/x86_64-unknown-linux-gnu/debug/swift-collectionsPackageDiscoveredTests.derived/BasicContainersTests.swift
 29 [ra] [thunk]  0x00005619836052cc thunk for @escaping @callee_guaranteed () -> () + 11 in swift-collectionsPackageTests.xctest at //<compiler-generated>
...

Thread 1 "-qos.overcommit":

  0      0x00007fcaae656e9e <unknown> in libc.so.6
  1 [ra] 0x00007fcaaf9a0ff2 _dispatch_mgr_invoke + 129 in libdispatch.so
  2 [ra] 0x00007fcaaf9a0f5d _dispatch_mgr_thread + 108 in libdispatch.so
  3 [ra] 0x00007fcaaf9a4195 _dispatch_worker_thread + 676 in libdispatch.so
...

Thread 2 "r-initiated-qos":

  0  0x00007fcaae5c2117 <unknown> in libc.so.6
...

Thread 3 "-qos.overcommit":

  0  0x00007fcaae5c2117 <unknown> in libc.so.6
...


Registers:

rax 0x0000000000000000  0
rdx 0x0000000000000368  872
rcx 0x0000000000000003  3
rbx 0x00000005619ba10f  23112425743
rsi 0x0000000000000007  7
rdi 0x00005619b9fe0010  03 00 00 00 02 00 01 00 00 00 01 00 05 00 06 00  ················
rbp 0x00007ffc8bb1a4d0  80 a5 b1 8b fc 7f 00 00 88 4f f5 af ca 7f 00 00  ·¥±·ü····Oõ¯Ê···
rsp 0x00007ffc8bb1a4b0  31 49 0e ba 19 56 00 00 40 ce 35 b0 ca 7f 00 00  1I·º·V··@Î5°Ê···
 r8 0x00005619ba10f980  0f a1 9b 61 05 00 00 00 34 a3 c2 1b e0 07 1e 49  ·¡·a····4£Â·à··I
 r9 0x00007ffc8bb1a2a0  46 61 74 61 6c 20 65 72 72 6f 72 3a 20 62 75 66  Fatal error: buf
r10 0x0000000000000000  0
r11 0x491e07e01bc2a334  5268657273189868340
r12 0x00007ffc8bb1a500  40 ce 35 b0 ca 7f 00 00 31 49 0e ba 19 56 00 00  @Î5°Ê···1I·º·V··
r13 0x00007ffc8bb1a770  b8 93 0d ba 19 56 00 00 f8 93 0d ba 19 56 00 00  ¸··º·V··ø··º·V··
r14 0x0000000000000001  1
r15 0x000000000000000b  11
rip 0x00007fcaafdc9bdf  0f 0b 48 83 ec 08 48 8d 05 e4 93 49 00 48 8d 3d  ··H·ì·H··ä·I·H·=

rflags 0x0000000000010202  

cs 0x0033  fs 0x0000  gs 0x0000


Images (20 omitted):

0x000056198302c000–0x00005619837a4830 5e88640d993497e7edc52f0dfcf9c7e22394b81e swift-collectionsPackageTests.xctest /__w/swift-collections/swift-collections/.build/x86_64-unknown-linux-gnu/debug/swift-collectionsPackageTests.xctest
0x00007fcaae531000–0x00007fcaae6ed3c1 085164b7e9093b36e3be17226f0d94a35b818c55 libc.so.6                            /usr/lib/x86_64-linux-gnu/libc.so.6
0x00007fcaaf96f000–0x00007fcaaf9b7258 7f0314a07f785650b2b11739bbdbece3f3e254fc libdispatch.so                       /usr/lib/swift/linux/libdispatch.so
0x00007fcaafc5c000–0x00007fcab0233040 043a3b3afd93ceff019ce5df8b02aace817a96e8 libswiftCore.so                      /usr/lib/swift/linux/libswiftCore.so

Backtrace took 0.39s

[lorentey]

This turned out to be caused by a stdlib regression: swiftlang/swift#86863

rdar://169203950

[lorentey]

#565 will work around the issue on all toolchains. The workaround will need to be reverted once the upstream fix lands (and propagates to snapshots). This issue should remain open to track that.