Assertion failed with main-snapshot-2025-10-02 toolchain when compiling embedded project

[ebariaux]

Description

When compiling example projects from the https://github.com/swiftlang/swift-embedded-examples repository, compiler exits with "Assertion failed" error and stack trace.
Although initially identified when building the stm32-uart-echo, I've had it with several projects that use the Swift-MMIO package (and only those using that package) e.g. rpi-pico2-neopixel or stm32-lcd-logo.

Reproduction

Clone https://github.com/swiftlang/swift-embedded-examples
Update the .swift-version file to use main-snapshot-2025-10-02
Build stm32-uart-echo, rpi-pico2-neopixel or stm32-lcd-logo project using make as instructed in the
README

Stack dump

0.	Program arguments: /Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend -frontend -c /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/BitField.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/BitFieldProjectable.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/MMIOInterposer.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/MMIOMacros.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/Register.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/RegisterArray.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/RegisterProtocol.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/RegisterStorage.swift /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIO/RegisterValue.swift -supplementary-output-file-map /var/folders/09/ky71wbn96pz2r4z56b1l4b480000gn/T/TemporaryDirectory.OnjjgN/supplementaryOutputs-1 -target armv7em-apple-none-macho -disable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -I /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/Modules -I /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib -F /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -F /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/PrivateFrameworks -color-diagnostics -Xcc -fcolor-diagnostics -g -debug-info-format=dwarf -dwarf-version=5 -module-cache-path /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/ModuleCache -suppress-warnings -swift-version 6 -O -D SWIFT_PACKAGE -D SWIFT_MODULE_RESOURCE_BUNDLE_UNAVAILABLE -mergeable-symbols -disable-stack-protector -load-plugin-executable /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/arm64-apple-macosx/release/MMIOMacros-tool#MMIOMacros -enable-experimental-feature Embedded -empty-abi-descriptor -plugin-path /Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/lib/swift/host/plugins/testing -file-compilation-dir /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel -Xcc -fmodule-map-file=/Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/checkouts/swift-mmio/Sources/MMIOVolatile/module.modulemap -Xcc --sysroot -Xcc /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -Xcc -F -Xcc /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -Xcc -F -Xcc /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/PrivateFrameworks -Xcc -fPIC -Xcc -g -no-auto-bridging-header-chaining -module-name MMIO -package-name swift_mmio -target-sdk-version 15.5 -target-sdk-name macosx15.5 -external-plugin-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -external-plugin
-path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/local/lib/swift/host/plugins#/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/bin/swift-plugin-server -in-process-plugin-server-path /Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/lib/swift/host/libSwiftInProcPluginServer.dylib -plugin-path /Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/lib/swift/host/plugins -plugin-path /Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/local/lib/swift/host/plugins -enable-default-cmo -parse-as-library -num-threads 10 -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/BitField.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/BitFieldProjectable.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/MMIOInterposer.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/MMIOMacros.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/Register.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/RegisterArray.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/RegisterProtocol.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/RegisterStorage.swift.o -o /Users/ebariaux/Development/3rdParty/Apple/swift-embedded-examples/rpi-pico2-neopixel/.build/armv7em-apple-none-macho/release/MMIO.build/RegisterValue.swift.o
1.	Apple Swift version 6.3-dev (LLVM 0d0246569621d5b, Swift 199240b3fe97eda)
 #0 0x0000000109b2f2ac (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x105a672ac)
 #1 0x0000000109b2d9d8 (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x105a659d8)
 #2 0x0000000109b2f8f4 (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x105a678f4)
 #3 0x000000019fd956a4 (/usr/lib/system/libsystem_platform.dylib+0x1804ad6a4)
 #4 0x000000019fd5b88c (/usr/lib/system/libsystem_pthread.dylib+0x18047388c)
 #5 0x000000019fc64a3c (/usr/lib/system/libsystem_c.dylib+0x18037ca3c)
 #6 0x000000019fc63c70 (/usr/lib/system/libsystem_c.dylib+0x18037bc70)
 #7 0x0000000109b3bac0 (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x105a73ac0)
 #8 0x00000001045a5c58 (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x1004ddc58)
 #9 0x00000001043869cc (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x1002be9cc)
#10 0x00000001040fe500 (/Users/ebariaux/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-10-02-a.xctoolchain/usr/bin/swift-frontend+0x100036500)
#11 0x000000019f9bab98

Expected behavior

Builds the project properly, as it does e.g. when using Swift 6.2

Environment

Apple Swift version 6.3-dev (LLVM 0d0246569621d5b, Swift 199240b)
Target: arm64-apple-macosx15.0
Build config: +assertions

Additional information

No response