Dev version of zb crashing on build

[zombiezen]

Reported by @Abdiramen

Steps to reproduce

1. Run the first example in https://zb.256lights.llc/getting-started#first-steps using dev version (noted below)

Expected behavior

Build succeeds

Actual behavior

Backend panic:

[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb7fcf3]

goroutine 1302 [running]:
sync.(*WaitGroup).Go.func1.1()
        /home/vimto/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/src/sync/waitgroup.go:251 +0x45
panic({0xcd0460?, 0x14ffa20?})
        /home/vimto/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/src/runtime/panic.go:860 +0x13a
unique.Handle[...].Value(...)
        /home/vimto/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/src/unique/handle.go:27
zb.256lights.llc/pkg/internal/backend.equivalenceClass.toRealizationOutputReference(...)
        /home/vimto/zb/internal/backend/equivalence_class.go:44
zb.256lights.llc/pkg/internal/backend.(*builder).do-range4(...)
        /home/vimto/zb/internal/backend/realize.go:915
zb.256lights.llc/pkg/internal/backend.(*builder).do.Set[...].func10(...)
        /home/vimto/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/src/maps/iter.go:28
zb.256lights.llc/pkg/internal/backend.(*builder).do(0x1aa5b17b31c0, {0xe1e050, 0x1aa5b19624b0}, {0x1aa5b195c700, 0x3c}, 0x1aa5b17a7d70, 0x0)
        /home/vimto/zb/internal/backend/realize.go:912 +0x28b3
zb.256lights.llc/pkg/internal/backend.(*builder).realize(0x1aa5b17b31c0, {0xe1e050, 0x1aa5b19624b0}, 0x1aa5b1837a40, 0x0)
        /home/vimto/zb/internal/backend/realize.go:384 +0x8e7
zb.256lights.llc/pkg/internal/backend.(*Server).realize.func1()
        /home/vimto/zb/internal/backend/realize.go:113 +0x3b2
sync.(*WaitGroup).Go.func1()
        /home/vimto/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/src/sync/waitgroup.go:258 +0x4a
created by sync.(*WaitGroup).Go in goroutine 1300
        /home/vimto/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.1.linux-amd64/src/sync/waitgroup.go:238 +0x73

Version information

• zb version: 7428ce2
• Operating System and Version: Linux 6.12.85+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.85-1 (2026-04-30)
• CPU architecture: x86_64-unknown-linux