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Rust 1.98 compiler generates empty vtable slot for boxed async service causing segfault
Rust 1.98 introduces a code-generation bug that leaves a vtable slot empty for boxed async services, causing a null-pointer dereference and process crash at runtime.
The bug is classified as P-critical because it turns correct Rust code into a segfault, breaking stable-to-stable compatibility. It affects projects that use boxed async trait objects on aarch64-apple-darwin, as demonstrated by the Rama proxy example failing in CI. Users must either downgrade to 1.97.1, use a nightly toolchain, or avoid the affected pattern until a fix is released.
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Rust 1.98 generates a zero-filled method entry in the vtable for a boxed async service, leading to a SIGSEGV at address zero.
The issue was reproduced with the Rama repository, where 1.98 crashes while 1.97.1 and nightly-2026-07-16 work correctly.
Root cause analysis points to incorrect treatment of trait bounds as impossible, causing the compiler to emit a Vacant vtable entry instead of the actual method.
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What the cluster adds up to.
In Rust 1.98 the compiler incorrectly treats certain trait bounds as impossible when generating vtables for boxed async trait objects. As a result, the first method slot in the vtable is left zero instead of holding the actual function pointer. When the generated code dispatches through that slot, it jumps to address zero, triggering a segmentation fault. This manifests as a SIGSEGV with pc=0 on aarch64-apple-darwin targets.
Adopting Rust 1.98 in a project that uses the affected pattern can cause previously passing binaries to crash at runtime, forcing teams to roll back or invest debugging time. The Rama project observed the failure on every CI run after upgrading to 1.98, requiring a clean rebuild with an older toolchain to restore continuity. Until a patch is released, developers must either stay on Rust 1.97.1, switch to a nightly build, or refactor code to avoid boxed async service objects. This adds overhead to dependency management and release processes.
The bug stops working for any code path that constructs a boxed async service where the compiler deems the associated trait bounds impossible, which includes the Rama proxy’s EagerHttpProxyConnector::serve method. It is specific to the aarch64-apple-darwin target; other architectures were not reported in the provided material. The issue does not affect plain synchronous trait objects or non-boxed async calls. Consequently, only a subset of async-heavy, cross-platform libraries is exposed.
The problem is tracked as issue #161441 in the rust-lang/rust repository, labeled P-critical, regression-from-stable-to-stable, and marked with a bisection and an MCVE. Root-cause analysis indicates that the vtable creation routine places a Vacant entry because the predicate evaluation incorrectly marks the method’s bounds as impossible. The nightly toolchain from 2026-07-16 correctly includes both the erased caller and the concrete Service::serve method, showing the regression is isolated to the 1.98 stable release. A fix will need to adjust the trait-bound impossibility check in the codegen path for vtable layout.
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