Rust proposes cross-crate forward declarations for safer kernel code

A new Rust language feature called cross-crate forward declarations would let crates declare functions they need without defining them, with the compiler verifying that exactly one implementation exists at link time. This generalizes existing ad-hoc mechanisms like custom panic handlers and allocators. At Kangrejos 2026, Dönszelmann proposed applying this to the kernel's Rust integer overflow handling, which currently relies on a less clean workaround.

How forward declarations would replace weak symbols

• Current problem: Rust's standard library uses multiple one-off mechanisms (panic_handler, global_allocator) for user code to inject behavior; the kernel's Rust integer overflow handling similarly needs a hook but lacks a clean abstraction
• Proposed solution: A forward fn declaration in one crate, with exactly one impl fn required in the dependency graph; the compiler checks this at link time rather than leaving it to the linker
• Safety improvement: Unlike C weak symbols, mismatches are caught at compile or link time rather than silently failing or linking incorrectly
• Kernel application: Would replace the current workaround for Rust's overflow-checks in kernel code, where integer overflow panics need to be redirected to kernel-specific handling instead of aborting

Why it matters

If accepted, this would become a foundational Rust language feature that kernel and embedded developers could rely on for safe, late-bound configuration without unsafe code or linker-dependent behavior. It would also establish a pattern for future standard library extension points.