It is common for programmers to assemble their programs from a combination of trusted and untrusted components. In this context, a trusted program component is said to be robustly safe if it behaves safely when linked against arbitrary untrusted code. Prior work has shown how various encapsulation mechanisms (in both high- and low-level languages) can be used to protect code so that it is robustly safe, but none of the existing work has explored how robust safety can be achieved in a patently unsafe language like C. In this paper, we show how to bring robust safety to a simple yet representative C-like language we call Rec . Although Rec (like C) is inherently “dangerous” and thus not robustly safe, we can “save” Rec programs via compilation to Cap , a CHERI-like capability machine . To formalize the benefits of such a hardening compiler , we develop Reckon, a separation logic for verifying robust safety of Rec programs. Reckon is not sound under Rec ’s unsafe, C-like semantics, but it is sound when Rec programs are hardened via compilation and linked against untrusted code running on Cap . As a crucial step in proving soundness of Reckon, we introduce a novel technique of semantic back-translation , which we formalize by building on the DimSum framework for multi-language semantics. All our results are mechanized in the Rocq prover.

@article{paperbot3877,
  title = {Endangered by the Language But Saved by the Compiler: Robust Safety via Semantic Back-Translation},
  author = {Niklas Mück and Aïna Linn Georges and Derek Dreyer and Deepak Garg and Michael Sammler},
  journal = {Proceedings of the ACM on Programming Languages},
  volume = {10},
  number = {POPL},
  year = {2026},
  doi = {10.1145/3776682}
}