iddqd, or the Hardest Kind of Unsafe Rust
Oxide Computer Company, Tuesday, June 2nd, 2026
Rain Paharia explains iddqd, a Rust library for maps with borrowed keys, and discusses safe abstraction of unsafe code.
iddqd is a Rust library developed by Oxide Computer Company that allows keys to be borrowed from values in maps, solving the problem of key duplication and synchronization in standard library maps.
The library is extensively used in Omicron, Oxide's control plane software, to maintain in-memory indexes for large records like disks and sled inventories. Because correctness is critical for the control plane, the article explores how iddqd manages its unsafe code through sound abstractions and discusses the challenges of reasoning about unsafe Rust, including the fundamental undecidability of static analysis and the strict rules that must be followed to prevent undefined behavior.
The author emphasizes that unsafe code must be carefully encapsulated behind safe abstractions and that reasoning about such code requires understanding both the unsafe implementation and the surrounding safe Rust context.