Rust 1.93.0 release notes: SIMD, varargs, and the stuff that breaks builds

I’ve watched “minor” Rust upgrades stall a release train for one dumb reason. Emscripten flags. Rust 1.93.0 lands with real wins for low-level work (SIMD on s390x, C-style variadic functions), plus a few changes that can trip CI in under 60 seconds if you ship WebAssembly or rely on sloppy tests. The 30-second upgrade call Upgrade if you hit FFI edges, ship on IBM Z, or you want stricter diagnostics before prod. Wait a week if your WebAssembly pipeline depends on Emscripten and you cannot spare an afternoon to chase linker flags. High risk: Emscripten unwinding ABI change for panic=unwind. Your build can fail at link time. Medium risk: Stricter #[test] validation. Rust stops ignoring invalid placements and starts erroring. Low risk: New lints and Cargo quality-of-life changes. You will mostly see warnings. What actually changed (the parts you will notice) This bit me when a “harmless” std behavior change hid in a patch note. BTreeMap::append now stops overwriting existing keys when the