A Rust compiler optimization made a WebAssembly interpreter 50% slower. Finding that was the single biggest win in Wasmi 2.0. 🦀 Robin Freyler released Wasmi 2.0 today : 2.2x faster than 1.0, and among the fastest portable Wasm interpreters around. Used by Typst, Zellij, Stellar's Soroban and Ripple. The debugging story is the best part. While benchmarking competitors, he noticed Stitch : a different interpreter, not his own, had dropped 30% between Rust 1.91 and 1.92. The cause was a MIR optimization Rust enabled by default in 1.92, which merged two separate dispatch paths into a single branch site. Usually that's good. Here it was poison. The CPU's branch predictor went from two branch sites with clean histories to one site with mixed history, and accuracy fell apart. He fixed Stitch, someone else's project, and its score recovered. Then found the same pattern in his own. Fixing it took Wasmi's CoreMark from ~2800 to over 4200. Also in the release: accumulator registers keeping operands in actual hardware registers, a lock-free function map so calls do zero lookups, and SIMD support that no longer costs 5-10% just by being enabled. 🔗 wasmi-labs.github.io/blog/po… #RustLang #WebAssembly #Performance

The #2 most reported Rust annoyance (right after compile speed) is about to shrink. 🦀 Big target directories were the second most common complaint in the 2025 State of Rust survey. One real cause: Cargo has been storing the same crate metadata twice on disk, once in the .rmeta file (produced early, so dependent crates can start compiling sooner) and again inside the final .rlib file. Cargo will soon default to -Zembed-metadata=no on nightly, which stops embedding that duplicate metadata into .rlib files. Real measured numbers, compiling serde and Cargo itself: → Release profile, no debug info: up to 33% smaller target directory → Cargo's own release build: ~300 MiB saved in absolute terms → Dev profile with incremental + debuginfo: a more modest ~5-8%, since debug info and incremental artifacts dominate there anyway Most people won't notice anything except a smaller target directory. The only real edge case: if you link .rlib files manually outside of normal Cargo builds, you may now need to pass the matching .rmeta file explicitly. Nightly only for now, with stabilization planned if the rollout goes smoothly. Opt-out available via CARGO_UNSTABLE_EMBED_METADATA=true if you hit issues. Small on its own, but with survey data showing how many people this genuinely affects. 🔗 blog.rust-lang.org/inside-ru… #RustLang #Cargo #DevTools #OpenSource