Rust Compiler Achieves 4.57% Wall-Time Reduction in Two Months

Overall Performance Improvements

The Rust compiler achieved a 4.57% mean wall-time reduction between July 29 and September 28, 2026. This improvement was observed across 555 of 629 benchmarks, with only 74 showing regressions. Several benchmarks recorded double-digit percentage reductions in wall-time.

Specific Component Enhancements

Significant speed gains were made in rustdoc, detailed by Noah Lev. Clippy also saw performance improvements, with Jakub Beránek enabling Profile-Guided Optimization (PGO), leading to up to an 18% wall-time reduction in some benchmarks. An upgrade to LLVM 23 by Nikita Popov resulted in a 1.2% mean wall-time reduction across all benchmarks.

New Borrow Checker and Trait Solver

The new borrow checker, Polonius Alpha, has been enabled on Nightly. It offers greater precision, accepting some valid programs previously rejected. While it introduces minor compile-time increases in a minority of cases, such as for the `serde` crate, Jack Huey implemented optimizations that reduced instruction counts for `serde` by 3-5%.

The new trait solver, Penelope Hammertime, was also enabled on Nightly. Similar to the new borrow checker, it can be slower in some instances, and efforts are underway to improve its performance.

Ongoing Optimization Efforts

Further work is planned to address remaining performance regressions from Polonius Alpha and Penelope Hammertime. Despite these minor regressions, the overall compiler performance shows a net improvement due to numerous other optimizations implemented during this period.