> ICU has bindings for Rust that provide case-folding functionality, but not case-insensitive substring search.

> ICU has many bindings. The Rust one doesn’t expose any substring search functionality, but the Python one does:

Python's ICU support is based on ICU4C. Rust's ICU "bindings" are actually a new implementation called ICU4X, by developers who worked on i18n at Mozilla and Google and on ICU4C, with the goal of a cleaner, more performant implementation that is also memory safe. Maybe not relevant (as in substantially altering the benchmarks), but it's at least worth noting that the ICU backends aren't consistent throughout.