How can I case fold a string in Rust?

问题:

I'm writing a simple full text search library, and need case folding to check if two words are equal. For this use case, the existing .to_lowercase() and .to_uppercase() methods are not enough.

From a quick search of crates.io, I can find libraries for normalization and word splitting but not case folding. regex-syntax does have case folding code, but it's not exposed in its API.

If there aren't any existing solutions then I might have to roll my own