There are a lot in the new trademark policy that discourage using the word Rust in any possible way that is not explicitly mentioned as permitted. But you are right, stating "written in Rust" is explicitly mentioned as permitted.

    4.1.3 Statements about compatibility, interoperability or derivation

    You may use the Word Marks, but not the Logos, to truthfully describe the relationship between your software and ours. Our Marks should be used after a verb or preposition that describes the relationship between your software and ours. So you may say, for example, "the Dungeness tool for the Rust compiler" but may not say "the Dungeness Rust compiler," which suggests that Dungeness is the source of the Rust compiler. Some other examples that may work for you are:

    [Your software] is written in the Rust language
    [Your software] can compile software written in Rust 
    [Your software] can be used in the Rust compiler toolchain
    [Your software] is based on the official Rust compiler

https://docs.google.com/document/d/1ErZlwz9bbSI43dNo-rgQdkov...