https://wiki.hyperbola.info/doku.php?id=en:main:rusts_freedom_flaws

As free software activists, we all enjoy using the latest and greatest in free software, but we need to make sure that the software we are using really does respect our freedom. Many users have expressed to us their desire to run Rust, since it appears to be fully free software, but it still fails in several ways.

Rust and also Cargo (the Rust package manager) violate the freedom to redistribute without “explicit” approval. Their trademark license imposes requirements for the distribution of modified versions that make it inconvenient to exercise freedom 3. The Rust's Media Guide says it merely supplements the official Mozilla trademark policy; it doesn't replace it. Since their trademark policy applies, then everything in that list (including Rust and Cargo) pulls in the same issue as Firefox and Thunderbird.

In short, Mozilla won't be happy with us applying patches and modifications to their trademarked language without “explicit approval”, except for non-commercial usage, so it is a freedom issue.