Typically language developers make a pretty strong distinction between the compiler/runtime and the language specification itself, as is the case for C - or even C#, despite some usual Microsoft shittiness. The Rust Foundation says "modified version of the programming language, compiler, or the Cargo package manager," seeming to cover source-based forks from rustc and reimplementations of Rust that do things a bit differently. This really is overly restrictive compared to most other programming languages. You can create "Jim's Java Interpreter" or "Sarah's WASM-extended C# compiler" which use custom modifications of these languages, without asking permission from Sun or Microsoft; but the Rust Foundation seems to forbid this sort of thing. (Maybe I am misreading something but this has been my understanding for a while.) Rust appears to be using trademark claims in lieu of a formal specification, which is how C# protects its integrity against modifications. It is heavy-handed and seems intended to let the Rust Foundation hold an unusual amount of power over the language itself, and not just their implementation.

To be clear I believe they have good intentions: they think the book is a good enough specification and they don't want Rust to be sabotaged by bad ideas. But it's an unusual and somewhat untested arrangement: the Rust Foundation's general insistence that Rust = rustc + cargo + authorized backends like GCC seems like a significant technical risk that has gone badly underdiscussed. Too much of the community sneers at the idea of formal specification, but there is no magic spell that guarantees good management in perpetuity at any single organization. Obviously specification has its own problems and risks. But I think the issue has drawn a lot of snitty fights and bad-faith blogs - mostly Rust critics, to be fair - and very little serious discussion.