This will be another bad decision just like with Swift. From what I heard, Rust is notoriously bad at letting people define their own structure and instead beats you up until you satisfy the borrow checker. I think it'll make development slow and unpleasant. There are people out there who enjoy that, but it's not a fit for when you need to deliver a really huge codebase in reasonable time. I remember Andreas mentioning he just wanted something like C++, but with a GC and D would be absolutely perfect for this job.
Maybe, but will they have to fight with borrow checker for doing some other than (the very OOP) DOM components? They'll obviously use both for a long time in the future, so more functional places can get Rust, while more OOP places can benefit from C++