But then that shifts the issue. You've now got an opaque binary blob being injected into programs. What if it is malicious?

ABI stability is significantly different in a VM language to a native language, and reified generics necessarily require source compilation. Binary-only development for Rust would be exactly as 'sort of not really' as C++, for exactly the same reasons. (There is also no notion of 'installing' dependencies in Rust, and build-time code being malicious isn't much worse than runtime code being malicious.)