It does have an interpreter for the full language, that's what Miri uses [0]. In fact, Miri doesn't even have its own evaluator, it just adds additional features to the rustc const-evaluation. The big limitations are that it doesn't have much support for syscalls or other calls into non-Rust code, and it emulates all multithreaded code on a single thread.

[0] https://github.com/rust-lang/miri