Rust was designed to be optimized; this means unoptimized Rust code can easily be 2x-100x slower than optimized code. Given that speed is one of Rust's goals, yeah, you could implement a Rust compiler with no optimization, and that's fine. But it won't be usable to a significant chunk of users who rely on the speed.

Another way this can affect language design: Rust supports enums, which are a sum type. The memory layout of the enum is undefined at the language level. This is specifically so that compilers can do optimizations to make layout more compact than a naive implementation. In some sense, this is leaking optimization concerns out into language design.