The interesting thing to me is that this does not fully valid Rust code, e.g. not checking for borrow safety. Which would normally make no damn sense for a Rust compiler, but actually makes sense for what this particular project is trying to achieve.

This is not a compiler for developing Rust programs. This is a compiler to let platforms unsupported by Rust/LLVM build rust programs from source. So it takes a Rust program which already exists, assumes it can be compiled by rustc, and translates it to C so it can be handed off to your local compiler. Skipping the burrow checker probably lets this execute in environments which are more constrained than what rustc can fit into.

Of course there's a question of how much of Rust's safety survive translation into C. Certainly some. E.g. the emitted program would include bounds checks where not specifically elided in Rust, and references are statically guaranteed to be non-null before being lowered into pointers. But there could be gaps, especially in cases where Rust semantics mismatch with C in ways that LLVM hasn't exposed yet.