The rust compiler used to be pass based too, but then became on-demand as that architecture is more amenable for incremental compilation. In that time the performance has improved, although I'm not sure how much this architectural change was responsible for it, I think it were mostly unrelated changes.

How are you going to do borrow checking without MIR? Is GCC-Generic suitable for that?

Rust moved it from HIR to MIR to handle more edge cases.

isn't there basically some sort of static analysis going on during the rust compilation to ensure memory is accounted for?

There are a variety of static checks that Rust performs in order to ensure memory safety. rustc might perform these checks at various parts of the pipeline, but it might be possible for a different compiler to perform these checks at other points depending on what information its chosen IRs encode (for example, borrow checking requires a control-flow graph, which only exists at the MIR stage in rustc).