I'm personally very interested in "non-optimizing optimizers", essentially lint passes which point out places where you may optimize something without actually making that optimization. The advantage to this is that you get access to a lot of optimizations which would otherwise require you to make a lot of things undefined behavior to make work.

An example of this is the escape analysis lint in rust-clippy, which can detect if an allocation via Box/Vec/String is unnecessary (though right now it only supports one of those and I need to work on it more) and tell you how to remove it.