Rust 1.36 has been released, and we should migrate from mem::uninitialized to MaybeUninit. mem::uninitialized will be deprecated in Rust 1.38.
FWIW, Servo is planning to do this so we'll get most of this when they get to it. I plan to do a sync tomorrow to allow this not to get too messy.
I had just started on doing this, but noticed that the current offset_of macro used in servo_arc relies on UB, and would continue to do so when converted to MaybeUninit. I have a patch to compute the offset differently which I'll upload to a separate bug.
Servo PR doing this in Stylo: https://github.com/servo/servo/pull/23726
m-c has a few other uses of mem::uninitialized in WebRender and Cubeb.