Four levels of in-place initialization

IntroductionThe goal of in-place initialization is to enable the construction of types directly into a memory location without any additional moves or copies. When working with big types this can be more efficient and even prevent stack overflows. But some types are what we call address sensitive and so cannot be moved for correctness reasons. There is some disagreement about how we should encode in-place initialization in the language. There are conflicting requirements and constraints at play, and reconciling those is tricky. I believe that the right way to attack the problem space is not by introducing a single feature, but by introducing a 4-level feature hierarchy for in-place initialization. Level 0: Raw pointersAt the lowest level we have raw pointers and MaybeUninit. This is by far the most flexible way to encode emplacement, but it comes at the cost of virtually everything else. This is both how the pin-init crate and placing crate are implemented internally. To read more…