Rollup merge of #68701 - amosonn:patch-2, r=RalfJung
Rollup merge of #68701 - amosonn:patch-2, r=RalfJung

Improve #Safety of various methods in core::ptr

For `read`, `read_unaligned`,`read_volatile`, `replace`, and `drop_in_place`:

- The value they point to must be properly initialized

For `replace`, additionally:

- The pointer must be readable

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust