Both of those things can be done in Rust, but not in safe Rust, you have to use unsafe APIs that don't check lifetimes at compile time. Safe Rust assumes a clear distinction between memory allocations that are still live and those that have been deallocated, and that you never want to access the latter, which of course is true for most applications.

You can indeed write custom allocators, and you can read to or write from special addresses. The former will usually, and the latter will always, require some use of `unsafe` in order to declare to the compiler: "I have verified that the rules of ownership and borrowing are respected in this block of code".