Raw pointer

[ricked-twice]

During an internship studying Rust compilation phase, I encounter this problem with std::ptr::read function.
As I read this guide, I thought I could contribute to it.
So this is a presentation of my contribution to this guide (english and french).

I extended the chapter 4 of the book, and added a rule about the usage of the std::ptr::read function.
As explained, a usage with a type implementing the Drop trait could lead to double-drop and possibly a double-free if the value is stored on the heap and freed during drop execution.

This kind of contribution should have followed an issue, but I have been told that it would be easier for maintainers to directly go for a pull request. So here it is! 😄

[danielhenrymantilla]

I like the idea to insist on this point 💯

We could maybe even mention that in the rare case where ptr::read is needed, then:

• either use a pre-emptive ManuallyDrop of the original value;

• or ptr::write-overwrite the original value before it is used. This last case should be handled with great caution, though, given how minimal exception safety / unwind safety may be broken otherwise (e.g., a panic before the write will still lead to a double drop; thus the write should happen in a "finally" like block, such as one from ::scopeguard (or using catch_unwind)); code snippets of https://docs.rs/replace_with being very educational in that regard.

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I still think the suggested documentation should be a bit more harsh as to how bad misusing this function is, given how many things can go wrong, and how wrong that can be (UAF / double free can cause remote code execution), hence my request for changes 😉

• (Note that I am just a random user, I am not part of the maintainers of the repo).

[ricked-twice]

Okay, so finally I should have took last reviews into account (with a lot of commits, sorry about that...).
It only took me a year or so 👍