Shims cannot trust user-defined types, need to always use `deref_pointer_as`

[RalfJung]

Calling deref_pointer means we create a place at whatever type that argument was declared at by the user. But we can't trust that type, we should ideally entirely ignore it and use deref_pointer_as everywhere. Failure to do so can lead to ICEs like #3839.

To help with this, do the following:

• rg -w deref_pointer src/shims
• Pick one of the remaining places where we call deref_pointer
• Figure out which type that argument is supposed to have
• Replace the call with deref_pointer_as and the correct type

Getting the correct type will be the hardest part of this, and it depends on what the correct type is. Many basic types are available in this.machine.layouts, but struct types need to be looked up in libc/std or maybe we even have to construct one ourselves (or entirely change the code). If you are unsure about a concrete case, just ask here. :)

[oli-obk]

Once all of them are converted we should probably add a ./miri tidy (or make it part of ./miri check and similar commands) that automatically greps and asserts that there are no new ones introduced

[RalfJung]

Ah yes that's a good low-tech solution.

[badumbatish]

can i have a try at this?

[RalfJung]

Sure. :) I'd suggest to start with just one file and fix the deref_pointer there, to make sure you get the hang of this.

Unfortunately I won't have the time to provide a lot of mentoring. You can always try Zulip, there are other people there that can help.

[geetanshjuneja]

Hi, If no one working on this issue. Can I work on this?

[RalfJung]

@geetanshjuneja yes feel free to. :) Zulip is a great place to ask questions. For your first PR, please only change a few places so that we can make sure things are working as intended.

[geetanshjuneja]

@rustbot claim