Raw pointer field projection

1

If you have a raw pointer to a struct like *mut Range<_>, such as one might obtain from MaybeUininit::<Range<_>>::as_mut_ptr then we incur an unsafe operations when projecting from the raw pointer to a field:

unsafe fn partial_default(self: &mut MaybeUninit<Self>) {
    let self = self.as_mut_ptr();
    let start = 0;
    let start_ptr = unsafe { &mut (*self).start as *mut T };  // Maybe avoidable unsafe?
    unsafe { core::ptr::write(start_ptr,start) };  // Unavoidable unsafe
}

Are there proc macro that facilitate this? Should rustc provide an easier method somehow? Could self.start be given type *mut T here? Or is projection too risky somehow?

I noticed this in one of the optional fields discussions.

2

Can't help you with avoiding unsafe, but I have some criticisms of the code you wrote here.

let self = unsafe { self.assume_init_mut() }

https://doc.rust-lang.org/std/mem/union.MaybeUninit.html#method.assume_init_mut

Calling this when the content is not yet fully initialized causes undefined behavior


unsafe { &mut (*self).start as *mut T };

There's a better way to do this now, with std:::ptr::addr_of_mut. That line of code you wrote could be UB for uninitialized fields, I don't know if that's been decided yet.

I contributed some docs for how to initialize a MaybeUninit<Struct> in the Initializing a struct field-by-field section

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3

Can't you use this instead?

#![feature(maybe_uninit_ref)]
#![allow(unused_unsafe)]
use core::mem::MaybeUninit;

pub struct Range {
	pub start: i32
}

pub unsafe fn partial_default(this: &mut MaybeUninit<Range>) {
    let this = unsafe { &mut *this.assume_init_mut() };
    let start = 0;
    let start_ptr = &mut this.start;
    *start_ptr = start;
}
4

I made a copy-paste error from the other discussion, oops. Avoiding assume_init_mut is actually the point.

I'd missed that even *self breaks, not sure if it breaks Drop. Yes, addr_of_mut heads the right direction, thanks!

5

See also: Need for -> operator for Unsafe Code Guidelines

1 Like
6

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