Working with MaybeUninit<Struct> fields

1

The documentation for MaybeUninit says:

There is currently no supported way to create a raw pointer or reference to a field of a struct inside MaybeUninit<Struct> . That means it is not possible to create a struct by calling MaybeUninit::uninit::<Struct>() and then writing to its fields.

This is of course true for repr(Rust) structs because their layout is unknown. But shouldn't it be possible to partially initialize the fields of a repr(C) struct since the offset and size for each field is known.

Will this code cause undefined behavior?

#[repr(C)]
pub struct FooBar {
    pub foo: u32,
    pub bar: u32,
}

pub fn initFields() -> FooBar {
    let mut foobar = MaybeUninit::<FooBar>::uninit();
    let foobar_mut_ptr = foobar.as_mut_ptr() as *mut u32;
    unsafe {
        foobar_mut_ptr.offset(0).write(0u32);
        foobar_mut_ptr.offset(1).write(1u32);
        foobar.assume_init()
    }
}

Also, is it ok to read from a partially initialized MaybeUninit<Struct>?

pub fn readField() {
    let mut foobar = MaybeUninit::<FooBar>::uninit();
    let foobar_mut_ptr = foobar.as_mut_ptr() as *mut u32;
    unsafe {
        foobar_mut_ptr.offset(0).write(0u32);
        println!("foo={}", foobar_mut_ptr.offset(0).read());
    }
}
2

This should be fine

3

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