const_fn usage (with `Cell` fields)

[Gilnaa]

Previously on memoffset:

• const_fn usage #4

Constant evaluation mostly stable with the exception of the inability to take address of a Cell in const which still requires nightly and #![feature(const_refs_to_cell)].

[overlookmotel]

As far as I can see, this also blocks getting offset of any field of a struct with generics e.g.:

struct Foo<T> {
  t: T,
  n: u8,
}

// Both these produce compiler error
impl<T> Foo<T> {
  const OFFSET_T: usize = memoffset::offset_of!(Self, t);
  const OFFSET_N: usize = memoffset::offset_of!(Self, n);
}

Compiler error:

error[E0658]: cannot borrow here, since the borrowed element
may contain interior mutability

Bounding T to T: Copy + Clone (so it's definitely not a cell type) doesn't help.

The error seems to come from the let base_ptr = line:

const OFFSET_T: usize = {
  let uninit = memoffset::__priv::mem::MaybeUninit::<Self>::uninit();
  let base_ptr: *const Self = uninit.as_ptr();
  // ...
};

Is there any workaround for this on stable Rust? This comment says "Fortunately it can be gotten around by passing in a reference to the const fn", but I'm unclear what function they're referring to.

[RalfJung]

There will be a work-around soon: Rust 1.77 is slated to ship with a native offset_of! macro, making this crate largely obsolete. The scheduled release date is March 21st.

[overlookmotel]

Thanks for coming back so fast Ralf.

Amazing! Champagne for everyone! 🎉

While we're talking, can I ask you something else on the same sort of topic?
https://users.rust-lang.org/t/obtain-type-layout-information-at-compile-time/88380

The part which is relevant is where I asked:

Ralf Jung tantilizingly mentioned [on his blog about rustc_layout]
(https://www.ralfj.de/blog/2020/04/04/layout-debugging.html) "some time ago I wrote an awful hack for this based on rustc debug tracing". Ralf, if you're out there, can you share this awful hack please?!

[RalfJung]

The blog post says that the hack is no longer needed, it describes the much nicer way to do that. I don't have the hack any more, it was probably some random info! or trace! somewhere in rustc.

This is getting off-topic, please reach out to me on Zulip for further questions.