Tracking Issue for layout information behind pointers · Issue #69835 · rust-lang/rust (original) (raw)

The feature gate for the issue is #![feature(layout_for_ptr)].

This tracks three functions:

• core::mem::size_of_val_raw<T: ?Sized>(val: *const T) -> usize
• core::mem::align_of_val_raw<T: ?Sized>(val: *const T) -> usize
• core::alloc::Layout::for_value_raw<T: ?Sized>(t: *const T) -> Layout

These provide raw-pointer variants of the existing mem::size_of_val, mem::align_of_val, and Layout::for_value.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also uses as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Unresolved Questions

• What should the exact safety requirements of these functions be? It is currently possible to create raw pointers that have metadata that would make size wrap via ptr::slice_from_raw_parts. Trait vtable pointers are currently required to always be valid, but this is not guaranteed and an open question whether this is required of invalid pointers.
  • [A]Rc's current implementation requires calling Layout::for_value_raw on a pointer that has been drop_in_place'd, so this likely needs to stay possible.
• How should this interact with extern types? As this is a new API surface, we could potentially handle them properly whereas the _of_val cannot. Additionally, extern types may want to access the value to determine the size (e.g. a null terminated cstr).

rust-lang/lang-team#166 is tangentially related, as it serves to document what requirements currently exist on pointee types and getting a known layout from them.

Implementation history

• Allow calculating the layout behind a pointer #69079 implemented these functions, including the intrinsic adjustments required to support them.