Tracking Issue for [*const T|*mut T]::with_metadata_of

[oliver-giersch]

Feature gate: #![feature(set_ptr_value)]

Public API

impl<T: ?Sized> *mut T {
    pub fn with_metadata_of<U: ?Sized>(self, val: *const U) -> *mut U;
}

impl<T: ?Sized> *const T {
    pub fn with_metadata_of<U: ?Sized>(self, val: *const U) -> *const U;
}

Steps / History

• Implementation: adds [*mut|*const] ptr::set_ptr_value #74774
• Change type and add #[must_use]: Requested changes to [*mut T|*const T]::set_ptr_value #75407
• Reverse argument order and rename to with_metadata_of: Refactor set_ptr_value as with_metadata_of #95249
• Const argument for mutable with_metadata_of: Adjust argument type for mutable with_metadata_of (#75091) #103346
• Final commenting period (FCP)
• Stabilization PR

Unresolved Questions

• Correct argument type -- *mut ()? *mut u8? *mut Opaque?
• Is this even needed/useful given that there now also is Tracking Issue for pointer metadata APIs #81513?

[SimonSapin]

As discussed on Zulip this could be replaced with ptr::from_raw_parts(val, ptr::metadata(self)), tracked in #81513

[ChayimFriedman2]

Even if we want to keep these methods I think it's better to write them in terms of the metadata API as it's easy to reason about and less error prone than the current implementation.