Repository navigation
Tracking Issue for cast_maybe_uninit #145036
Description
Feature gate: #![feature(cast_maybe_uninit)]
This is a tracking issue for the cast_init and cast_uninit methods on pointers.
Public API
impl<T> *const T {
pub const fn cast_uninit(self) -> *const MaybeUninit<T>;
}
impl<T> *mut T {
pub const fn cast_uninit(self) -> *mut MaybeUninit<T>;
}
impl<T> NonNull<T> {
pub const fn cast_uninit(self) -> NonNull<MaybeUninit<T>>;
}
impl<T> *const MaybeUninit<T> {
pub const fn cast_init(self) -> *const T;
}
impl<T> *mut MaybeUninit<T> {
pub const fn cast_init(self) -> *mut T;
}
impl<T> NonNull<MaybeUninit<T>> {
pub const fn cast_init(self) -> NonNull<T>;
}Steps / History
(Remember to update the S-tracking-* label when checking boxes.)
- ACP: ACP:
cast_initmethod for pointers libs-team#627 - Implementation: Add
cast_initandcast_uninitmethods for pointers #145325 - Final comment period (FCP)1
- Stabilization PR
Unresolved Questions
- Naming: should
cast_initbeas_ptrto matchMaybeUninit::as_ptrthat does the same operation?
Footnotes
Reacted by Taylor Cramer and zachs18
Activity
theemathas commentedon Aug 7, 2025
Presumably, we'd want a
NonNullversion of this too? And this should probably work on?Sizedtypes?ChrisDenton commentedon Aug 7, 2025
Personally I'm not terribly convinced by
cast_uninittbh. It's very common to want to get a "normal" pointer from a&[MaybeUninit<T>](for ffi or to write through, etc) but a*mut MaybeUninit<T>has no special properties over*mut T. And*const MaybeUninit<T>just seems like a bit of a contradiction in types. After all an&MaybeUninit<T>would be mostly useless, right?To be clear, I'm not saying they have no usage whatsoever. Just that it feels quite rare to actually want this.
clarfonthey commentedon Aug 7, 2025
Yes; I forgot that when converting the proposed API into a full list.
clarfonthey commentedon Aug 7, 2025
@scottmcm's example on the original ACP felt pretty compelling: rust-lang/libs-team#627 (comment)
Essentially: while APIs operating on
MaybeUninit<T>will presumably want easy ways to cast things into ordinary pointers, there will also be APIs which store the ordinary pointers and want to convert them back intoMaybeUninit<T>. The example inVecis just one, but it feels natural that for any API that might want to do one conversion, there will probably be a similar API that would like to do the reverse.clarfonthey commentedon Aug 12, 2025
Actually taking this back:
MaybeUninitisn't allowed for unsized types, so, only sized for now.Rollup merge of rust-lang#145325 - clarfonthey:cast-init, r=scottmcm
Unrolled build for #145325
Rollup merge of rust-lang#145325 - clarfonthey:cast-init, r=scottmcm
papill9 commentedon Mar 30, 2026
Is anything blocking this to move to stable?