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Tracking Issue for raw-pointer-to-reference conversion methods #122034

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RalfJung opened this issue Mar 5, 2024 · 0 comments
Open
1 of 3 tasks

Tracking Issue for raw-pointer-to-reference conversion methods #122034

RalfJung opened this issue Mar 5, 2024 · 0 comments
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C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. T-libs Relevant to the library team, which will review and decide on the PR/issue.

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@RalfJung
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RalfJung commented Mar 5, 2024 •
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Feature gate: #![feature(as_ref_unchecked)] (or similar)

This is a tracking issue for raw-pointer-to-reference conversion methods (rust-lang/libs-team#342).

With #106116, we have methods for almost all casts/conversions one wants to do on references and pointers, to avoid as casts and prefix operators. Just one direction is missing: turning raw pointers into references. Here we have as_ref/as_mut, but they behave different from &*ptr/&mut *ptr: they return an Option and perform a null-check. (These are the only methods on raw pointers that perform a null check.)

Public API

impl<T> *const T {
  unsafe fn as_ref_unchecked<'a>(self) -> &'a T {
    &*self
  }
}
impl<T> *mut T {
  unsafe fn as_ref_unchecked<'a>(self) -> &'a T {
    &*self
  }
  unsafe fn as_mut_unchecked<'a>(self) -> &'a mut T {
    &mut *self
  }
}

Motivating examples or use cases

Some random examples from a quick grep of the rustc sources:

&mut *(out as *mut &mut dyn PrintBackendInfo)
&mut *(state as *mut &mut dyn FnMut(&[u8]) -> io::Result<()>)
&mut *(self.0 as *mut _)
&mut *(vec as *mut Vec<Library>)
&mut *(value as *mut T as *mut UnsafeCell<T>)
&mut *(s as *mut T).cast::<[T; 1]>()

The examples above then become

out.cast::<&mut dyn PrintBackendInfo>().as_mut_unchecked()
state.cast::<&mut dyn FnMut(&[u8]) -> io::Result<()>>().as_mut_unchecked()
self.0.cast().as_mut_unchecked()
vec.cast::<Vec<Library>>().as_mut_unchecked()
ptr::from_mut(value).cast::<UnsafeCell<T>>().as_mut_unchecked()
ptr::from_mut(s).cast::<[T; 1]>().as_mut_unchecked()

Unfortunately, since the as_mut name is already taken, these are all longer than the prefix variants. But they can be read left-to-right which is an advantage.

Steps / History

Unresolved Questions

  • Is there a shorter name we could use?

Footnotes

  1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩
@RalfJung RalfJung added C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Mar 5, 2024
GrigorenkoPV added a commit to GrigorenkoPV/rust that referenced this issue Mar 14, 2024
@GrigorenkoPV
Implement ptr_as_ref_unchecked (rust-lang#122034)
d6a1b36
@GrigorenkoPV GrigorenkoPV mentioned this issue Mar 14, 2024
Merged
@GrigorenkoPV GrigorenkoPV mentioned this issue May 3, 2024
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workingjubilee added a commit to workingjubilee/rustc that referenced this issue May 3, 2024
@workingjubilee
Rollup merge of rust-lang#122492 - GrigorenkoPV:ptr_as_ref_unchecked,…
cce23b1 
… r=workingjubilee

Implement ptr_as_ref_unchecked

Implementation of rust-lang#122034.

Prefixed the feature name with `ptr_` for clarity.

Linked const-unstability to rust-lang#91822, so the post there should probably be updated to mentions the 3 new methods when/if this PR is merged.
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue May 3, 2024
@matthiaskrgr
Rollup merge of rust-lang#122492 - GrigorenkoPV:ptr_as_ref_unchecked,…
96c3f44 
… r=workingjubilee

Implement ptr_as_ref_unchecked

Implementation of rust-lang#122034.

Prefixed the feature name with `ptr_` for clarity.

Linked const-unstability to rust-lang#91822, so the post there should probably be updated to mentions the 3 new methods when/if this PR is merged.
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue May 3, 2024
@matthiaskrgr
Rollup merge of rust-lang#122492 - GrigorenkoPV:ptr_as_ref_unchecked,…
a296693 
… r=workingjubilee

Implement ptr_as_ref_unchecked

Implementation of rust-lang#122034.

Prefixed the feature name with `ptr_` for clarity.

Linked const-unstability to rust-lang#91822, so the post there should probably be updated to mentions the 3 new methods when/if this PR is merged.
rust-timer added a commit to rust-lang-ci/rust that referenced this issue May 3, 2024
@rust-timer
Unrolled build for rust-lang#122492
2e596f5 
Rollup merge of rust-lang#122492 - GrigorenkoPV:ptr_as_ref_unchecked, r=workingjubilee

Implement ptr_as_ref_unchecked

Implementation of rust-lang#122034.

Prefixed the feature name with `ptr_` for clarity.

Linked const-unstability to rust-lang#91822, so the post there should probably be updated to mentions the 3 new methods when/if this PR is merged.
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