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Adjust argument type for mutable with_metadata_of (#75091) #103346

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Merged
merged 2 commits into from
Oct 22, 2022
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Adjust argument type for mutable with_metadata_of (#75091) #103346

merged 2 commits into from
Oct 22, 2022

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197g
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@197g 197g commented Oct 21, 2022 •
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The method takes two pointer arguments: one self supplying the pointer value, and a second pointer supplying the metadata.

The new parameter type more clearly reflects the actual requirements. The provenance of the metadata parameter is disregarded completely. Using a mutable pointer in the call site can be coerced to a const pointer while the reverse is not true.

In some cases, the current parameter type can thus lead to a very slightly confusing additional cast. Example.

// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);

This could then instead be simplified to:

// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = uninit.as_ptr().with_metadata_of(&**val);

Tracking issue: #75091

@dtolnay you're reviewed #95249, would you mind chiming in?

Andreas Molzer added 2 commits October 21, 2022 14:46
Argument type for mutable with_metadata_of (rust-lang#75091)
71c39de 
The method takes two pointer arguments: one `self` supplying the pointer
value, and a second pointer supplying the metadata.

The new parameter type more clearly reflects the actual requirements.
The provenance of the metadata parameter is disregarded completely.
Using a mutable pointer in the call site can be coerced to a const
pointer while the reverse is not true.

An example of the current use:

```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);
```

This could then instead be simplified to:

```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = uninit.as_ptr().with_metadata_of(&**val);
```
Reduce mutability in std-use of with_metadata_of
e3606b2
@rustbot rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Oct 21, 2022
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rustbot commented Oct 21, 2022

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

  • Stabilizing library features
  • Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
  • Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
  • Changing public documentation in ways that create new stability guarantees
  • Changing observable runtime behavior of library APIs

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r? @joshtriplett

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 21, 2022
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197g commented Oct 21, 2022

r? dtolnay

@dtolnay dtolnay assigned dtolnay and unassigned joshtriplett Oct 21, 2022
@dtolnay dtolnay added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 21, 2022
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:) I like it. Thank you for sharing the real-world example, that is compelling.

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dtolnay commented Oct 21, 2022

@bors r+

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bors commented Oct 21, 2022

📌 Commit e3606b2 has been approved by dtolnay

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 21, 2022
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197g commented Oct 21, 2022

@dtolnay Would you also adjust the tracking issue to reflect the current API when the merge has gone through? I'm afraid it's somewhat out-of-date and doesn't reflect the full history.

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dtolnay commented Oct 21, 2022

Done.

@Dylan-DPC Dylan-DPC mentioned this pull request Oct 22, 2022
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Dylan-DPC added a commit to Dylan-DPC/rust that referenced this pull request Oct 22, 2022
@Dylan-DPC
Rollup merge of rust-lang#103346 - HeroicKatora:metadata_of_const_poi…
91722f2 
…nter_argument, r=dtolnay

Adjust argument type for mutable with_metadata_of (rust-lang#75091)

The method takes two pointer arguments: one `self` supplying the pointer value, and a second pointer supplying the metadata.

The new parameter type more clearly reflects the actual requirements. The provenance of the metadata parameter is disregarded completely. Using a mutable pointer in the call site can be coerced to a const pointer while the reverse is not true.

In some cases, the current parameter type can thus lead to a very slightly confusing additional cast. [Example](197g/static-alloc@cad9377).

```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);
```

This could then instead be simplified to:

```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = uninit.as_ptr().with_metadata_of(&**val);
```

Tracking issue: rust-lang#75091

`@dtolnay` you're reviewed rust-lang#95249, would you mind chiming in?
@Dylan-DPC Dylan-DPC mentioned this pull request Oct 22, 2022
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bors added a commit to rust-lang-ci/rust that referenced this pull request Oct 22, 2022
@bors
Auto merge of rust-lang#103398 - Dylan-DPC:rollup-cj6w00o, r=Dylan-DPC
289b2b8 
Rollup of 7 pull requests

Successful merges:

 - rust-lang#102602 (Slightly tweak comments wrt `lint_overflowing_range_endpoint`)
 - rust-lang#103190 (rustdoc: render bounds of cross-crate GAT params)
 - rust-lang#103224 (Allow semicolon after closure within parentheses in macros)
 - rust-lang#103280 ((rust-lang#102929) Implement `String::leak` (attempt 2))
 - rust-lang#103329 (Add a forgotten check for NonNull::new_unchecked's precondition)
 - rust-lang#103346 (Adjust argument type for mutable with_metadata_of (rust-lang#75091))
 - rust-lang#103360 (Reduce false positives in msys2 detection)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit b22559f into rust-lang:master Oct 22, 2022
@rustbot rustbot added this to the 1.66.0 milestone Oct 22, 2022
@197g 197g deleted the metadata_of_const_pointer_argument branch October 22, 2022 16:34
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