Implement DerefMut for PathBuf

[zertosh]

Without this, there's no way to get a &mut Path from PathBuf without
going through into_boxed_path. This is relevant now that #105002 adds
PathBuf::as_mut_os_string and Path::as_mut_os_str.

[rustbot]

r? @thomcc

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[rustbot]

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.

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• Changing observable runtime behavior of library APIs

[zertosh]

Now sure if I should include these here or make separate PRs, but the following impls would also be useful now but are missing:

• impl BorrowMut<Path> for PathBuf
• impl AsMut<Path> for PathBuf
• impl AsMut<Path> for Path
• impl AsMut<OsStr> for Path
• impl From<&mut Path> for PathBuf
• impl From<&mut OsStr> for PathBuf

[dtolnay]

Deref from &mut PathBuf to &mut Path seems non-problematic given that the same thing is already expressible less conveniently as:

fn mutate_path_buf(path_buf: &mut PathBuf) {
    let tmp: PathBuf = mem::take(path_buf);
    let mut tmp: Box<Path> = tmp.into_boxed_path();
    mutate_path(&mut *tmp);
    *path_buf = tmp.to_path_buf();
}

fn mutate_path(path: &mut Path) {
    // ...
}

@rust-lang/libs-api:
@rfcbot fcp merge

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[riking]

I don't see a #[repr(transparent)] on the struct definition - I don't think this transmute is correct.

[dtolnay]

This is correct for standard library code.

[BurntSushi]

@dtolnay Can you say more? What property of std lets it do this?

[dtolnay]

The fact that if any future compiler change caused these types to be given different layout or disrupted the present behavior of this code in any other way, the same change would atomically adjust std to accommodate that. In other words the standard library is written to function under exactly one compiler version ever, whereas downstream code is required to be sound under all possible future compilers which are non-breaking evolutions of the current implicit "spec".

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[dtolnay]

@bors r+

[bors]

📌 Commit 2c54178 has been approved by dtolnay

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2c54178 with merge 9c07efe...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 9c07efe to master...

[rust-timer]

Finished benchmarking commit (9c07efe): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	3.2%	[2.8%, 3.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-3.6%, -3.6%]	1
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Cycles

This benchmark run did not return any relevant results for this metric.