impl<T: AsRawFd> AsRawFd for {Arc,Box}<T>

[jyn514]

This allows implementing traits that require a raw FD on Arc and Box.

Previously, you'd have to add the function to the trait itself:

trait MyTrait {
    fn as_raw_fd(&self) -> RawFd;
}

impl<T: MyTrait> MyTrait for Arc<T> {
    fn as_raw_fd(&self) -> RawFd {
        (**self).as_raw_fd()
    }
}

In particular, this leads to lots of "multiple applicable items in scope" errors because you have to disambiguate MyTrait::as_raw_fd from AsRawFd::as_raw_fd at each call site. In generic contexts, when passing the type to a function that takes impl AsRawFd it's also sometimes required to use T: MyTrait + AsRawFd, which wouldn't be necessary if I could write MyTrait: AsRawFd.

After this PR, the code can be simpler:

trait MyTrait: AsRawFd {}

impl<T: MyTrait> MyTrait for Arc<T> {}

[rust-highfive]

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 r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. 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

[rust-highfive]

r? @kennytm

(rust-highfive has picked a reviewer for you, use r? to override)

[jyn514]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[sunfishcode]

Would it make sense for these to implement AsFd as well?

[joshtriplett]

@rfcbot merge

[rfcbot]

Team member @joshtriplett 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. 🔔

[dtolnay]

@bors r+

[bors]

📌 Commit 70a350264bfdc3f4e3c11ac00248c5974ff8768e has been approved by dtolnay

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

⌛ Testing commit cf483a1 with merge b04bfb4...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing b04bfb4 to master...

[rust-timer]

Finished benchmarking commit (b04bfb4): comparison url.

Instruction count

• Primary benchmarks: 😿 relevant regressions found
• Secondary benchmarks: 😿 relevant regressions found

	mean1	max	count2
Regressions 😿 (primary)	0.9%	1.0%	2
Regressions 😿 (secondary)	2.1%	2.5%	6
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	0.9%	1.0%	2

Max RSS (memory usage)

Results

• Primary benchmarks: 🎉 relevant improvements found
• Secondary benchmarks: 🎉 relevant improvements found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	-2.6%	-3.6%	2
Improvements 🎉 (secondary)	-3.4%	-4.0%	3
All 😿🎉 (primary)	-2.6%	-3.6%	2

Cycles

Results

• Primary benchmarks: 🎉 relevant improvement found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	3.7%	3.7%	1
Improvements 🎉 (primary)	-2.4%	-2.4%	1
Improvements 🎉 (secondary)	-3.7%	-3.7%	1
All 😿🎉 (primary)	-2.4%	-2.4%	1

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³

[Mark-Simulacrum]

The two regressed benchmarks have recently been particularly sensitive and the regressions don't seem related to this PR (not in doc builds, for example). Dropping the regression label.