Add Default impls for Pinned Box, Rc, Arc

[Jules-Bertholet]

Fixes #143688.

@rustbot label T-libs-api needs-fcp

Also needs a crater run, as the Box impls could theoretically be breaking due to #[fundamental] (though a cursory search suggests this is unlikely to cause issues).

[rustbot]

r? @tgross35

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[tgross35]

@bors2 try

[rust-bors]

⌛ Trying commit 9060b6b with merge 5ef65a4…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5ef65a4 (5ef65a4ed75279dc1b7a26cedc27293a9f4ca8db, parent: 32cd9114712a24010b0583624dc52ac302194128)

[tgross35]

@craterbot check

[craterbot]

👌 Experiment pr-143717 created and queued.
🤖 Automatically detected try build 5ef65a4
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-143717 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-143717 is completed!
📊 7 regressed and 5 fixed (662328 total)
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[Jules-Bertholet]

All spurious failures. “prepare-fail (112811)” seems not-great though. Looks like a lot of “no space left on device” errors? If an issue with crater is preventing a large proportion of the crates from being properly tested…

[Skgland]

All spurious failures. “prepare-fail (112811)” seems not-great though. Looks like a lot of “no space left on device” errors? If an issue with crater is preventing a large proportion of the crates from being properly tested…

Note that the prepare-fail category is a relatively recent change, they used to be categorized as error. Also prepare-fail results are included in the retry-regressed-list.txt

[Amanieu]

@rfcbot merge

[rfcbot]

Team member @Amanieu 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

Concerns:

• Box<T>: Default (Add Default impls for Pinned Box, Rc, Arc #143717 (comment))

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.

[dtolnay]

Consider generalizing the Pin<Box<>> impl like this:

  impl<T> Default for Pin<Box<T>>
  where
-     T: Default
+     Box<T>: Default

This would apply to types like Pin<Box<serde_json::value::RawValue>>, where RawValue does not implement Default but Box<RawValue> does.

@rfcbot concern Box<T>: Default