Impl Default for PhantomPinned

[petertodd]

PhantomPinned is just a marker type, with an obvious default value (the only value). So I can't think of a reason not to do this. Sure, it's used in exotic situations with unsafe code. But the people writing that code can decide for themselves if they can derive Default, and in many situations the derived impl will make sense:

#[derive(Default)]
struct NeedsPin {
   marker: PhantomPinned,
   buf: [u8; 1024],
   ptr_to_data: Option<*const u8>,
}

[rust-highfive]

r? @shepmaster

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[shepmaster]

r? @withoutboats

[dtolnay]

@rust-lang/libs: This PR adds Default for https://doc.rust-lang.org/std/marker/struct.PhantomPinned.html.
The type is a unit struct so it has a single possible value and is safe to construct, so Default seems like a no-brainer.

@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
• @KodrAus
• @dtolnay
• @sfackler
• @withoutboats

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. 🔔

[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.

The RFC will be merged soon.

[dtolnay]

@bors r+

[bors]

📌 Commit 28f8e62 has been approved by dtolnay

[bors]

⌛ Testing commit 28f8e62 with merge 068320b...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 068320b to master...

[pthariensflame]

relnotes needed?