Add Default implementation for std::sync::Once

[tbu-]

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[rustbot]

r? @clarfonthey

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[clarfonthey]

Technically insta-stable and requires an FCP, although this seems very reasonable to me.

@rustbot label +needs-fcp +waiting-on-fcp -waiting-on-review +T-libs-api -T-libs

[rustbot]

Error: Unknown labels: waiting-on-fcp

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[joshtriplett]

@rfcbot merge libs-api

[rust-rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

No concerns currently listed.

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[rust-rfcbot]

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