Implement core::future::lazy

[ibraheemdev]

Lazy executes a closure the first time it is polled:

let a = std::future::lazy(|_| 1);
assert_eq!(a.await, 1);

Prior art: https://docs.rs/futures/latest/futures/future/fn.lazy.html

The argument for exposing task::Context to the closure is that lazy can serve as a one-time poll_fn that accepts an FnOnce, as opposed to an FnMut. See here for details.

(Would a better name be once?)

[rust-highfive]

r? @joshtriplett

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[bors]

☔ The latest upstream changes (presumably #91691) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

Triage: merge conflicts

[JohnCSimon]

@ibraheemdev can you please resolve the merge conflicts?

[JohnCSimon]

Ping from triage:
@joshtriplett - what is the status of reviewing this PR? Thanks.

[thomcc]

@ibraheemdev Mind making an ACP for this.

r=me on the impl if the API gets a second.

r? @thomcc

[thomcc]

@rustbot author

[ibraheemdev]

After thinking about it, I'm not sure there's any benefit of this over an async block and something like #92116 (comment) to access the context. I'm going to close this for now.