std::sync::ONCE_INIT is deprecated

[JacobEberhardt]

std::sync::ONCE_INIT is deprecated which leads to warnings when building.
Would be great, if this could be replaced.

Built with
rustc 1.38.0-nightly (6e310f2ab 2019-07-07)

Reproducible by building the example from readme.md.
This results in the following warning:
warning: use of deprecated item 'lazy::std::sync::ONCE_INIT': the new function is now preferred --> src/inline_lazy.rs:14:9 | 14 | pub use self::std::sync::ONCE_INIT; | ^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: #[warn(deprecated)] on by default

[BurntSushi]

ONCE_INIT won't actually be deprecated for another two releases.

We should just add a targeted allow(deprecated) to its use of ONCE_INIT. There's not much point in bumping the MSRV from 1.24 to 1.31 just so Once::new can be used.

[AxelNennker]

Sorry for commenting on a closed issue...
I am wondering whether you considered using https://github.com/dtolnay/rustversion
So if the rustc version is 1.31 or newer use Once::new otherwise use ONCE_INIT
Or something.
Thanks for maintaining lazy_static