Stabilization does not instantly progress the code from nightly through beta to stable. That progression occurs once every six weeks, late on a Thursday U.S. time. Rust 1.43.0 was released on 23 April 2020. Thus I would expect this feature to become available in beta on 4 June with the release of Rust 1.44.0, and in stable on 16 July with the release of Rust 1.45.0.