> How mature does a feature need to be before it's permitted to use them? Each project has its own answer for that question; the acronym for it is MSRV (Minimu

> How mature does a feature need to be before it's permitted to use them? Each project has its own answer for that question; the acronym for it is MSRV (Minimum Supported Rust Version), and there's a field in the Cargo.toml for it ("rust-version"). It's not unusual to have the CI test the project with both the MSRV and the latest stable Rust version, to catch accidental use of new features. Unfortunately, there's as of yet no consensus on how to choose the MSRV. Some projects are more conservative ("the MSRV has to be at least one year old", "the MSRV is whatever comes with the oldest still supported LTS of Linux distros X and Y", "I set the MSRV once in the past and don't intend to increase it ever"), some projects are more aggressive ("we support only latest Rust and the previous one"), other projects are somewhere in the middle. (Edit: there's at the moment a very interesting discussion about the MSRV of the "libc" crate, which a lot of other crates depend on, and MSRV in general, at https://github.com/rust-lang/libs-team/issues/72 https://github.com/rust-lang/libs-team/issues/72)