Disallow !Sync types in #[once] fixtures

[narpfel]

!Sync types must not be shared to different threads by reference.

Note: This PR is an API break (because !Sync types were allowed before). If !Sync types are required for #[once] fixtures, they can be wrapped in a Mutex to prevent data races.

This PR also bumps the MSRV to Rust 1.60 due to the usage of once_cell. Rust 1.60 was released on 2022-04-07.

Resolves #235.

[la10736]

THX for this PR. When I implemented this feature once_cell was not stable yet. I noted just now your're not using OnceCell from std::cell::once_cell. I'm not incline to use an external dependency for it.

[la10736]

Ok, we can remove this external dependency and use std::sync::OnceLock instead.

Moreover your changes to errors.rs file introduce an annoying regression 😢. The compiler stop t show some of the errors in the first lines of the file... this kind of tests are fragile. I solved it by separate in 2 files: I'll share my branch if you like.

Finally, can you add a log line in changelog file?

[la10736]

You can find my branch here https://github.com/la10736/rstest/tree/temp-once-fixture-not-sync-types

[narpfel]

I'm not incline to use an external dependency for it.

👍 I was not sure what the MSRV policy for rstest is, so I picked the option that hat a lower MSRV.

Moreover your changes to errors.rs file introduce an annoying regression 😢.

I only tested with nightly locally. 😊

Finally, can you add a log line in changelog file?

Done.

[la10736]

LGTM!!! THX