Edition and MSRV with trybuild

[TedDriggs]

derive_builder uses trybuild and has started getting CI failures on its MSRV of 1.40 because once_cell is using the 2021 edition. Happily, we're only using trybuild with 1.59, so I think this problem goes away if I can find a way to avoid 1.40 ever encountering the trybuild dependency.

Unfortunately, rust-lang/cargo#1596 doesn't seem to have any movement, so I'm a bit stumped on how to proceed.

My first thought was to move these tests to their own crate, but I don't know if there's a good way to conditionally include a crate in a workspace so I'm not sure if that actually changes anything for me.

[dtolnay]

rust-lang/cargo#1596 (comment) describes an idiom for optional dev-dependencies.

[TedDriggs]

Can that be used to target a particular rust version?

[dtolnay]

Yes if your CI only enables cfg(skip_ui_tests) on particular rust versions.