Set an msrv requirement and test it on CI

[Veetaha]

The current code is tested with 1.80.1 on CI, but I tried to keep it compatible with older versions manually.
We need to set the required-rust-version in Cargo.toml and the MSRV version to the tests on CI. CI should have matrix jobs for each check that validate the code on both MSRV and latest stable

[EdJoPaTo]

Personally I would try to get 1.63 (or earlier) as MSRV in order to support current Debian bullseye. That would allow for tools depending on bon to be packaged natively for Debian Bullseye.

Also, -Zmsrv-policy (part of the upcoming resolver = "3" / edition 2024) should help with that. See rust-lang/cargo#13873

The following graphic contains the Rust version in currently supported (no out of support) distros known to repology.org:

[Veetaha]

Thank you for the insight! derive-builder supports 1.56, although it depends on syn 2.0 that supports 1.61+. I believe it's better to follow syn in this regard, since any change to syn may accidentally break its current untested compatibility with rustc 1.56.

[Veetaha]

Correction syn 2.0.56 supports rust version 1.56, but syn 2.0.57 bumped that requirement. I think to be able to replace derive-builder we need to do the same then (1.56 MSRV). Not sure how hard it'll be and if it will be hard I'm fine with using 1.61 as MSRV to follow the cutting edge of syn. The advantage of derive-builder that makes it easy for them to support 1.56 is that the crate is mostly feature complete and slow-moving, while bon is moving fast.

[EdJoPaTo]

Personally I think it's fine to bump MSRV when the rust-version on the published versions on crates.io is correct. That way an older version of bon can still be used when an older version of Rust (like on Debian) is required for some reason.

Also, something can have an older MSRV when a dependency of it has a newer one. -Zmsrv-policy will still be able to figure it out. And crate patches also exist to adapt / remove code from some crate.

[Veetaha]

Personally I think it's fine to bump MSRV when the rust-version on the published versions on crates.io is correct

What do you mean by "when the rust-version is correct"? If it's incorrect, then it's a bug

[Veetaha]

I've set MSRV to 1.70.0 for now. The problem with making it lower is that crates we use for testing (such as trybuild, for example), don't compile on versions lower than that. I may consider lowering this requirement in the future if a compelling use case emerges or once I just have enough will to fight with this

[Veetaha]

I created a followup issue to keep lowering the MSRV (#102). This first iteration is just a good enough first attempt at lowering it. So no reason to be sad

[Veetaha]

This feature is now available in bon 2.2. Here is the release blog post on Reddit