Add MSRV; add MSRV+minimal-versions check

[Finomnis]

This is an alternative to #26 and #27.

It combines the MSRV and minimal-versions check. This has several advantages:

• No external dependencies can break the MSRV test through an update.
• Building with the oldest compatible dependency versions has the potential to find the actual minimal Rust version this crate can be built with.
• Even if an update of a dependency breaks this crate for the MSRV (through an MSRV update of itself, for example), we can guarantee to the user that he can still build this crate by reverting to the minimal dependency versions.

[Finomnis]

@mciantyre I also fixed it so that we don't force the user to upgrade to a newer dependency version.

[Finomnis]

I think that points one and two are true.

In my opinion the point where I disagree is that the user should be able install a version with a defect. If a dependency has a defect, it should be yanked. All non-yanked versions should work fine, just have a different amount of features.

I'm concerned I can't provide a guarantee like "imxrt-usbd works with 1.74 for 8 months" if I need to force a dependency update.

I'm not sure how you could ever guarantee that. As you mentioned, if there is a defect in a dependency, and a new version of the dependency fixes the defect but features a new MSRV, then you need to update, no matter what. I guess that's a general problem of not having the control over all dependencies.

So I'm unsure what to do now... Do you object to the changes I proposed in general? Just don't have an MSRV?

[Finomnis]

@mciantyre The official guidelines seem to be:

• https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field
• https://doc.rust-lang.org/cargo/guide/continuous-integration.html#verifying-rust-version

There don't seem to be more than that, so it's kind of open to interpretation.

In my own projects, I check all four of:

• MSRV with newest dependencies
• MSRV with oldest dependencies
• Latest Rust with newest dependencies
• Latest Rust with oldest dependencies

Although I do suppose this is a little overkill.

[Finomnis]

As more references, this is how tokio does the min-versions thing: https://github.com/tokio-rs/tokio/blob/59579465bee804b65a66f95c810c3ea01bb923a1/.github/workflows/ci.yml#L277-L307

And here is some more discussion around that topic: https://users.rust-lang.org/t/psa-please-specify-precise-dependency-versions-in-cargo-toml/71277

[Finomnis]

Closed due to inactivity.