version conflicts

[pdgilbert]

I am building some things that work with multiple device crates and have just started getting cargo update errors because one crate has update bxcan while another has not:

error: failed to select a version for `defmt`.
    ... required by package `bxcan v0.5.0`
    ... which is depended on by `stm32f7xx-hal v0.2.0 (https://github.com/stm32-rs/stm32f7xx-hal#c0bb1f2b)`
    ... which is depended on by `eg_stm_hal v0.1.0 (/home/paul/githubClones/eg_stm_hal/boards/none-stm32f722)`
versions that meet the requirements `^0.2.0` are: 0.2.1, 0.2.0

the package `defmt` links to the native library `defmt`, but it conflicts with a previous package which links to `defmt` as well:
package `defmt v0.1.3`
    ... which is depended on by `bxcan v0.4.0`
    ... which is depended on by `stm32f0xx-hal v0.17.1 (https://github.com/stm32-rs/stm32f0xx-hal#e9c99650)`
    ... which is depended on by `eg_stm_hal v0.1.0 (/home/paul/githubClones/eg_stm_hal/boards/none-stm32f722)`

failed to select a version for `defmt` which could resolve this conflict

It seems that bxcan demands a specific version of defmt and also in the README.md usage suggests demanding a specific version of bxcan (now bxcan = "0.5.0" but I see it is updated automatically).

Any chance you can be more flexible by depending on defmt = "^..." or ">=..." and suggest similar flexibility regarding bxcan in your README?

[jonas-schievink]

Yeah, it would be good to improve this. I think it's mostly up to defmt and Cargo to provide better handling of dependencies that use links here, see knurling-rs/defmt#426 and rust-lang/cargo#7880.

[pdgilbert]

Ok, I see the defmt issue is a bit messy. But I think crates using bxcan can help by specifying bxcan = ">=0.4" rather than `bxcan = "=0.5.0" ? Could you at least change the README.md?

[jonas-schievink]

I've opened #35 to make defmt support opt-in, and to allow patch versions of bxcan to update defmt (which normally would be considered a breaking change). This should lift some of the burden from HALs, since they no longer need to update to a new breaking bxcan release every time a new defmt version comes out.

Naturally, the drawback is that the defmt version may change, so that means that mostly only applications (which check in their lockfile) would want to use the defmt feature.