Migrate to clap 3.0.0

[robinkrahl]

Current status:

• 2021-12-10: compiles with clap 3.0.0 and should already work quite well

To do:

• double-check extension handling
• check version output and test case
• update help text tests
  • remove help text tests?
• use clap::ArgEnum
• replace StructOpt/structopt with Parser/clap
• re-evaluate overall argument handling with regard to the new clap API
• consider making clap_generate optional
• re-evaluate clap feature selection

[d-e-s-o]

Thanks for the pull request! Will take a look this weekend.

[d-e-s-o]

Looks good so far, @robinkrahl. It seems we may have to bump the minimum supported Rust version to at least 1.51 with these changes.

[robinkrahl]

Thanks for the review! Do you have an opinion on whether we should keep the help text tests after the migration? I’m still undecided. They might be a good reminder for updating the man page if the CLI changes, but they also create some noise.

[d-e-s-o]

Do you have an opinion on whether we should keep the help text tests after the migration?

It's not a particularly strongly held opinion, but I'd remove them. As you say, there may some value as a reminder, but ultimately I suspect that will only work until we are conditioned to accepting changes in those tests without questioning them much. I also believe we should be able to get similar value through peer reviews.