Port mdBook to clap3

[epage]

This includes resolving the deprecation warnings, one warning per commit. This was a fairly straightforward conversion, sticking to simpler parts of clap. App::arg_from_usage was probably the most involved. The replacement, arg!(), doesn't seem to support kebab-case in value names.

I tested by

• Adding the App::debug_assert() test
• Inspecting before/after of --help for each subcommand

I'm assuming there is some end-to-end tests to at least catch egregious cases.

Differences I noticed (all cosmetic)

• Colored help is now on all subcommands (ColoredHelp was set only on the parent before rather than with global_setting, now its the default)
• Combining of the Flags and Options sections (UnifiedHelpMessage is now the only behavior)
• Some spacing is slightly different so more lines of text wrap.

[epage]

mdBook's MSRV is at 1.46.0. clap is at 1.54. We are open to experimenting with lowering the MSRV (clap-rs/clap#3267) though it isn't a priority for us.

Should we up mdBook's MSRV or hold off on this until either (1) clap's lowers or (2) mdBook is ready to bump?

[ehuss]

I think we can probably bump the MSRV. We don't have a fixed policy, but I think 8 months is probably ok. Does 3.1.6 still work on 1.54?

[ehuss]

Can this also update

mdBook/guide/src/guide/installation.md

Line 23 in 46345b8

mdBook currently requires at least Rust version 1.46.

?

[ehuss]

Oh, and maybe add a comment in main.yml to keep them in sync (that was added in #1719, but that PR seemed to have stall).

[epage]

Updated this with the MSRV bump with the comment linking the locations.

And yes, clap is still on 1.54. We had a request to extend our MSRV to a 6 month period. We are doing this on a trial basis

[ehuss]

Thanks!