For most users, this won't be worth doing, they can just specify the
parser if needed. Where this has value is crates that integrate custom
types into clap, like creating click-like file integration. See
https://click.palletsprojects.com/en/8.0.x/arguments/#file-arguments
Clap has focused on reporting development errors through assertions
rather than mixing user errors with development errors. Sometimes,
developers need to handle things more flexibly so included in #3732 was
the reporting of value accessor failures as internal errors with a
distinct type. I've been going back and forth on whether the extra
error pessimises the usability in the common case vs dealing with the
proliferation of different function combinations. In working on
deprecating the `value_of` functions, I decided that it was going to be
worth duplicating so long as we can keep the documentation focused.
The remove functions no longer return `Arc` but the core type, at the
cost of requiring `Clone`. I originally held off on this
in #3732 in the hope of gracefully transition the derive and requiring
`Clone` would have been a breaking change but when it came to #3734, I didn't
find a way to make it work without a breaking change, so I made it
opt-in. This means I can force the `Clone` requirement now.
I added the requirement for `Clone` everywhere else in the hopes that in
the future, we can drop the `Arc` without a breaking change.
When to show usage? We are currently mixed about it. For `validator`,
we didn't show it at all. Sometimes we show the used arguments and
sometimes we don't.
With `ValueParser`, I ran into the problem that we weren't showing the
used arguments like we had previously in some cases. In deciding how to
solve this, I went with the simplest route for now and removed it as the
usage likely doesn't add much context to help people solve their
problem, more so the recommendation for help. We'll see how the
feedback is on this and adjust.
For clap 3, its opt-in as a precaution against breaking
compatibility in some weird cases.
This does require the types to implement `Clone`.
Fixes#3734Fixes#3496Fixes#3589
