Before #2005, `Clap` was a special trait that derived all clap traits it
detected were relevant (including an enum getting both `ArgEnum`,
`Clap`, and `Subcommand`). Now, we have elevated `Clap`, `Args`,
`Subcommand`, and `ArgEnum` to be user facing but the name `Clap` isn't
very descriptive.
This also helps further clarify the relationships so a crate providing
an item to be `#[clap(flatten)]` or `#[clap(subcommand)]` is more likely
to choose the needed trait to derive.
Also, my proposed fix fo #2785 includes making `App` attributes almost
exclusively for `Clap`. Clarifying the names/roles will help
communicate this.
For prior discussion, see #2583
Before, partial command lines would panic at runtime. Now it'll be a
compile error
For example:
```
pub enum Opt {
Daemon(DaemonCommand),
}
pub enum DaemonCommand {
Start,
Stop,
}
```
Gives:
```
error[E0277]: the trait bound `DaemonCommand: clap::Args` is not satisfied
--> clap_derive/tests/subcommands.rs:297:16
|
297 | Daemon(DaemonCommand),
| ^^^^^^^^^^^^^ the trait `clap::Args` is not implemented for `DaemonCommand`
|
= note: required by `augment_args`
```
To nest this, you currently need `enum -> struct -> enum`. A later
change will make it so you can use the `subcommand` attribute within
enums to cover this case.
This is a part of #2005
