`Parser::parse_from` will call `exit` on failure and we don't just lose
backtrace information but we don't even know which of the tests running
in parallel panicked. I ran into this when experimenting with
`clap_derive` and I couldn't tell what actually failed.
So let's switch to `Parse::try_parse_from`.
Errors went from:
```
test option_option ... ok
error: Found argument 'bar' which wasn't expected, or isn't valid in this context
USAGE:
clap_derive [OPTIONS]
For more information try --help
error: test failed, to rerun pass '--test arg_enum'
```
To:
```
test option_option ... ok
test variant_with_defined_casing ... ok
test skip_variant ... ok
test default_value ... ok
test vector ... FAILED
test option_vector ... ok
failures:
---- vector stdout ----
thread 'vector' panicked at 'called `Result::unwrap()` on an `Err` value: Error { message: Formatted(Colorizer { use_stderr: true, color_when: Auto
, pieces: [("error:", Some(Red)), (" ", None), ("Found argument '", None), ("bar", Some(Yellow)), ("' which wasn't expected, or isn't valid in this
context", None), ("\n\n", None), ("USAGE:\n clap_derive [OPTIONS]", None), ("\n\nFor more information try ", None), ("--help", Some(Green)), ("
\n", None)] }), kind: UnknownArgument, info: ["bar"], source: None, backtrace: Backtrace }', clap_derive/tests/arg_enum.rs:388:56
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
vector
test result: FAILED. 15 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
error: test failed, to rerun pass '--test arg_enum'
```
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
