An arg can now be conditionally required (i.e. it's only required if arg
A is used with a value of V).
For example:
```rust
let res = App::new("ri")
.arg(Arg::with_name("cfg")
.required_if("extra", "val")
.takes_value(true)
.long("config"))
.arg(Arg::with_name("extra")
.takes_value(true)
.long("extra"))
.get_matches_from_safe(vec![
"ri", "--extra", "val"
]);
assert!(res.is_err());
assert_eq!(res.unwrap_err().kind, ErrorKind::MissingRequiredArgument);
```
Relates to #764
An arg can now conditionally require additional arguments if it's value matches a specific value.
For example, arg A can state that it only requires arg B if the value X was used with arg A. If any
other value is used with A, arg B isn't required.
Relates to #764
One can now implement conditional default values. I.e. a default value that is only applied in
certain conditions, such as another arg being present, or another arg being present *and*
containing a specific value.
Now it's possible to say, "Only apply this default value if arg X is present" or "Only apply this
value if arg X is present, but also only if arg X's value is equal to Y"
This new method is fully compatible with the current `Arg::default_value`, which gets set only if
the arg wasn't used at runtime *and* none of the specified conditions were met.
Releates to #764
Instead of using Format::Error directly, we need to use a Colorizer to
make sure that the message only gets colorized when it is appropriate
to do so.
Fixes#775
One can now use `Arg::allow_hyphen_values(true)` which will enable `--opt -val` style values only
for the specific arg and not command wide.
Closes#742
Flags, Opts, and Positionals now store their internals using compartmented Base, Valued, and
Switched structs to keep the code duplication down and make it easier to maintain.
Iniside the src/app/parser.rs there have been several changes to make reasoning about the code
easier. Primarily moving related sections out of the large get_matches_with into their own
functions.
Now one can build CLIs that support things like `mv <files>... <target>`
There are a few requirements and caveats;
* The final positional argument (and all positional arguments prior) *must* be required
* Only one positional argument may be `multiple(true)`
* Only the second to last, or last positional argument may be `multiple(true)`
Closes#725
Prior to this commit, conflicting error messages and the suggeseted usage would depend on whether
you defined the conflict on both arguments, or just one, and the order in which you specified the
conflicting arguments at runtime.
Now they are symetrical, meaning the suggestions from the error message are consistent, and it no
longer matters if you specify the conflict in one, or both arguments.
Closes#718
Don't attempt to change the display order of flags/options until any app
settings have been propagated down from a parent App in case DeriveDisplayOrder
and/or UnifiedHelpMessage are propagated.
Fixes#706
One can now use `AppSettings::AllowNegativeNumbers` which functions similar to `AllowLeadingHyphen`
with the exception that only unknown numbers are allowed have a preceding hyphen character.
See the documentation for details and examples.
Closes#696
