Custom error message support

[inf17101]

Hello,

I am using clap_derive and I have the problem that when the attribute env is used and a user provides an invalid value there then clap does output the default error message. It does not say anything to the user that the parsed environment variable was used. It outputs the error message like the user would provide the argument directly via cli option.

I would ask if there is a feature or derive attribute to change the error message so that a custom error message is print. I know that clap itself does provide such functionality but we use derive attributes and then a single Arguments::parse() call and this won't work together without changing much code.

So to explain in detail what would be great to have for better useability:

• if the user sets an environment variable and you are using the derive env attribute, then clap_derive shall output the context that the environment variable was used in case of an error
• if the user uses the cli argument directly then it is ok to output the default context that the provided cli parameter is invalid in case of an error

Also workarounds are welcome. :-)

[epage]

Looks like most places have the relevant context that this could be baked directly into clap. Feel free to make an issue for this.