Differentiate between default_value and env in ArgMatches

[drystone]

Maintainer's notes:

Proposed implementation:

1. Rename ValueType to ArgSource
2. Add a function to look up the ArgSource

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I've a situation where it would be nice to know whether a value has come from a default value or from an environment variable. The workaround is simple but involves rechecking the ArgMatches::value_of against the environment variable.

[swfsql]

While still an open issue, I think a viable alternative would be to load values from a configuration file using serde, where no args nor env vars would be involved.

[drystone]

I'd agree if the value from a configuration file is mandatory. Perhaps, though, it would be nice to have a default so it's not required in the configs?

In order of decreasing precedence a value can come from command line, environment, configuration file, default. If a value is set in the configuration file, a clap match will only override it if it came from the environment, not if it was a default. So it would be nice to be able to differentiate.

[swfsql]

I'd agree if the value from a configuration file is mandatory. Perhaps, though, it would be nice to have a default so it's not required in the configs?

Serde does offer #[serde(default)] to use the Default::default() implementation as a fallback, but in this case you'd probably need to move all default values to functions that may be accessed by both Default::default() implementation and clap default_value binding.

But I think this would prevent the avoidance of the default as a fallback from being used, which may be valuable during production, I think.

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I agree it'd be nice to fine pick precedence between various inputs.
This crate has some merging operations of structopt/clap and deserialized information, may worth take a look: https://github.com/dalance/structopt-toml