Introduce possible_values_t

[Gordon01]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

3.0.0-rc.7

Describe your use case

possible_values are not chacked for ranges at compile time, like a default_value_t and this slightly reduce the robustness of the code.

For example, this code compiles and fails at runtime:

#[clap(short, long, default_value_t = 16, possible_values = ["16", "512"])]
fat: u8,

Describe the solution you'd like

The following code should result in error like this:

#[clap(short, long, default_value_t = 16, possible_values_t = [16, 512])]
fat: u8,

error: literal out of range for `u8`
note: the literal `512` does not fit into the type `u8` whose range is `0..=255`

Alternatives, if applicable

No response

Additional Context

No response

[epage]

A not-great workaround until this is implemented is to use ArgEnum where you #[clap(name = "N")] each variant.

I've normally thought of possible values for enums. It makes sense to also support numbers. I am curious though, what is your use case for only accepting specific numbers?

[Gordon01]

I am curious though, what is your use case for only accepting specific numbers?

FAT filesystem type (12, 16 or 32)

[epage]

#3732 will have an impact on how we design / expose this

[epage]

With #3732 and friends, possible_values has been deprecated and replaced by value_parser, so I'm closing this out