Support something similar to serde(default), clap(default_value_t).

[cbeck88]

clap-derive has a parameter called default_value_t, used like arg(default_value_t = expr), or arg(default_value_t). (discussion)

serde has slightly different take, the syntax is serde(default) or serde(default = "path-to-function"). (docu: https://serde.rs/attr-default.html)

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We currently have default_value = "..." which is basically the same clap.

I haven't found arg(default_value_t = ...) to be that useful. serde(default) is very useful -- I actually didn't know that arg(default_value_t) worked like that in clap.

The naming default_value_t kind of irks me -- it clashes with traditional systems programming conventions.

Traditionally, in C and C++, _t indicates that something is a type.

For example, size_t is a type used to represent a size.

Many more examples from C standard library and POSIX: https://stackoverflow.com/a/231807/3598119

In JSON for modern C++, the _t things in the API are types.

In clap however, default_value_t = ... is assigned a value, and is not a type.

I find this confusing, and so I'd rather break with clap's naming and follow serde, but follow clap's semantics. So we would have arg(default) means that it is initialized with Default::default(), and arg(default = expr) mean that it is initialized with the expression.

(We would not, like serde, require that you create a named function somewhere else, when there is no good reason to make you do that afaict. We are already wrapping whatever expr you supply in a hidden function in the implementation of many things like value_parser and validation_predicate.)