`serde`-like default value behavior

[epage]

Maintainer's notes

• We can use the field's Default::default with #[clap(default_value_t)]
• Design work is needed for using the container's Default::default

---

Issue by epage Thursday May 03, 2018 at 13:17 GMT Originally opened as https://github.com/TeXitoi/structopt/issues/105

---

It'd be nice to have annotations like serde for default values

• Use the struct's Default impl
• Use the field's Default impl

[epage]

Comment by TeXitoi Thursday May 03, 2018 at 13:38 GMT

---

The problem is that, most of the time, clap will not be able to show the default value (because just a few types implement Display).

[epage]

default_value_t allows you to do the type's Default impl or a custom value of the type, so long as the type implements Display.

[epage]

I missed the case of using the container's Default::default. The question is how to specify this. Serde does this by having a container-level default attribute but for us to do that, we would make all fields optional. This would require a weird handling of structs to use flatten to leverage it or a new way to specify it at the field level