FR: derive Default also for Structs with non implementers of Default

[thkoch2001]

Given a struct:

struct Outlink {
    rel: Option<String>,
    context: Context,
    url: Url,                // Url does not implement Default !!!
    redirect_count: usize,
    content_type: Option<String>,
    // ... more members implementing Default
}

I want to be able to:

#[derive(Default)]
struct Outlink {
    rel: Option<String>,
    context: Context,
    #[default_from=EXPRESSION]
    url: Url,
    redirect_count: usize,
    content_type: Option<String>,
    // ... more members implementing Default
}

Where EXPRESSION is any valid expression of type Url, e.g. Url.parse("file:///dummy").unwrap().

This satisfies at least two scenarios:

a) There is no need anymore for trivil constructors, as they can instead be written as:

    let url = ...;
    let outlink = Outlink {
        url .. Outlink::Default()
    }

b) Types that have no intrinsic default value but do have a sensible default value in the context of another type.

[kennytm]

if Url::parse is a const fn, then #3681 already covered it.

[thkoch2001]

Yes! Thank you! The syntax in #3681 is also much less surprising than mine.

For the record, I actually ended up with splitting the struct in two, which also makes more sense semantically in my app:

#[derive(Default)]
struct Inlink {
    rel: Option<String>,
    context: Context,
    redirect_count: usize,
    content_type: Option<String>,
}

struct Outlink {
    url: Url,
    i: Inlink,
}