Space age

[adnilsson]

Description

Solutions to the Space Age exercise.

Mentoring

First iteration

The solution in b380536 manually implements the trait, copy-paste style.
I never requested mentoring on this since it was so straightforward and there were hints to use a macro to simplify.

Second iteration

I thought: "the usual way to auto-implement traits is to use a derive-directive", and so I did in 1674489

This, more or less, requires that you use additional crates--in particular, syn--to parse the token stream into a structured syntax tree.

I realized afterward that they were looking to use a declarative macro to both define the struct and implement the trait.
However, a derive directive has its benefits. For example, you can slap a Planet implementation onto any struct you want.
Or, maybe we could rename the trait more generally as SunOrbiter so Pluto can join as well =)

I consciously added the seconds field to the Duration struct because I find it less ambiguous compared to a tuple struct. It's helpful to be specific about the intended unit. But this also made me think that an enum might be more suited: then one could choose if the duration is in seconds, days, or years.

The mentor thought I did good:

Your solution is great - I agree its much nicer than the declarative macro.

They added a suggestion that I could use a parse_meta method to improve:

    let orbital_period_meta = orbital_period_attr.parse_meta().expect("'orbital_period' is not a valid meta attribute");
    let orbital_period_expr = match orbital_period_meta {
        syn::Meta::NameValue(nv) => nv.lit,
        _ => panic!("'orbital_period' attribute must be a NameValue"),
    };

Third iteration

I thought I'd take the suggestion and see how it went.

However, the suggestion is only valid for syn 1.0, but the package is currently in 2.0 where this behavior has changed.
The replacement module syn::meta and the related method Attribute::parse_nested_meta only allow Meta::List as attributes. I'd have to redefine my helper to be #[orbit_period(years = 0.123)].

I find this strange since, to me, it seems that a MetaItem in an attribute according to the Rust reference allows SimplePath = Expression.

In any case, it might be nice to also have the unit of the orbital period in the macro, so I submitted a modified version.

The mentor did not offer any additional constructive comments.