Question: Serialize the profile name when extracting about figment HOT 5 CLOSED

Yes, this is possible via magic. You can already mostly do this by using the magic Tagged value. It would be straightforward to implement a SelectedProfile type that can be used in place of the String in your example (the existence of such a type would also mean you can keep the string and use a serde attribute to convert the magic value into a String). I'll look into implementing this soon.

One problem with this idea - perhaps I've even implemented it before - is that any such value would only have meaning during extraction. It's not clear what you'd set the value to if you want to create the structure without an extraction, for example.

One alternative is to set the value yourself directly. That is, perform the extraction, and then set config.profile = figment.profile(). You can ask serde to skip deserializing the value so that it's not set during extraction. This is the approach Rocket takes, for example.

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Closing as this isn't really possible to implement with today's serde, and work-arounds (above) exist.

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Yes, the alternative you mention is the one I am using. I tell serde to ignore the profile variable in the structure and then assign it manually as you say, but it would be interesting (or more elegant) if the extract did it for you. In addition for what you comment this something that you also use in Rocket so it makes sense.

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Alright, so I looked into this, as I had already implemented this before. It's actually sitting in the magic module right now:

#[derive(Debug, Clone, PartialEq, Deserialize, Serialize)]
#[serde(rename = "___figment_selected_profile")]
pub struct SelectedProfile {
    profile: Profile,
}

/// TODO: This doesn't work when it's in a map and the config doesn't contain a
/// value for the corresponding field; we never get to call `deserialize` on the
/// field's value. We can't fabricate this from no value. We either need to fake
/// the field name, somehow, or just not have this.
impl Magic for SelectedProfile {
    const NAME: &'static str = "___figment_selected_profile";
    const FIELDS: &'static [&'static str] = &["profile"];

    fn deserialize_from<'de: 'c, 'c, V: de::Visitor<'de>>(
        de: ConfiguredValueDe<'c>,
        visitor: V
    ) -> Result<V::Value, Error>{
        let mut map = crate::value::Map::new();
        map.insert(Self::FIELDS[0].into(), de.config.profile().to_string().into());
        visitor.visit_map(MapDe::new(&map, |v| ConfiguredValueDe::from(de.config, v)))
    }
}

impl Deref for SelectedProfile {
    type Target = Profile;

    fn deref(&self) -> &Self::Target {
        &self.profile
    }
}

As the comment indicates, this works, but it has some downsides. In particular, as the comment indicates, any kind of nesting will fail:

#[derive(serde::Deserialize)]
struct Base {
    profile: SelectedProfile,
}

#[derive(serde::Deserialize)]
struct Testing {
    base: Base,
    value: usize
}

// This panics.
let testing: Testing = Figment::from(("value", 123)).extract().unwrap();

The issue is that there's no base field in the figment - magic only works at the top-level, since that's the only case in which serde asks to deserialize a SelectedProfile directly instead of asking to deserialize a Base.

As a result of this gotcha, I've opted to not included this functionality thus far.

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Completely true, I hadn't noticed that use case in which there is no profile.

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