Macro config_namespace

macro_rules! config_namespace {
    (
     $(#[doc = $struct_d:tt])*
     $vis:vis struct $struct_name:ident {
        $(
        $(#[doc = $d:tt])*
        $field_vis:vis $field_name:ident : $field_type:ty, default = $default:expr
        )*$(,)*
    }
    ) => { ... };
}

A macro that wraps a configuration struct and automatically derives Default and ConfigField for it, allowing it to be used in the ConfigOptions configuration tree

For example,

config_namespace! {
   /// Amazing config
   pub struct MyConfig {
       /// Field 1 doc
       field1: String, default = "".to_string()

       /// Field 2 doc
       field2: usize, default = 232

       /// Field 3 doc
       field3: Option<usize>, default = None
   }
}

Will generate

/// Amazing config
#[derive(Debug, Clone)]
#[non_exhaustive]
pub struct MyConfig {
    /// Field 1 doc
    field1: String,
    /// Field 2 doc
    field2: usize,
    /// Field 3 doc
    field3: Option<usize>,
}
impl ConfigField for MyConfig {
    fn set(&mut self, key: &str, value: &str) -> Result<()> {
        let (key, rem) = key.split_once('.').unwrap_or((key, ""));
        match key {
            "field1" => self.field1.set(rem, value),
            "field2" => self.field2.set(rem, value),
            "field3" => self.field3.set(rem, value),
            _ => _internal_err!(
                "Config value \"{}\" not found on MyConfig",
                key
            ),
        }
    }

    fn visit<V: Visit>(&self, v: &mut V, key_prefix: &str, _description: &'static str) {
        let key = format!("{}.field1", key_prefix);
        let desc = "Field 1 doc";
        self.field1.visit(v, key.as_str(), desc);
        let key = format!("{}.field2", key_prefix);
        let desc = "Field 2 doc";
        self.field2.visit(v, key.as_str(), desc);
        let key = format!("{}.field3", key_prefix);
        let desc = "Field 3 doc";
        self.field3.visit(v, key.as_str(), desc);
    }
}

impl Default for MyConfig {
    fn default() -> Self {
        Self {
            field1: "".to_string(),
            field2: 232,
            field3: None,
        }
    }
}

NB: Misplaced commas may result in nonsensical errors