Derive Macro FunctionConfig 

#[derive(FunctionConfig)]
{
    // Attributes available to this derive:
    #[config]
    #[output]
}

Derives FunctionConfig (and ConfigField, so a derived config nests). Derives FunctionConfig for a function’s typed configuration, and ConfigField, so a derived config nests inside another.

It applies to a struct with named fields. Each field is compiled from the key of its name through the field type’s ConfigField implementation, every issue at its own path, unknown keys first.

Attribute	On	Meaning
#[config(rename = "k")]	field	The JSON key; the default is the field’s name
#[config(default = <literal>)]	field	A string, integer, float or bool, compiled through the field’s type when the key is absent
#[config(default_json = "<json>")]	field	Any JSON default, as text; text that is not JSON is a compile error
#[config(serde)]	field	Read with serde_json; an error is INPUT_PARSE, and an absent key is read from null. Resolved as it serializes, so the type is Serialize too
#[config(records)]	field	The engine records the field’s result; inherited by everything below it
#[output(default = "p")]	output	The default when the key is absent or null
#[output(primary)]	output	Required on exactly one output when there are several
#[config(check = path)]	struct	fn(&Self, &mut CompileCx<'_>), called only when every field compiled

A field is an output when it carries #[output(...)], or its type’s last path segment is Output, or it is an Option of such a type. A field’s doc comment is its description.

The primary output, the one marked or the only one, is what FunctionConfig::primary_output returns, so its type is Output or Option<Output>; any other is refused.

FunctionConfig::resolve gives every field with a value, set by the input or by its default, keyed by its key, in declaration order, each through its type’s ConfigField::resolve, and a serde field as it serializes; an Option left unset is absent.