Allow using `enum`s in form easily

[m4tx]

Now with #351 merged, we have an easy way to derive SelectChoice for user-defined enums. However, its usefulness is quite limited, as there is still no obvious way to use them in forms, because they do not implement AsFormField.

For Weekday, we implement AsFormField manually:

cot/cot/src/form/fields/chrono.rs

Lines 8 to 54 in 144a377

	impl AsFormField for Weekday {
	type Type = SelectField<Self>;
	fn clean_value(field: &Self::Type) -> Result<Self, FormFieldValidationError> {
	let value = check_required(field)?;
	Self::from_str(value)
	}
	fn to_field_value(&self) -> String {
	<Self as SelectChoice>::to_string(self)
	}
	}
	macro_rules! impl_as_form_field_mult {
	($field_type:ty) => {
	impl_as_form_field_mult_collection!(::std::vec::Vec<$field_type>, $field_type);
	impl_as_form_field_mult_collection!(::std::collections::VecDeque<$field_type>, $field_type);
	impl_as_form_field_mult_collection!(
	::std::collections::LinkedList<$field_type>,
	$field_type
	);
	impl_as_form_field_mult_collection!(::std::collections::HashSet<$field_type>, $field_type);
	impl_as_form_field_mult_collection!(::indexmap::IndexSet<$field_type>, $field_type);
	};
	}
	macro_rules! impl_as_form_field_mult_collection {
	($collection_type:ty, $field_type:ty) => {
	impl AsFormField for $collection_type {
	type Type = SelectMultipleField<$field_type>;
	fn clean_value(field: &Self::Type) -> Result<Self, FormFieldValidationError> {
	let value = check_required_multiple(field)?;
	value.iter().map(|id| <$field_type>::from_str(id)).collect()
	}
	fn to_field_value(&self) -> String {
	String::new()
	}
	}
	};
	}
	impl_as_form_field_mult!(Weekday);
	impl_as_form_field_mult_collection!(WeekdaySet, Weekday);

For user-defined enums, I think there are two possible ways to tackle this:

1. Add a derive macro that would create generic AsFormField impls for the enum as well as Vec<T>, HashSet<T>, etc., similarly to the ones for Weekday.
2. Add wrapper structs (e.g. Choice and ChoiceMultiple) that would contain the enum implementing SelectChoice, and a Vec of these, respectively.

Which of these would be more appropriate is subject to discussion.

[melroy12]

@m4tx
I'm sorry If my understanding is bit shallow !
I think the solution 1 is good !
why!
We can work directly with our Enum types
But the only downside I think is that the macro needs to generate a lot f code
I think solution 1 is good because It eliminates the boilerplate completely!
Let me know what you think !

[xerion12]

hey @m4tx, nice work here and also in #397, i’ve been working quite a bit with rust procedural macros like syn, quote, darling and form stuff in frameworks like axum / utoipa, so i’d really like to help a bit with improving the derive-based AsFormField part, maybe around enum and multi-select handling, if that’s something still open.