feat(derive): check form parser availability

Introduce FormValue as the parser-availability bound for #[slhx::form] models, blanket it for FromStr types, and add compile-fail coverage for a form field whose domain type lacks a parser.

req: form/004

req: form/006

slhx-derive/src/lib.rs

@@ -91,7 +91,11 @@ pub fn form(attr: TokenStream, item: TokenStream) -> TokenStream {
     let errors = form_contract_errors(&form_name, &form_struct);
     if errors.is_empty() {
         let ident = &form_struct.ident;
-        let (impl_generics, ty_generics, where_clause) = form_struct.generics.split_for_impl();
+        let mut generics = form_struct.generics.clone();
+        for ty in form_parser_types(&form_struct) {
+            generics.make_where_clause().predicates.push(parse_quote!(#ty: ::slhx::FormValue));
+        }
+        let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
         quote!(
             #form_struct
             impl #impl_generics ::slhx::FormModel for #ident #ty_generics #where_clause {}
@@ -342,6 +346,40 @@ fn form_contract_errors(form_name: &str, form_struct: &ItemStruct) -> Vec<String
     errors
 }
 
+fn form_parser_types(form_struct: &ItemStruct) -> Vec<Type> {
+    let Fields::Named(fields) = &form_struct.fields else {
+        return Vec::new();
+    };
+    fields
+        .named
+        .iter()
+        .map(|field| parser_type(&field.ty).clone())
+        .collect()
+}
+
+fn parser_type(ty: &Type) -> &Type {
+    generic_inner_type(ty, "Option")
+        .or_else(|| generic_inner_type(ty, "Vec"))
+        .unwrap_or(ty)
+}
+
+fn generic_inner_type<'a>(ty: &'a Type, name: &str) -> Option<&'a Type> {
+    let Type::Path(path) = ty else {
+        return None;
+    };
+    let segment = path.path.segments.last()?;
+    if segment.ident != name {
+        return None;
+    }
+    let PathArguments::AngleBracketed(args) = &segment.arguments else {
+        return None;
+    };
+    args.args.iter().find_map(|arg| match arg {
+        GenericArgument::Type(ty) => Some(ty),
+        _ => None,
+    })
+}
+
 fn form_fields(path: &PathBuf, form_name: &str) -> Vec<GeneratedFormField> {
     let Ok(syms) = std::fs::read_to_string(path) else {
         return Vec::new();
@@ -484,7 +522,8 @@ fn compile_error(message: &str) -> TokenStream {
 
 #[cfg(test)]
 mod tests {
-    use super::{component_handler_names, handle_params, handle_requires_form, has_form_param, has_non_unit_return, is_form_type, missing_component_handlers, missing_handle_params, syms_contains_handle};
+    use super::{component_handler_names, handle_params, handle_requires_form, has_form_param, has_non_unit_return, is_form_type, missing_component_handlers, missing_handle_params, parser_type, syms_contains_handle};
+    use quote::quote;
     use syn::{parse_quote, ItemFn, Type};
 
     #[test]
@@ -519,6 +558,22 @@ mod tests {
         assert!(!has_non_unit_return(&empty));
     }
 
+    #[test]
+    fn form_parser_type_uses_option_and_vec_inner_types() {
+        // req: form/004
+        let required: Type = parse_quote!(Email);
+        let optional: Type = parse_quote!(Option<Email>);
+        let multiple: Type = parse_quote!(Vec<Email>);
+
+        let required_parser = parser_type(&required);
+        let optional_parser = parser_type(&optional);
+        let multiple_parser = parser_type(&multiple);
+
+        assert_eq!(quote!(#required_parser).to_string(), quote!(#required).to_string());
+        assert_eq!(quote!(#optional_parser).to_string(), quote!(#required).to_string());
+        assert_eq!(quote!(#multiple_parser).to_string(), quote!(#required).to_string());
+    }
+
     #[test]
     fn form_param_matches_form_type_not_name_suffix() {
         // req: form/004 req: form/006