Support desc element in field

From [XEP-0004: Data Forms](https://xmpp.org/extensions/xep-0004.html#protocol-field):

> ...
> The <field/> element MAY contain any of the following child elements:
>
> <desc/>
> The XML character data of this element provides a natural-language
> description of the field, intended for presentation in a
> user-agent (e.g., as a "tool-tip", help button, or explanatory text
> provided near the field). The <desc/> element SHOULD NOT contain
> newlines (the \n and \r characters), since layout is the
> responsibility of a user agent, and any handling of
> newlines (e.g., presentation in a user interface) is unspecified
> herein. (Note: To provide a description of a field, it
> is RECOMMENDED to use a <desc/> element rather than
> a separate <field/> element of type "fixed".)
> ...

parsers/src/data_forms.rs

@@ -82,6 +82,9 @@ pub struct Field {
     /// The form will be rejected if this field isn’t present.
     pub required: bool,
 
+    /// The natural-language description of the field, intended for presentation in a user-agent
+    pub desc: Option<String>,
+
     /// A list of allowed values.
     pub options: Vec<Option_>,
 
@@ -100,6 +103,7 @@ impl Field {
             type_,
             label: None,
             required: false,
+            desc: None,
             options: Vec::new(),
             media: Vec::new(),
             values: Vec::new(),
@@ -174,6 +178,7 @@ impl TryFrom<Element> for Field {
             type_: get_attr!(elem, "type", Default),
             label: get_attr!(elem, "label", Option),
             required: false,
+            desc: None,
             options: vec![],
             values: vec![],
             media: vec![],
@@ -204,6 +209,10 @@ impl TryFrom<Element> for Field {
             } else if element.is("media", ns::MEDIA_ELEMENT) {
                 let media_element = MediaElement::try_from(element.clone())?;
                 field.media.push(media_element);
+            } else if element.is("desc", ns::DATA_FORMS) {
+                check_no_children!(element, "value");
+                check_no_attributes!(element, "value");
+                field.desc = Some(element.text());
             } else {
                 return Err(Error::ParseError(
                     "Field child isn’t a value, option or media element.",
@@ -374,7 +383,7 @@ mod tests {
     fn test_size() {
         assert_size!(Option_, 24);
         assert_size!(FieldType, 1);
-        assert_size!(Field, 64);
+        assert_size!(Field, 76);
         assert_size!(DataFormType, 1);
         assert_size!(DataForm, 52);
     }
@@ -384,7 +393,7 @@ mod tests {
     fn test_size() {
         assert_size!(Option_, 48);
         assert_size!(FieldType, 1);
-        assert_size!(Field, 128);
+        assert_size!(Field, 152);
         assert_size!(DataFormType, 1);
         assert_size!(DataForm, 104);
     }
@@ -428,6 +437,7 @@ mod tests {
                 type_: FieldType::Fixed,
                 label: None,
                 required: false,
+                desc: None,
                 options: vec![],
                 values: vec!["Section 1: Bot Info".to_string()],
                 media: vec![],
@@ -435,6 +445,30 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_desc() {
+        let elem: Element =
+            "<x xmlns='jabber:x:data' type='form'><field type='jid-multi' label='People to invite' var='invitelist'><desc>Tell all your friends about your new bot!</desc></field></x>"
+                .parse()
+                .unwrap();
+        let form = DataForm::try_from(elem).unwrap();
+        assert_eq!(form.type_, DataFormType::Form);
+        assert!(form.form_type.is_none());
+        assert_eq!(
+            form.fields,
+            vec![Field {
+                var: Some("invitelist".to_string()),
+                type_: FieldType::JidMulti,
+                label: Some("People to invite".to_string()),
+                required: false,
+                desc: Some("Tell all your friends about your new bot!".to_string()),
+                options: vec![],
+                values: vec![],
+                media: vec![],
+            }]
+        );
+    }
+
     #[test]
     fn test_invalid() {
         let elem: Element = "<x xmlns='jabber:x:data'/>".parse().unwrap();

parsers/src/pubsub/owner.rs

@@ -233,6 +233,7 @@ mod tests {
                     type_: FieldType::ListSingle,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![String::from("whitelist")],
                     media: vec![],
@@ -280,6 +281,7 @@ mod tests {
                     type_: FieldType::ListSingle,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![String::from("whitelist")],
                     media: vec![],

parsers/src/pubsub/pubsub.rs

@@ -625,6 +625,7 @@ mod tests {
                         type_: FieldType::ListSingle,
                         label: None,
                         required: false,
+                        desc: None,
                         options: vec![],
                         values: vec![String::from("whitelist")],
                         media: vec![],

parsers/src/server_info.rs

@@ -91,6 +91,7 @@ pub fn generate_address_field<S: Into<String>>(var: S, values: Vec<String>) -> F
         type_: FieldType::ListMulti,
         label: None,
         required: false,
+        desc: None,
         options: vec![],
         values,
         media: vec![],
@@ -126,6 +127,7 @@ mod tests {
                     type_: FieldType::ListMulti,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![],
                     media: vec![],
@@ -135,6 +137,7 @@ mod tests {
                     type_: FieldType::ListMulti,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![
                         String::from("xmpp:admin@foo.bar"),
@@ -148,6 +151,7 @@ mod tests {
                     type_: FieldType::ListMulti,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![],
                     media: vec![],
@@ -157,6 +161,7 @@ mod tests {
                     type_: FieldType::ListMulti,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![],
                     media: vec![],
@@ -166,6 +171,7 @@ mod tests {
                     type_: FieldType::ListMulti,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![
                         String::from("xmpp:security@foo.bar"),
@@ -178,6 +184,7 @@ mod tests {
                     type_: FieldType::ListMulti,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![String::from("mailto:support@foo.bar")],
                     media: vec![],