parsers: use Error type from xso

This is a large change and as such, it needs good motivation. Let me
remind you of the ultimate goal: we want a derive macro which allows us
to FromXml/IntoXml, and that derive macro should be usable from
`xmpp_parsers` and other crates.

For that, any code generated by the derive macro mustn't depend on any
code in the `xmpp_parsers` crate, because you cannot name the crate you
are in portably (`xmpp_parsers::..` wouldn't resolve within
`xmpp_parsers`, and `crate::..` would point at other crates if the macro
was used in other crates).

We also want to interoperate with code already implementing
`TryFrom<Element>` and `Into<Element>` on structs. This ultimately
requires that we have an error type which is shared by the two
implementations and that error type must be declared in the `xso` crate
to be usable by the macros.

Thus, we port the error type over to use the type declared in `xso`.

This changes the structure of the error type greatly; I do not think
that `xso` should have to know about all the different types we are
parsing there and they don't deserve special treatment. Wrapping them in
a `Box<dyn ..>` seems more appropriate.

parsers/src/mam.rs

@@ -11,9 +11,9 @@ use crate::message::MessagePayload;
 use crate::ns;
 use crate::pubsub::NodeName;
 use crate::rsm::{SetQuery, SetResult};
-use crate::util::error::Error;
 use crate::Element;
 use minidom::Node;
+use xso::error::{Error, FromElementError};
 
 generate_id!(
     /// An identifier matching a result message to the query requesting it.
@@ -41,8 +41,8 @@ impl IqSetPayload for Query {}
 impl IqResultPayload for Query {}
 
 impl TryFrom<Element> for Query {
-    type Error = Error;
-    fn try_from(elem: Element) -> Result<Query, Error> {
+    type Error = FromElementError;
+    fn try_from(elem: Element) -> Result<Query, FromElementError> {
         check_self!(elem, "query", MAM);
         check_no_unknown_attributes!(elem, "query", ["queryid", "node"]);
 
@@ -52,32 +52,34 @@ impl TryFrom<Element> for Query {
         for child in elem.children() {
             if child.is("x", ns::DATA_FORMS) {
                 if form.is_some() {
-                    return Err(Error::ParseError(
-                        "Element query must not have more than one x child.",
-                    ));
+                    return Err(
+                        Error::Other("Element query must not have more than one x child.").into(),
+                    );
                 }
                 form = Some(DataForm::try_from(child.clone())?);
                 continue;
             }
             if child.is("set", ns::RSM) {
                 if set.is_some() {
-                    return Err(Error::ParseError(
+                    return Err(Error::Other(
                         "Element query must not have more than one set child.",
-                    ));
+                    )
+                    .into());
                 }
                 set = Some(SetQuery::try_from(child.clone())?);
                 continue;
             }
             if child.is("flip-page", ns::MAM) {
                 if flip_page.is_some() {
-                    return Err(Error::ParseError(
+                    return Err(Error::Other(
                         "Element query must not have more than one flip-page child.",
-                    ));
+                    )
+                    .into());
                 }
                 flip_page = Some(true);
                 continue;
             }
-            return Err(Error::ParseError("Unknown child in query element."));
+            return Err(Error::Other("Unknown child in query element.").into());
         }
         Ok(Query {
             queryid: match elem.attr("queryid") {
@@ -296,7 +298,7 @@ mod tests {
             .unwrap();
         let error = Query::try_from(elem).unwrap_err();
         let message = match error {
-            Error::ParseError(string) => string,
+            FromElementError::Invalid(Error::Other(string)) => string,
             _ => panic!(),
         };
         assert_eq!(message, "Unknown child in query element.");