Use associated accumulator type to deserialize into

instant-xml-macros/src/de.rs

@@ -68,7 +68,7 @@ fn deserialize_scalar_enum(
 
             fn deserialize<'cx>(
                 deserializer: &mut ::instant_xml::Deserializer<'cx, 'xml>,
-                into: &mut Option<Self>,
+                into: &mut Self::Accumulator,
             ) -> Result<(), ::instant_xml::Error> {
                 use ::instant_xml::Error;
 
@@ -88,6 +88,7 @@ fn deserialize_scalar_enum(
                 Ok(())
             }
 
+            type Accumulator = Option<Self>;
             const KIND: ::instant_xml::Kind = ::instant_xml::Kind::Scalar;
         }
     )
@@ -163,10 +164,10 @@ fn deserialize_forward_enum(
 
             fn deserialize<'cx>(
                 deserializer: &mut ::instant_xml::Deserializer<'cx, 'xml>,
-                into: &mut Option<Self>,
+                into: &mut Self::Accumulator,
             ) -> Result<(), ::instant_xml::Error> {
                 use ::instant_xml::de::Node;
-                use ::instant_xml::{Error, FromXml};
+                use ::instant_xml::{Accumulate, Error, FromXml};
 
                 let id = deserializer.parent();
                 #variants else {
@@ -180,6 +181,7 @@ fn deserialize_forward_enum(
                 Ok(())
             }
 
+            type Accumulator = Option<Self>;
             const KIND: ::instant_xml::Kind = ::instant_xml::Kind::Element;
         }
     )
@@ -283,10 +285,10 @@ fn deserialize_struct(
 
             fn deserialize<'cx>(
                 deserializer: &mut ::instant_xml::Deserializer<'cx, 'xml>,
-                into: &mut Option<Self>,
+                into: &mut Self::Accumulator,
             ) -> Result<(), ::instant_xml::Error> {
                 use ::instant_xml::de::Node;
-                use ::instant_xml::{Error, FromXml, Id, Kind};
+                use ::instant_xml::{Accumulate, Error, FromXml, Id, Kind};
 
                 enum __Elements {
                     #elements_enum
@@ -336,6 +338,7 @@ fn deserialize_struct(
                 Ok(())
             }
 
+            type Accumulator = Option<Self>;
             const KIND: ::instant_xml::Kind = ::instant_xml::Kind::Element;
         }
     )
@@ -399,7 +402,7 @@ fn named_field(
     }
 
     declare_values.extend(quote!(
-        let mut #enum_name: Option<#no_lifetime_type> = None;
+        let mut #enum_name = <#no_lifetime_type as FromXml>::Accumulator::default();
     ));
 
     let deserialize_with = field_meta
@@ -434,7 +437,7 @@ fn named_field(
             direct.extend(quote!(
                 Node::Text(text) => {
                     let mut nested = deserializer.for_node(Node::Text(text));
-                    FromXml::deserialize(&mut nested, &mut #enum_name)?;
+                    <#no_lifetime_type>::deserialize(&mut nested, &mut #enum_name)?;
                 }
             ));
         } else {
@@ -442,11 +445,11 @@ fn named_field(
                 __Elements::#enum_name => match <#no_lifetime_type as FromXml>::KIND {
                     Kind::Element => {
                         let mut nested = deserializer.nested(data);
-                        FromXml::deserialize(&mut nested, &mut #enum_name)?;
+                        <#no_lifetime_type>::deserialize(&mut nested, &mut #enum_name)?;
                     }
                     Kind::Scalar => {
                         let mut nested = deserializer.nested(data);
-                        FromXml::deserialize(&mut nested, &mut #enum_name)?;
+                        <#no_lifetime_type>::deserialize(&mut nested, &mut #enum_name)?;
                         nested.ignore()?;
                     }
                 },
@@ -479,10 +482,7 @@ fn named_field(
 
     let field_str = format!("{type_name}::{field_name}");
     return_val.extend(quote!(
-        #field_name: match #enum_name {
+        #field_name: #enum_name.try_done(#field_str)?,
-            Some(v) => v,
-            None => <#no_lifetime_type as FromXml>::missing(#field_str)?,
-        },
     ));
 
     Ok(())
@@ -540,10 +540,10 @@ fn deserialize_tuple_struct(
 
             fn deserialize<'cx>(
                 deserializer: &mut ::instant_xml::Deserializer<'cx, 'xml>,
-                into: &mut Option<Self>,
+                into: &mut Self::Accumulator,
             ) -> Result<(), ::instant_xml::Error> {
                 use ::instant_xml::de::Node;
-                use ::instant_xml::{Error, FromXml, Id, Kind};
+                use ::instant_xml::{Accumulate, Error, FromXml, Id, Kind};
 
                 #declare_values
                 deserializer.ignore()?;
@@ -552,6 +552,7 @@ fn deserialize_tuple_struct(
                 Ok(())
             }
 
+            type Accumulator = Option<Self>;
             const KIND: ::instant_xml::Kind = ::instant_xml::Kind::Element;
         }
     )
@@ -576,7 +577,7 @@ fn unnamed_field(
             Kind::Element => match deserializer.next() {
                 Some(Ok(Node::Open(data))) => {
                     let mut nested = deserializer.nested(data);
-                    let mut value: Option<#no_lifetime_type> = None;
+                    let mut value = <#no_lifetime_type as FromXml>::Accumulator::default();
                     <#no_lifetime_type as FromXml>::deserialize(&mut nested, &mut value)?;
                     nested.ignore()?;
                     value
@@ -586,7 +587,7 @@ fn unnamed_field(
                 None => return Err(Error::MissingValue(#field_str)),
             }
             Kind::Scalar => {
-                let mut value: Option<#no_lifetime_type> = None;
+                let mut value = <#no_lifetime_type as FromXml>::Accumulator::default();
                 <#no_lifetime_type as FromXml>::deserialize(deserializer, &mut value)?;
                 value
             }
@@ -595,10 +596,7 @@ fn unnamed_field(
 
     let field_str = format!("{type_name}::{index}");
     return_val.extend(quote!(
-        match #name {
+        #name.try_done(#field_str)?,
-            Some(v) => v,
-            None => <#no_lifetime_type as FromXml>::missing(#field_str)?,
-        },
     ));
 }
 
@@ -620,13 +618,14 @@ fn deserialize_unit_struct(input: &syn::DeriveInput, meta: &ContainerMeta) -> To
 
             fn deserialize<'cx>(
                 deserializer: &mut ::instant_xml::Deserializer<'cx, 'xml>,
-                into: &mut Option<Self>,
+                into: &mut Self::Accumulator,
             ) -> Result<(), ::instant_xml::Error> {
                 deserializer.ignore()?;
                 *into = Some(Self);
                 Ok(())
             }
 
+            type Accumulator = Option<Self>;
             const KIND: ::instant_xml::Kind = ::instant_xml::Kind::Element;
         }
     )

instant-xml/src/impls.rs

@@ -1,13 +1,13 @@
-use std::any::type_name;
 use std::borrow::Cow;
 use std::fmt;
 use std::net::IpAddr;
 use std::str::FromStr;
+use std::{any::type_name, marker::PhantomData};
 
 #[cfg(feature = "chrono")]
 use chrono::{DateTime, NaiveDate, Utc};
 
-use crate::{Deserializer, Error, FromXml, Id, Kind, Serializer, ToXml};
+use crate::{Accumulate, Deserializer, Error, FromXml, Id, Kind, Serializer, ToXml};
 
 // Deserializer
 
@@ -49,7 +49,7 @@ impl<'xml, T: FromStr> FromXml<'xml> for FromXmlStr<T> {
 
     fn deserialize(
         deserializer: &mut Deserializer<'_, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -72,6 +72,7 @@ impl<'xml, T: FromStr> FromXml<'xml> for FromXmlStr<T> {
         }
     }
 
+    type Accumulator = Option<FromXmlStr<T>>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -86,7 +87,7 @@ impl<'xml> FromXml<'xml> for bool {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -111,6 +112,7 @@ impl<'xml> FromXml<'xml> for bool {
         Ok(())
     }
 
+    type Accumulator = Option<bool>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -180,7 +182,7 @@ macro_rules! from_xml_for_number {
 
             fn deserialize<'cx>(
                 deserializer: &mut Deserializer<'cx, 'xml>,
-                into: &mut Option<Self>,
+                into: &mut Self::Accumulator,
             ) -> Result<(), Error> {
                 if into.is_some() {
                     return Err(Error::DuplicateValue);
@@ -195,6 +197,7 @@ macro_rules! from_xml_for_number {
                 Ok(())
             }
 
+            type Accumulator = Option<Self>;
             const KIND: Kind = Kind::Scalar;
         }
     };
@@ -224,7 +227,7 @@ impl<'xml> FromXml<'xml> for char {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -239,6 +242,7 @@ impl<'xml> FromXml<'xml> for char {
         Ok(())
     }
 
+    type Accumulator = Option<Self>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -253,7 +257,7 @@ impl<'xml> FromXml<'xml> for String {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -267,6 +271,7 @@ impl<'xml> FromXml<'xml> for String {
         Ok(())
     }
 
+    type Accumulator = Option<String>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -281,7 +286,7 @@ impl<'xml> FromXml<'xml> for &'xml str {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -304,6 +309,7 @@ impl<'xml> FromXml<'xml> for &'xml str {
         Ok(())
     }
 
+    type Accumulator = Option<&'xml str>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -322,21 +328,19 @@ where
 
     fn deserialize(
         deserializer: &mut Deserializer<'_, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
         }
 
-        let mut value = None;
+        let mut value = <T::Owned as FromXml<'xml>>::Accumulator::default();
         T::Owned::deserialize(deserializer, &mut value)?;
-        if let Some(value) = value {
+        *into = Some(Cow::Owned(value.try_done("Cow<T>")?));
-            *into = Some(Cow::Owned(value));
-        }
-
         Ok(())
     }
 
+    type Accumulator = Option<Cow<'a, T>>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -348,31 +352,39 @@ impl<'xml, T: FromXml<'xml>> FromXml<'xml> for Option<T> {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
-        match into.as_mut() {
+        <T>::deserialize(deserializer, &mut into.value)?;
-            Some(value) => {
-                <T>::deserialize(deserializer, value)?;
-            }
-            None => {
-                let mut value = None;
-                <T>::deserialize(deserializer, &mut value)?;
-                if let Some(value) = value {
-                    *into = Some(Some(value));
-                }
-            }
-        }
-
         Ok(())
     }
 
-    fn missing(_: &'static str) -> Result<Self, Error> {
+    type Accumulator = OptionAccumulator<T, T::Accumulator>;
-        Ok(None)
-    }
-
     const KIND: Kind = <T>::KIND;
 }
 
+pub struct OptionAccumulator<T, A: Accumulate<T>> {
+    value: A,
+    marker: PhantomData<T>,
+}
+
+impl<T, A: Accumulate<T>> Default for OptionAccumulator<T, A> {
+    fn default() -> Self {
+        Self {
+            value: A::default(),
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<T, A: Accumulate<T>> Accumulate<Option<T>> for OptionAccumulator<T, A> {
+    fn try_done(self, field: &'static str) -> Result<Option<T>, Error> {
+        match self.value.try_done(field) {
+            Ok(value) => Ok(Some(value)),
+            Err(_) => Ok(None),
+        }
+    }
+}
+
 to_xml_for_number!(i8);
 to_xml_for_number!(i16);
 to_xml_for_number!(i32);
@@ -556,22 +568,15 @@ impl<'xml, T: FromXml<'xml>> FromXml<'xml> for Vec<T> {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
-        let mut value = None;
+        let mut value = T::Accumulator::default();
         T::deserialize(deserializer, &mut value)?;
-        let dst = into.get_or_insert(Vec::new());
+        into.push(value.try_done("Vec<T>")?);
-        if let Some(value) = value {
-            dst.push(value);
-        }
-
         Ok(())
     }
 
-    fn missing(_: &'static str) -> Result<Self, Error> {
+    type Accumulator = Vec<T>;
-        Ok(Vec::new())
-    }
-
     const KIND: Kind = T::KIND;
 }
 
@@ -636,7 +641,7 @@ impl<'xml> FromXml<'xml> for DateTime<Utc> {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -656,6 +661,7 @@ impl<'xml> FromXml<'xml> for DateTime<Utc> {
         }
     }
 
+    type Accumulator = Option<Self>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -696,7 +702,7 @@ impl<'xml> FromXml<'xml> for NaiveDate {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -716,6 +722,7 @@ impl<'xml> FromXml<'xml> for NaiveDate {
         }
     }
 
+    type Accumulator = Option<Self>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -730,12 +737,13 @@ impl<'xml> FromXml<'xml> for () {
 
     fn deserialize<'cx>(
         _: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         *into = Some(());
         Ok(())
     }
 
+    type Accumulator = Option<Self>;
     const KIND: Kind = Kind::Scalar;
 }
 
@@ -760,7 +768,7 @@ impl<'xml> FromXml<'xml> for IpAddr {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error> {
         if into.is_some() {
             return Err(Error::DuplicateValue);
@@ -775,6 +783,7 @@ impl<'xml> FromXml<'xml> for IpAddr {
         Ok(())
     }
 
+    type Accumulator = Option<Self>;
     const KIND: Kind = Kind::Scalar;
 }
 

instant-xml/src/lib.rs

@@ -41,18 +41,36 @@ pub trait FromXml<'xml>: Sized {
 
     fn deserialize<'cx>(
         deserializer: &mut Deserializer<'cx, 'xml>,
-        into: &mut Option<Self>,
+        into: &mut Self::Accumulator,
     ) -> Result<(), Error>;
 
-    // If the missing field is of type `Option<T>` then treat is as `None`,
+    type Accumulator: Accumulate<Self>;
-    // otherwise it is an error.
-    fn missing(field: &'static str) -> Result<Self, Error> {
-        Err(Error::MissingValue(field))
-    }
-
     const KIND: Kind;
 }
 
+/// A type implementing `Accumulate<T>` is used to accumulate a value of type `T`.
+pub trait Accumulate<T>: Default {
+    fn try_done(self, field: &'static str) -> Result<T, Error>;
+}
+
+impl<T> Accumulate<T> for Option<T> {
+    fn try_done(self, field: &'static str) -> Result<T, Error> {
+        self.ok_or(Error::MissingValue(field))
+    }
+}
+
+impl<T> Accumulate<Vec<T>> for Vec<T> {
+    fn try_done(self, _: &'static str) -> Result<Vec<T>, Error> {
+        Ok(self)
+    }
+}
+
+impl<T> Accumulate<Option<T>> for Option<T> {
+    fn try_done(self, _: &'static str) -> Result<Option<T>, Error> {
+        Ok(self)
+    }
+}
+
 pub fn from_str<'xml, T: FromXml<'xml>>(input: &'xml str) -> Result<T, Error> {
     let (mut context, root) = Context::new(input)?;
     let id = context.element_id(&root)?;
@@ -67,12 +85,9 @@ pub fn from_str<'xml, T: FromXml<'xml>>(input: &'xml str) -> Result<T, Error> {
         }));
     }
 
-    let mut value = None;
+    let mut value = T::Accumulator::default();
     T::deserialize(&mut Deserializer::new(root, &mut context), &mut value)?;
-    match value {
+    value.try_done("root element")
-        Some(value) => Ok(value),
-        None => T::missing("<root>"),
-    }
 }
 
 pub fn to_string(value: &(impl ToXml + ?Sized)) -> Result<String, Error> {