Add support for Cow<'_, [T]>

instant-xml/src/de.rs

@@ -3,7 +3,7 @@ use std::collections::{BTreeMap, VecDeque};
 
 use xmlparser::{ElementEnd, Token, Tokenizer};
 
-use crate::impls::decode;
+use crate::impls::{decode, CowStrAccumulator};
 use crate::{Error, Id};
 
 pub struct Deserializer<'cx, 'xml> {
@@ -344,19 +344,21 @@ impl<'xml> Iterator for Context<'xml> {
     }
 }
 
-pub fn borrow_cow_str<'xml>(
-    into: &mut Option<Cow<'xml, str>>,
+pub fn borrow_cow_str<'a, 'xml: 'a>(
+    into: &mut CowStrAccumulator<'xml, 'a>,
     _: &'static str,
     deserializer: &mut Deserializer<'_, 'xml>,
 ) -> Result<(), Error> {
-    if into.is_some() {
+    if into.inner.is_some() {
         return Err(Error::DuplicateValue);
     }
 
-    if let Some(value) = deserializer.take_str()? {
-        *into = Some(decode(value)?);
+    let value = match deserializer.take_str()? {
+        Some(value) => value,
+        None => return Ok(()),
     };
 
+    into.inner = Some(decode(value)?);
     deserializer.ignore()?;
     Ok(())
 }

instant-xml/src/impls.rs

@@ -319,11 +319,7 @@ impl<'xml> FromXml<'xml> for &'xml str {
     const KIND: Kind = Kind::Scalar;
 }
 
-impl<'xml, 'a, T: ?Sized> FromXml<'xml> for Cow<'a, T>
-where
-    T: ToOwned,
-    T::Owned: FromXml<'xml>,
-{
+impl<'xml, 'a> FromXml<'xml> for Cow<'a, str> {
     #[inline]
     fn matches(id: Id<'_>, field: Option<Id<'_>>) -> bool {
         match field {
@@ -334,20 +330,65 @@ where
 
     fn deserialize(
         into: &mut Self::Accumulator,
-        field: &'static str,
+        _: &'static str,
         deserializer: &mut Deserializer<'_, 'xml>,
     ) -> Result<(), Error> {
-        if into.is_some() {
+        if into.inner.is_some() {
             return Err(Error::DuplicateValue);
         }
 
-        let mut value = <T::Owned as FromXml<'xml>>::Accumulator::default();
-        T::Owned::deserialize(&mut value, field, deserializer)?;
-        *into = Some(Cow::Owned(value.try_done(field)?));
+        let value = match deserializer.take_str()? {
+            Some(value) => value,
+            None => return Ok(()),
+        };
+
+        into.inner = Some(decode(value)?.into_owned().into());
         Ok(())
     }
 
-    type Accumulator = Option<Cow<'a, T>>;
+    type Accumulator = CowStrAccumulator<'xml, 'a>;
+    const KIND: Kind = Kind::Scalar;
+}
+
+#[derive(Default)]
+pub struct CowStrAccumulator<'xml, 'a> {
+    pub(crate) inner: Option<Cow<'a, str>>,
+    marker: PhantomData<&'xml str>,
+}
+
+impl<'xml, 'a> Accumulate<Cow<'a, str>> for CowStrAccumulator<'xml, 'a> {
+    fn try_done(self, field: &'static str) -> Result<Cow<'a, str>, Error> {
+        match self.inner {
+            Some(inner) => Ok(inner),
+            None => Err(Error::MissingValue(field)),
+        }
+    }
+}
+
+// The `FromXml` implementation for `Cow<'a, [T]>` always builds a `Cow::Owned`:
+// it is not possible to deserialize into a `Cow::Borrowed` because there's no
+// place to store the originating slice (length only known at run-time).
+impl<'xml, 'a, T: FromXml<'xml>> FromXml<'xml> for Cow<'a, [T]>
+where
+    [T]: ToOwned<Owned = Vec<T>>,
+{
+    #[inline]
+    fn matches(id: Id<'_>, field: Option<Id<'_>>) -> bool {
+        T::matches(id, field)
+    }
+
+    fn deserialize(
+        into: &mut Self::Accumulator,
+        field: &'static str,
+        deserializer: &mut Deserializer<'_, 'xml>,
+    ) -> Result<(), Error> {
+        let mut value = T::Accumulator::default();
+        T::deserialize(&mut value, field, deserializer)?;
+        into.push(value.try_done(field)?);
+        Ok(())
+    }
+
+    type Accumulator = Vec<T>;
     const KIND: Kind = Kind::Scalar;
 }
 

instant-xml/src/lib.rs

@@ -1,4 +1,4 @@
-use std::fmt;
+use std::{borrow::Cow, fmt};
 
 use thiserror::Error;
 
@@ -66,6 +66,15 @@ impl<T> Accumulate<Vec<T>> for Vec<T> {
     }
 }
 
+impl<'a, T> Accumulate<Cow<'a, [T]>> for Vec<T>
+where
+    [T]: ToOwned<Owned = Vec<T>>,
+{
+    fn try_done(self, _: &'static str) -> Result<Cow<'a, [T]>, Error> {
+        Ok(Cow::Owned(self))
+    }
+}
+
 impl<T> Accumulate<Option<T>> for Option<T> {
     fn try_done(self, _: &'static str) -> Result<Option<T>, Error> {
         Ok(self)

instant-xml/tests/scalar.rs

@@ -25,13 +25,14 @@ struct StructDeserializerScalars<'a, 'b> {
     nested: NestedLifetimes<'a>,
     cow: Cow<'a, str>,
     option: Option<&'a str>,
+    slice: Cow<'a, [u8]>,
 }
 
 #[test]
 fn scalars() {
     assert_eq!(
         from_str(
-            "<StructDeserializerScalars xmlns=\"URI\"><bool_type>true</bool_type><i8_type>1</i8_type><u32_type>42</u32_type><string_type>string</string_type><str_type_a>lifetime a</str_type_a><str_type_b>lifetime b</str_type_b><char_type>c</char_type><f32_type>1.20</f32_type><NestedLifetimes><flag>true</flag><str_type_a>asd</str_type_a></NestedLifetimes><cow>123</cow></StructDeserializerScalars>"
+            "<StructDeserializerScalars xmlns=\"URI\"><bool_type>true</bool_type><i8_type>1</i8_type><u32_type>42</u32_type><string_type>string</string_type><str_type_a>lifetime a</str_type_a><str_type_b>lifetime b</str_type_b><char_type>c</char_type><f32_type>1.20</f32_type><NestedLifetimes><flag>true</flag><str_type_a>asd</str_type_a></NestedLifetimes><cow>123</cow><slice>1</slice><slice>2</slice><slice>3</slice></StructDeserializerScalars>"
         ),
         Ok(StructDeserializerScalars{
             bool_type: true,
@@ -48,13 +49,14 @@ fn scalars() {
             },
             cow: Cow::from("123"),
             option: None,
+            slice: Cow::Borrowed(&[1, 2, 3]),
         })
     );
 
     // Option none
     assert_eq!(
         from_str(
-            "<StructDeserializerScalars xmlns=\"URI\"><bool_type>true</bool_type><i8_type>1</i8_type><u32_type>42</u32_type><string_type>string</string_type><str_type_a>lifetime a</str_type_a><str_type_b>lifetime b</str_type_b><char_type>c</char_type><f32_type>1.2</f32_type><NestedLifetimes><flag>true</flag><str_type_a>asd</str_type_a></NestedLifetimes><cow>123</cow><option>asd</option></StructDeserializerScalars>"
+            "<StructDeserializerScalars xmlns=\"URI\"><bool_type>true</bool_type><i8_type>1</i8_type><u32_type>42</u32_type><string_type>string</string_type><str_type_a>lifetime a</str_type_a><str_type_b>lifetime b</str_type_b><char_type>c</char_type><f32_type>1.2</f32_type><NestedLifetimes><flag>true</flag><str_type_a>asd</str_type_a></NestedLifetimes><cow>123</cow><option>asd</option><slice>1</slice><slice>2</slice><slice>3</slice></StructDeserializerScalars>"
         ),
         Ok(StructDeserializerScalars{
             bool_type: true,
@@ -71,6 +73,7 @@ fn scalars() {
             },
             cow: Cow::from("123"),
             option: Some("asd"),
+            slice: Cow::Borrowed(&[1, 2, 3]),
         })
     );
 }