Improve optional support for borsh serialization

- Do not allocate when deserializing ArrayString
- Serialize length as u32, not as u64, to be consistent with
  serialization of [T] and str
- Add tests

src/array_string.rs

@@ -640,13 +640,22 @@ impl<const CAP: usize> borsh::BorshSerialize for ArrayString<CAP> {
 /// Requires crate feature `"borsh"`
 impl<const CAP: usize> borsh::BorshDeserialize for ArrayString<CAP> {
     fn deserialize_reader<R: borsh::io::Read>(reader: &mut R) -> borsh::io::Result<Self> {
-        let s = <String as borsh::BorshDeserialize>::deserialize_reader(reader)?;
-        ArrayString::from(&s).map_err(|_| {
-            borsh::io::Error::new(
+        let len = <u32 as borsh::BorshDeserialize>::deserialize_reader(reader)? as usize;
+        if len > CAP {
+            return Err(borsh::io::Error::new(
                 borsh::io::ErrorKind::InvalidData,
-                format!("expected a string no more than {} bytes long", CAP),
-            )
-        })
+                format!("Expected a string no more than {} bytes long", CAP),
+            ))
+        }
+
+        let mut buf = [0u8; CAP];
+        let buf = &mut buf[..len];
+        reader.read_exact(buf)?;
+
+        let s = str::from_utf8(&buf).map_err(|err| {
+            borsh::io::Error::new(borsh::io::ErrorKind::InvalidData, err.to_string())
+        })?;
+        Ok(Self::from(s).unwrap())
     }
 }
 

src/arrayvec.rs

@@ -1309,12 +1309,7 @@ where
     T: borsh::BorshSerialize,
 {
     fn serialize<W: borsh::io::Write>(&self, writer: &mut W) -> borsh::io::Result<()> {
-        let vs = self.as_slice();
-        <usize as borsh::BorshSerialize>::serialize(&vs.len(), writer)?;
-        for elem in vs {
-            <T as borsh::BorshSerialize>::serialize(elem, writer)?;
-        }
-        Ok(())
+        <[T] as borsh::BorshSerialize>::serialize(self.as_slice(), writer)
     }
 }
 
@@ -1326,13 +1321,13 @@ where
 {
     fn deserialize_reader<R: borsh::io::Read>(reader: &mut R) -> borsh::io::Result<Self> {
         let mut values = Self::new();
-        let len = <usize as borsh::BorshDeserialize>::deserialize_reader(reader)?;
+        let len = <u32 as borsh::BorshDeserialize>::deserialize_reader(reader)?;
         for _ in 0..len {
             let elem = <T as borsh::BorshDeserialize>::deserialize_reader(reader)?;
             if let Err(_) = values.try_push(elem) {
                 return Err(borsh::io::Error::new(
                     borsh::io::ErrorKind::InvalidData,
-                    format!("expected an array with no more than {} items", CAP),
+                    format!("Expected an array with no more than {} items", CAP),
                 ));
             }
         }