Add a Bytes type for more efficient byte sequences

The `Bytes` type is heavily inspired by `serde_bytes` and ports it to the
serde_as system.

```rust
value: Vec<u8>,
```

Compared to `serde_bytes` these improvements are available

1. Integration with the `serde_as` annotation.
    /cc serde-rs/bytes#14
2. Implementation for arrays of arbitrary size (Rust 1.51+).
    /cc serde-rs/bytes#26

src/de/const_arrays.rs

@@ -2,6 +2,7 @@ use super::*;
 use crate::utils::{MapIter, SeqIter};
 use serde::de::*;
 use std::collections::{BTreeMap, HashMap};
+use std::convert::TryInto;
 use std::fmt;
 use std::mem::MaybeUninit;
 
@@ -146,3 +147,55 @@ macro_rules! tuple_seq_as_map_impl_intern {
 }
 tuple_seq_as_map_impl_intern!([(K, V); N], BTreeMap<KAs, VAs>);
 tuple_seq_as_map_impl_intern!([(K, V); N], HashMap<KAs, VAs>);
+
+impl<'de, const N: usize> DeserializeAs<'de, [u8; N]> for Bytes {
+    fn deserialize_as<D>(deserializer: D) -> Result<[u8; N], D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct ArrayVisitor<const M: usize>;
+
+        impl<'de, const M: usize> Visitor<'de> for ArrayVisitor<M> {
+            type Value = [u8; M];
+
+            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                formatter.write_fmt(format_args!("an byte array of size {}", M))
+            }
+
+            fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                array_from_iterator(SeqIter::new(seq), &self)
+            }
+
+            fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                v.try_into()
+                    .map_err(|_| Error::invalid_length(v.len(), &self))
+            }
+
+            fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                v.as_bytes()
+                    .try_into()
+                    .map_err(|_| Error::invalid_length(v.len(), &self))
+            }
+        }
+
+        deserializer.deserialize_bytes(ArrayVisitor::<N>)
+    }
+}
+
+impl<'de, const N: usize> DeserializeAs<'de, Box<[u8; N]>> for Bytes {
+    fn deserialize_as<D>(deserializer: D) -> Result<Box<[u8; N]>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        <Bytes as DeserializeAs<'de, [u8; N]>>::deserialize_as(deserializer).map(Box::new)
+    }
+}

src/de/impls.rs

@@ -4,6 +4,7 @@ use crate::rust::StringWithSeparator;
 use crate::utils;
 use crate::utils::duration::DurationSigned;
 use serde::de::*;
+use std::borrow::Cow;
 use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, LinkedList, VecDeque};
 use std::convert::From;
 use std::fmt::{self, Display};
@@ -702,3 +703,166 @@ where
         Ok(Option::<U>::deserialize_as(deserializer)?.unwrap_or_default())
     }
 }
+
+impl<'de> DeserializeAs<'de, &'de [u8]> for Bytes {
+    fn deserialize_as<D>(deserializer: D) -> Result<&'de [u8], D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        <&'de [u8]>::deserialize(deserializer)
+    }
+}
+
+// serde_bytes implementation for ByteBuf
+// https://github.com/serde-rs/bytes/blob/cbae606b9dc225fc094b031cc84eac9493da2058/src/bytebuf.rs#L196
+//
+// Implements:
+// * visit_seq
+// * visit_bytes
+// * visit_byte_buf
+// * visit_str
+// * visit_string
+impl<'de> DeserializeAs<'de, Vec<u8>> for Bytes {
+    fn deserialize_as<D>(deserializer: D) -> Result<Vec<u8>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct VecVisitor;
+
+        impl<'de> Visitor<'de> for VecVisitor {
+            type Value = Vec<u8>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                formatter.write_str("a byte array")
+            }
+
+            fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                utils::SeqIter::new(seq).collect::<Result<_, _>>()
+            }
+
+            fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(v.to_vec())
+            }
+
+            fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(v)
+            }
+
+            fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(v.as_bytes().to_vec())
+            }
+
+            fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(v.into_bytes())
+            }
+        }
+
+        deserializer.deserialize_byte_buf(VecVisitor)
+    }
+}
+
+impl<'de> DeserializeAs<'de, Box<[u8]>> for Bytes {
+    fn deserialize_as<D>(deserializer: D) -> Result<Box<[u8]>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        <Bytes as DeserializeAs<'de, Vec<u8>>>::deserialize_as(deserializer)
+            .map(|vec| vec.into_boxed_slice())
+    }
+}
+
+// serde_bytes implementation for Cow<'a, [u8]>
+// https://github.com/serde-rs/bytes/blob/cbae606b9dc225fc094b031cc84eac9493da2058/src/de.rs#L77
+//
+// Implements:
+// * visit_borrowed_bytes
+// * visit_borrowed_str
+// * visit_bytes
+// * visit_str
+// * visit_byte_buf
+// * visit_string
+// * visit_seq
+impl<'de> DeserializeAs<'de, Cow<'de, [u8]>> for Bytes {
+    fn deserialize_as<D>(deserializer: D) -> Result<Cow<'de, [u8]>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct CowVisitor;
+
+        impl<'de> Visitor<'de> for CowVisitor {
+            type Value = Cow<'de, [u8]>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                formatter.write_str("a byte array")
+            }
+
+            fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(Cow::Borrowed(v))
+            }
+
+            fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(Cow::Borrowed(v.as_bytes()))
+            }
+
+            fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(Cow::Owned(v.to_vec()))
+            }
+
+            fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(Cow::Owned(v.as_bytes().to_vec()))
+            }
+
+            fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(Cow::Owned(v))
+            }
+
+            fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+            where
+                E: Error,
+            {
+                Ok(Cow::Owned(v.into_bytes()))
+            }
+
+            fn visit_seq<V>(self, seq: V) -> Result<Self::Value, V::Error>
+            where
+                V: SeqAccess<'de>,
+            {
+                Ok(Cow::Owned(
+                    utils::SeqIter::new(seq).collect::<Result<_, _>>()?,
+                ))
+            }
+        }
+
+        deserializer.deserialize_bytes(CowVisitor)
+    }
+}

src/lib.rs

@@ -1320,3 +1320,107 @@ pub struct TimestampNanoSecondsWithFrac<
     FORMAT: formats::Format = f64,
     STRICTNESS: formats::Strictness = formats::Strict,
 >(PhantomData<(FORMAT, STRICTNESS)>);
+
+/// Optimized handling of owned and borrowed byte representations.
+///
+/// Serialization of byte sequences like `&[u8]` or `Vec<u8>` is quite inefficient since each value will be serialized individually.
+/// This converter type optimizes the serialization and deserialization.
+///
+/// This is a port of the `serde_bytes` crate making it compatible with the `serde_as`-annotation, which allows it to be used in more cases than provided by `serde_bytes`.
+///
+/// The type provides de-/serialization for these types:
+///
+/// * `[u8; N]`, Rust 1.51+, not possible using `serde_bytes`
+/// * `&[u8]`
+/// * `Box<[u8; N]>`, Rust 1.51+, not possible using `serde_bytes`
+/// * `Box<[u8]>`
+/// * `Vec<u8>`
+/// * `Cow<'_, [u8]>`
+///
+/// # Examples
+///
+/// ```
+/// # #[cfg(feature = "macros")] {
+/// # use serde::{Deserialize, Serialize};
+/// # use serde_with::{serde_as, Bytes};
+/// # use std::borrow::Cow;
+/// #
+/// #[serde_as]
+/// # #[derive(Debug, PartialEq)]
+/// #[derive(Deserialize, Serialize)]
+/// struct Test<'a> {
+///     #[serde_as(as = "Bytes")]
+///     array: [u8; 15],
+///     #[serde_as(as = "Bytes")]
+///     boxed: Box<[u8]>,
+///     #[serde_as(as = "Bytes")]
+///     #[serde(borrow)]
+///     cow: Cow<'a, [u8]>,
+///     #[serde_as(as = "Bytes")]
+///     vec: Vec<u8>,
+/// }
+///
+/// let value = Test {
+///     array: b"0123456789ABCDE".clone(),
+///     boxed: b"...".to_vec().into_boxed_slice(),
+///     cow: Cow::Borrowed(b"FooBar"),
+///     vec: vec![0x41, 0x61, 0x21],
+/// };
+/// let expected = r#"(
+///     array: "MDEyMzQ1Njc4OUFCQ0RF",
+///     boxed: "Li4u",
+///     cow: "Rm9vQmFy",
+///     vec: "QWEh",
+/// )"#;
+///
+/// assert_eq!(expected, ron::ser::to_string_pretty(&value, Default::default()).unwrap());
+/// assert_eq!(value, ron::from_str(&expected).unwrap());
+/// # }
+/// ```
+///
+/// ## Alternative to [`BytesOrString`]
+///
+/// The [`Bytes`] can replace [`BytesOrString`].
+/// [`Bytes`] is implemented for more types, which makes it better.
+/// The serialization behavior of [`Bytes`] differes from [`BytesOrString`], therefore only `deserialize_as` should be used.
+///
+/// ```rust
+/// # #[cfg(feature = "macros")] {
+/// # use serde::Deserialize;
+/// # use serde_json::json;
+/// # use serde_with::{serde_as, Bytes};
+/// #
+/// #[serde_as]
+/// # #[derive(Debug, PartialEq)]
+/// #[derive(Deserialize, serde::Serialize)]
+/// struct Test {
+///     #[serde_as(deserialize_as = "Bytes")]
+///     from_bytes: Vec<u8>,
+///     #[serde_as(deserialize_as = "Bytes")]
+///     from_str: Vec<u8>,
+/// }
+///
+/// // Different serialized values ...
+/// let j = json!({
+///     "from_bytes": [70,111,111,45,66,97,114],
+///     "from_str": "Foo-Bar",
+/// });
+///
+/// // can be deserialized ...
+/// let test = Test {
+///     from_bytes: b"Foo-Bar".to_vec(),
+///     from_str: b"Foo-Bar".to_vec(),
+/// };
+/// assert_eq!(test, serde_json::from_value(j).unwrap());
+///
+/// // and serialization will always be a byte sequence
+/// # assert_eq!(json!(
+/// {
+///     "from_bytes": [70,111,111,45,66,97,114],
+///     "from_str": [70,111,111,45,66,97,114],
+/// }
+/// # ), serde_json::to_value(&test).unwrap());
+/// # }
+/// ```
+#[derive(Copy, Clone, Debug, Default)]
+pub struct Bytes;

src/ser/const_arrays.rs

@@ -42,3 +42,21 @@ macro_rules! tuple_seq_as_map_impl_intern {
 }
 tuple_seq_as_map_impl_intern!([(K, V); N], BTreeMap<K, V>);
 tuple_seq_as_map_impl_intern!([(K, V); N], HashMap<K, V>);
+
+impl<'a, const N: usize> SerializeAs<[u8; N]> for Bytes {
+    fn serialize_as<S>(bytes: &[u8; N], serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_bytes(bytes)
+    }
+}
+
+impl<'a, const N: usize> SerializeAs<Box<[u8; N]>> for Bytes {
+    fn serialize_as<S>(bytes: &Box<[u8; N]>, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_bytes(&**bytes)
+    }
+}