From e1f53b2d4b5298763f207af2f32466a45ca14d24 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Tue, 26 Dec 2023 17:45:17 -0800
Subject: [PATCH] Support deserializing &'de [u8; N]

---
 src/bytearray.rs     | 41 +++++++++++++++++++++++++++++++++++++++++
 src/de.rs            | 10 ++++++++++
 tests/test_derive.rs |  8 +++++++-
 3 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/src/bytearray.rs b/src/bytearray.rs
index 216b000..0d97d23 100644
--- a/src/bytearray.rs
+++ b/src/bytearray.rs
@@ -35,6 +35,7 @@ use serde::ser::{Serialize, Serializer};
 /// # }
 /// ```
 #[derive(Copy, Clone, Eq, Ord)]
+#[cfg_attr(not(doc), repr(transparent))]
 pub struct ByteArray<const N: usize> {
     bytes: [u8; N],
 }
@@ -49,6 +50,10 @@ impl<const N: usize> ByteArray<N> {
     pub fn into_array(self) -> [u8; N] {
         self.bytes
     }
+
+    fn from_ref(bytes: &[u8; N]) -> &Self {
+        unsafe { &*(bytes as *const [u8; N] as *const ByteArray<N>) }
+    }
 }
 
 impl<const N: usize> Debug for ByteArray<N> {
@@ -218,3 +223,39 @@ impl<'de, const N: usize> Deserialize<'de> for ByteArray<N> {
         deserializer.deserialize_bytes(ByteArrayVisitor::<N>)
     }
 }
+
+struct BorrowedByteArrayVisitor<const N: usize>;
+
+impl<'de, const N: usize> Visitor<'de> for BorrowedByteArrayVisitor<N> {
+    type Value = &'de ByteArray<N>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        write!(formatter, "a borrowed byte array of length {}", N)
+    }
+
+    fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        let borrowed_byte_array: &'de [u8; N] = v
+            .try_into()
+            .map_err(|_| E::invalid_length(v.len(), &self))?;
+        Ok(ByteArray::from_ref(borrowed_byte_array))
+    }
+
+    fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        self.visit_borrowed_bytes(v.as_bytes())
+    }
+}
+
+impl<'a, 'de: 'a, const N: usize> Deserialize<'de> for &'a ByteArray<N> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_bytes(BorrowedByteArrayVisitor::<N>)
+    }
+}
diff --git a/src/de.rs b/src/de.rs
index 5933264..179c961 100644
--- a/src/de.rs
+++ b/src/de.rs
@@ -83,6 +83,16 @@ impl<'de, const N: usize> Deserialize<'de> for ByteArray<N> {
     }
 }
 
+impl<'de: 'a, 'a, const N: usize> Deserialize<'de> for &'a ByteArray<N> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // Via the serde::Deserialize impl for &ByteArray.
+        serde::Deserialize::deserialize(deserializer)
+    }
+}
+
 #[cfg(any(feature = "std", feature = "alloc"))]
 impl<'de> Deserialize<'de> for ByteBuf {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
diff --git a/tests/test_derive.rs b/tests/test_derive.rs
index dd818e4..57616e7 100644
--- a/tests/test_derive.rs
+++ b/tests/test_derive.rs
@@ -22,6 +22,9 @@ struct Test<'a> {
     #[serde(with = "serde_bytes")]
     byte_array: ByteArray<314>,
 
+    #[serde(with = "serde_bytes")]
+    borrowed_byte_array: &'a ByteArray<314>,
+
     #[serde(with = "serde_bytes")]
     byte_buf: ByteBuf,
 
@@ -67,6 +70,7 @@ fn test() {
         vec: b"...".to_vec(),
         bytes: Bytes::new(b"..."),
         byte_array: ByteArray::new([0; 314]),
+        borrowed_byte_array: &ByteArray::new([0; 314]),
         byte_buf: ByteBuf::from(b"...".as_ref()),
         cow_slice: Cow::Borrowed(b"..."),
         cow_bytes: Cow::Borrowed(Bytes::new(b"...")),
@@ -84,7 +88,7 @@ fn test() {
         &[
             Token::Struct {
                 name: "Test",
-                len: 15,
+                len: 16,
             },
             Token::Str("slice"),
             Token::BorrowedBytes(b"..."),
@@ -96,6 +100,8 @@ fn test() {
             Token::BorrowedBytes(b"..."),
             Token::Str("byte_array"),
             Token::Bytes(&[0; 314]),
+            Token::Str("borrowed_byte_array"),
+            Token::BorrowedBytes(&[0; 314]),
             Token::Str("byte_buf"),
             Token::Bytes(b"..."),
             Token::Str("cow_slice"),