Add "const-generics" feature

Author: c410-f3r

serde/Cargo.toml

@@ -31,6 +31,9 @@ targets = ["x86_64-unknown-linux-gnu"]
 [features]
 default = ["std"]
 
+# Supports arrays of arbitrary length
+const-generics = []
+
 # Provide derive(Serialize, Deserialize) macros.
 derive = ["serde_derive"]
 

serde/src/de/impls.rs

@@ -943,6 +943,7 @@ impl<A> ArrayVisitor<A> {
     }
 }
 
+#[cfg(not(feature = "const-generics"))]
 impl<'de, T> Visitor<'de> for ArrayVisitor<[T; 0]> {
     type Value = [T; 0];
 
@@ -960,6 +961,7 @@ impl<'de, T> Visitor<'de> for ArrayVisitor<[T; 0]> {
 }
 
 // Does not require T: Deserialize<'de>.
+#[cfg(not(feature = "const-generics"))]
 impl<'de, T> Deserialize<'de> for [T; 0] {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
@@ -969,6 +971,7 @@ impl<'de, T> Deserialize<'de> for [T; 0] {
     }
 }
 
+#[cfg(not(feature = "const-generics"))]
 macro_rules! array_impls {
     ($($len:expr => ($($n:tt)+))+) => {
         $(
@@ -1047,6 +1050,7 @@ macro_rules! array_impls {
     }
 }
 
+#[cfg(not(feature = "const-generics"))]
 array_impls! {
     1 => (0)
     2 => (0 1)
@@ -1082,6 +1086,117 @@ array_impls! {
     32 => (0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31)
 }
 
+#[cfg(feature = "const-generics")]
+struct ArrayGuard<T, const N: usize> {
+    dst: *mut T,
+    initialized: usize,
+}
+
+#[cfg(feature = "const-generics")]
+impl<T, const N: usize> Drop for ArrayGuard<T, N> {
+    fn drop(&mut self) {
+        debug_assert!(self.initialized <= N);
+        let initialized_part = core::ptr::slice_from_raw_parts_mut(self.dst, self.initialized);
+        #[allow(unsafe_code)]
+        unsafe {
+            core::ptr::drop_in_place(initialized_part);
+        }
+    }
+}
+
+#[cfg(feature = "const-generics")]
+fn try_create_array<E, F, T, const N: usize>(mut cb: F) -> Result<[T; N], E>
+where
+    F: FnMut(usize) -> Result<T, E>,
+{
+    let mut array: core::mem::MaybeUninit<[T; N]> = core::mem::MaybeUninit::uninit();
+    let mut guard: ArrayGuard<T, N> = ArrayGuard {
+        dst: array.as_mut_ptr() as _,
+        initialized: 0,
+    };
+    #[allow(unsafe_code)]
+    unsafe {
+        for (idx, value_ptr) in (&mut *array.as_mut_ptr()).iter_mut().enumerate() {
+            core::ptr::write(value_ptr, cb(idx)?);
+            guard.initialized += 1;
+        }
+        core::mem::forget(guard);
+        Ok(array.assume_init())
+    }
+}
+
+#[cfg(feature = "const-generics")]
+impl<'de, T, const N: usize> Visitor<'de> for ArrayVisitor<[T; N]>
+where
+    T: Deserialize<'de>,
+{
+    type Value = [T; N];
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("array")
+    }
+
+    #[inline]
+    fn visit_seq<S>(self, mut seq: S) -> Result<Self::Value, S::Error>
+    where
+        S: SeqAccess<'de>,
+    {
+        try_create_array(|idx| {
+            seq.next_element()?.ok_or(Error::invalid_length(idx, &self))
+        })
+    }
+}
+
+#[cfg(feature = "const-generics")]
+impl<'a, 'de, T, const N: usize> Visitor<'de> for ArrayInPlaceVisitor<'a, [T; N]>
+where
+    T: Deserialize<'de>,
+{
+    type Value = ();
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("array")
+    }
+
+    #[inline]
+    fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+    where
+        A: SeqAccess<'de>,
+    {
+        let mut fail_idx = None;
+        for (idx, dest) in self.0[..].iter_mut().enumerate() {
+            if seq.next_element_seed(InPlaceSeed(dest))?.is_none() {
+                fail_idx = Some(idx);
+                break;
+            }
+        }
+        if let Some(idx) = fail_idx {
+            return Err(Error::invalid_length(idx, &self));
+        }
+        Ok(())
+    }
+}
+
+#[cfg(feature = "const-generics")]
+impl<'de, T, const N: usize> Deserialize<'de> for [T; N]
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_tuple(N, ArrayVisitor::<[T; N]>::new())
+    }
+
+    fn deserialize_in_place<D>(deserializer: D, place: &mut Self) -> Result<(), D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_tuple(N, ArrayInPlaceVisitor(place))
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 macro_rules! tuple_impls {

serde/src/lib.rs

@@ -134,8 +134,9 @@
     )
 )]
 // Rustc lints.
-#![forbid(unsafe_code)]
-#![deny(missing_docs, unused_imports)]
+#![deny(missing_docs, unused_imports, unsafe_code)]
+// Constant generics
+#![cfg_attr(feature = "const-generics", feature(min_const_generics))]
 
 ////////////////////////////////////////////////////////////////////////////////
 

serde/src/ser/impls.rs

@@ -127,6 +127,7 @@ impl<T: ?Sized> Serialize for PhantomData<T> {
 ////////////////////////////////////////////////////////////////////////////////
 
 // Does not require T: Serialize.
+#[cfg(not(feature = "const-generics"))]
 impl<T> Serialize for [T; 0] {
     #[inline]
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
@@ -137,6 +138,7 @@ impl<T> Serialize for [T; 0] {
     }
 }
 
+#[cfg(not(feature = "const-generics"))]
 macro_rules! array_impls {
     ($($len:tt)+) => {
         $(
@@ -160,13 +162,32 @@ macro_rules! array_impls {
     }
 }
 
+#[cfg(not(feature = "const-generics"))]
 array_impls! {
     01 02 03 04 05 06 07 08 09 10
     11 12 13 14 15 16 17 18 19 20
     21 22 23 24 25 26 27 28 29 30
     31 32
 }
 
+#[cfg(feature = "const-generics")]
+impl<T, const N: usize> Serialize for [T; N]
+where
+    T: Serialize,
+{
+    #[inline]
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut seq = serializer.serialize_tuple(N)?;
+        for e in self.iter() {
+            seq.serialize_element(e)?;
+        }
+        seq.end()
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 
 impl<T> Serialize for [T]

test_suite/Cargo.toml

@@ -7,6 +7,7 @@ publish = false
 build = "build.rs"
 
 [features]
+const-generics = ["serde/const-generics"]
 expandtest = []
 unstable = ["serde/unstable"]
 

test_suite/tests/test_gen.rs

@@ -347,6 +347,13 @@ fn test_gen() {
     #[serde(deny_unknown_fields)]
     struct UnitDenyUnknown;
 
+    #[cfg(feature = "const-generics")]
+    #[derive(Serialize, Deserialize)]
+    struct ArbitraryArrayLength {
+        empty: [u8; 123],
+    }
+
+    #[cfg(not(feature = "const-generics"))]
     #[derive(Serialize, Deserialize)]
     struct EmptyArray {
         empty: [X; 0],