Create array in-place without an intermediate Vec

src/lib.rs

@@ -83,7 +83,7 @@ use serde::{
     de::{self, Deserialize, Deserializer, SeqAccess, Visitor},
     ser::{Serialize, SerializeTuple, Serializer},
 };
-use std::{convert::TryInto, fmt, marker::PhantomData};
+use std::{fmt, marker::PhantomData};
 
 /// Serialize const generic or arbitrarily-large arrays
 ///
@@ -113,6 +113,89 @@ struct ArrayVisitor<T, const N: usize> {
     _marker: PhantomData<T>,
 }
 
+#[cfg(inhibit_unsafe)]
+fn into_array<T, E: serde::de::Error, const N: usize>(
+    mut source: impl Iterator<Item = Result<T, E>>,
+) -> Result<[T; N], E> {
+    use std::convert::TryInto;
+
+    // Build a temporary container to hold our data as we deserialize it
+    // We can't rely on a Default<T> implementation, so we can't use an array here
+    let mut arr = Vec::with_capacity(N);
+
+    while let Some(val) = source.next() {
+        arr.push(val?);
+    }
+
+    // We can convert a Vec into an array via TryInto, which will fail if the length of the Vec
+    // doesn't match that of the array.
+    match arr.try_into() {
+        Ok(arr) => Ok(arr),
+
+        Err(arr) => Err(de::Error::invalid_length(
+            arr.len(),
+            &format!("an array of size {}", N).as_str(),
+        )),
+    }
+}
+
+#[cfg(not(inhibit_unsafe))]
+fn into_array<T, E: serde::de::Error, const N: usize>(
+    mut source: impl Iterator<Item = Result<T, E>>,
+) -> Result<[T; N], E> {
+    use std::mem::MaybeUninit;
+
+    // Safety: `assume_init` is sound because the type we are claiming to have
+    // initialized here is a bunch of `MaybeUninit`s, which do not require
+    // initialization.
+    let mut arr: [MaybeUninit<T>; N] = unsafe { MaybeUninit::uninit().assume_init() };
+
+    // Iterate over the array and fill the elemenets with the ones obtained from
+    // `seq`.
+    let mut place_iter = arr.iter_mut();
+    let mut cnt_filled = 0;
+    let err = loop {
+        match (source.next(), place_iter.next()) {
+            (Some(Ok(val)), Some(place)) => *place = MaybeUninit::new(val),
+            // no error, we're done
+            (None, None) => break None,
+            // error from serde, propagate it
+            (Some(Err(e)), _) => break Some(e),
+            // lengths do not match, report invalid_length
+            (None, Some(_)) | (Some(Ok(_)), None) => {
+                break Some(de::Error::invalid_length(
+                    cnt_filled,
+                    &format!("an array of size {}", N).as_str(),
+                ))
+            }
+        }
+        cnt_filled += 1;
+    };
+    if let Some(err) = err {
+        if std::mem::needs_drop::<T>() {
+            for elem in std::array::IntoIter::new(arr).take(cnt_filled) {
+                // Safety: `assume_init()` is sound because we did initialize CNT_FILLED
+                // elements. We call it to drop the deserialized values.
+                unsafe {
+                    elem.assume_init();
+                }
+            }
+        }
+        return Err(err);
+    }
+
+    // Safety: everything is initialized and we are ready to transmute to the
+    // initialized array type.
+
+    // See https://github.com/rust-lang/rust/issues/62875#issuecomment-513834029
+    //let ret = unsafe { std::mem::transmute::<_, [T; N]>(arr) };
+
+    let ret = unsafe { std::mem::transmute_copy(&arr) };
+    std::mem::forget(arr);
+
+    Ok(ret)
+}
+
 impl<'de, T, const N: usize> Visitor<'de> for ArrayVisitor<T, N>
 where
     T: Deserialize<'de>,
@@ -129,20 +212,7 @@ where
     where
         A: SeqAccess<'de>,
     {
-        // Build a temporary container to hold our data as we deserialize it
-        // We can't rely on a Default<T> implementation, so we can't use an array here
-        let mut arr = Vec::with_capacity(N);
-
-        while let Some(val) = seq.next_element()? {
-            arr.push(val);
-        }
-
-        // We can convert a Vec into an array via TryInto, which will fail if the length of the Vec
-        // doesn't match that of the array.
-        match arr.try_into() {
-            Ok(arr) => Ok(arr),
-            Err(arr) => Err(de::Error::invalid_length(arr.len(), &self)),
-        }
+        into_array(std::iter::from_fn(move || seq.next_element().transpose()))
     }
 }