Remove ArrayEncoding usage from Digest and delete ArrayEncoding.

Note: I originally tried an alternative implementation using `flat_map` that
ended up being materially slower. To fix that performance regression I had to
make the following change:

```
     let mut output = Output([0; MAX_OUTPUT_LEN]);
     output
         .0
-        .iter_mut()
-        .zip(input.iter().copied().flat_map(|Wrapping(w)| f(w)))
+        .chunks_mut(N)
+        .zip(input.iter().copied().map(|Wrapping(w)| f(w)))
         .for_each(|(o, i)| {
-            *o = i;
+            o.copy_from_slice(&i);
         });
     output
 }
```

I verified that this generates the same assembly code as the original code
on x86-64 using Rust 1.74.0, except that there are two additional 128-bit
moves in `sha256_formta_output` to zero out the latter half of `Output`,
which was intended.

src/digest.rs

@@ -24,11 +24,7 @@
 // The goal for this implementation is to drive the overhead as close to zero
 // as possible.
 
-use crate::{
-    c, cpu, debug,
-    endian::{ArrayEncoding, BigEndian},
-    polyfill,
-};
+use crate::{c, cpu, debug, polyfill};
 use core::num::Wrapping;
 
 mod sha1;
@@ -248,8 +244,7 @@ impl Digest {
 impl AsRef<[u8]> for Digest {
     #[inline(always)]
     fn as_ref(&self) -> &[u8] {
-        let as64 = unsafe { &self.value.as64 };
-        &as64.as_byte_array()[..self.algorithm.output_len]
+        &self.value.0[..self.algorithm.output_len]
     }
 }
 
@@ -456,10 +451,7 @@ union State {
 }
 
 #[derive(Clone, Copy)]
-union Output {
-    as64: [BigEndian<u64>; 512 / 8 / core::mem::size_of::<BigEndian<u64>>()],
-    as32: [BigEndian<u32>; 256 / 8 / core::mem::size_of::<BigEndian<u32>>()],
-}
+struct Output([u8; MAX_OUTPUT_LEN]);
 
 /// The maximum block length ([`Algorithm::block_len()`]) of all the algorithms
 /// in this module.
@@ -474,17 +466,30 @@ pub const MAX_OUTPUT_LEN: usize = 512 / 8;
 pub const MAX_CHAINING_LEN: usize = MAX_OUTPUT_LEN;
 
 fn sha256_format_output(input: State) -> Output {
-    let input = unsafe { &input.as32 };
-    Output {
-        as32: input.map(BigEndian::from),
-    }
+    let input = unsafe { input.as32 };
+    format_output::<_, _, { core::mem::size_of::<u32>() }>(input, u32::to_be_bytes)
 }
 
 fn sha512_format_output(input: State) -> Output {
-    let input = unsafe { &input.as64 };
-    Output {
-        as64: input.map(BigEndian::from),
-    }
+    let input = unsafe { input.as64 };
+    format_output::<_, _, { core::mem::size_of::<u64>() }>(input, u64::to_be_bytes)
+}
+
+#[inline]
+fn format_output<T, F, const N: usize>(input: [Wrapping<T>; sha2::CHAINING_WORDS], f: F) -> Output
+where
+    F: Fn(T) -> [u8; N],
+    T: Copy,
+{
+    let mut output = Output([0; MAX_OUTPUT_LEN]);
+    output
+        .0
+        .chunks_mut(N)
+        .zip(input.iter().copied().map(|Wrapping(w)| f(w)))
+        .for_each(|(o, i)| {
+            o.copy_from_slice(&i);
+        });
+    output
 }
 
 /// The length of the output of SHA-1, in bytes.

src/endian.rs

@@ -11,12 +11,6 @@ where
     const ZERO: Self;
 }
 
-/// Work around the inability to implement `AsRef` for arrays of `Encoding`s
-/// due to the coherence rules.
-pub trait ArrayEncoding<T> {
-    fn as_byte_array(&self) -> &T;
-}
-
 macro_rules! define_endian {
     ($endian:ident) => {
         #[derive(Clone, Copy)]
@@ -25,22 +19,6 @@ macro_rules! define_endian {
     };
 }
 
-macro_rules! impl_array_encoding {
-    ($endian:ident, $base:ident, $elems:expr) => {
-        impl ArrayEncoding<[u8; $elems * core::mem::size_of::<$base>()]>
-            for [$endian<$base>; $elems]
-        {
-            #[inline]
-            fn as_byte_array(&self) -> &[u8; $elems * core::mem::size_of::<$base>()] {
-                let as_bytes_ptr = self
-                    .as_ptr()
-                    .cast::<[u8; $elems * core::mem::size_of::<$base>()]>();
-                unsafe { &*as_bytes_ptr }
-            }
-        }
-    };
-}
-
 macro_rules! impl_endian {
     ($endian:ident, $base:ident, $to_endian:ident, $from_endian:ident, $size:expr) => {
         impl Encoding<$base> for $endian<$base> {
@@ -81,18 +59,11 @@ macro_rules! impl_endian {
                 $base::$from_endian(value)
             }
         }
-
-        impl_array_encoding!($endian, $base, 1);
-        impl_array_encoding!($endian, $base, 2);
-        impl_array_encoding!($endian, $base, 3);
-        impl_array_encoding!($endian, $base, 4);
-        impl_array_encoding!($endian, $base, 8);
     };
 }
 
 define_endian!(BigEndian);
 impl_endian!(BigEndian, u32, to_be, from_be, 4);
-impl_endian!(BigEndian, u64, to_be, from_be, 8);
 
 #[cfg(test)]
 mod tests {