Implement ByteEq and ByteHash derives (#144)

* Implement `ByteEq` and `ByteHash` derives

This adds the derives `ByteEq` and `ByteHash` that can be used as an
alternative to the `Eq` / `PartialEq` and `Hash` derives from the
standard library. The difference is that these variants use `bytemuck`
to convert their values to byte slices before comparing / hashing them.
This allows the comparisons to turn into a simple `memcmp` / `bcmp` (or
completely inlined as a few vector instructions) and allows hashers to
process all bytes at once, possibly allowing for some vector operations
as well.

Here's a quick comparison of the generated assembly:
![https://i.imgur.com/CGTSWTZ.png](https://i.imgur.com/CGTSWTZ.png)

* Address review comments

derive/src/lib.rs

@@ -224,6 +224,93 @@ pub fn derive_contiguous(
   proc_macro::TokenStream::from(expanded)
 }
 
+/// Derive the `PartialEq` and `Eq` trait for a type
+///
+/// The macro implements `PartialEq` and `Eq` by casting both sides of the
+/// comparison to a byte slice and then compares those.
+///
+/// ## Warning
+///
+/// Since this implements a byte wise comparison, the behavior of floating point
+/// numbers does not match their usual comparison behavior. Additionally other
+/// custom comparison behaviors of the individual fields are also ignored. This
+/// also does not implement `StructuralPartialEq` / `StructuralEq` like
+/// `PartialEq` / `Eq` would. This means you can't pattern match on the values.
+///
+/// ## Example
+///
+/// ```rust
+/// # use bytemuck_derive::{ByteEq, NoUninit};
+/// #[derive(Copy, Clone, NoUninit, ByteEq)]
+/// #[repr(C)]
+/// struct Test {
+///   a: u32,
+///   b: char,
+///   c: f32,
+/// }
+/// ```
+#[proc_macro_derive(ByteEq)]
+pub fn derive_byte_eq(
+  input: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+  let input = parse_macro_input!(input as DeriveInput);
+  let ident = input.ident;
+
+  proc_macro::TokenStream::from(quote! {
+    impl ::core::cmp::PartialEq for #ident {
+      #[inline]
+      #[must_use]
+      fn eq(&self, other: &Self) -> bool {
+        ::bytemuck::bytes_of(self) == ::bytemuck::bytes_of(other)
+      }
+    }
+    impl ::core::cmp::Eq for #ident { }
+  })
+}
+
+/// Derive the `Hash` trait for a type
+///
+/// The macro implements `Hash` by casting the value to a byte slice and hashing
+/// that.
+///
+/// ## Warning
+///
+/// The hash does not match the standard library's `Hash` derive.
+///
+/// ## Example
+///
+/// ```rust
+/// # use bytemuck_derive::{ByteHash, NoUninit};
+/// #[derive(Copy, Clone, NoUninit, ByteHash)]
+/// #[repr(C)]
+/// struct Test {
+///   a: u32,
+///   b: char,
+///   c: f32,
+/// }
+/// ```
+#[proc_macro_derive(ByteHash)]
+pub fn derive_byte_hash(
+  input: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+  let input = parse_macro_input!(input as DeriveInput);
+  let ident = input.ident;
+
+  proc_macro::TokenStream::from(quote! {
+    impl ::core::hash::Hash for #ident {
+      #[inline]
+      fn hash<H: ::core::hash::Hasher>(&self, state: &mut H) {
+        ::core::hash::Hash::hash_slice(::bytemuck::bytes_of(self), state)
+      }
+
+      #[inline]
+      fn hash_slice<H: ::core::hash::Hasher>(data: &[Self], state: &mut H) {
+        ::core::hash::Hash::hash_slice(::bytemuck::cast_slice::<_, u8>(data), state)
+      }
+    }
+  })
+}
+
 /// Basic wrapper for error handling
 fn derive_marker_trait<Trait: Derivable>(input: DeriveInput) -> TokenStream {
   derive_marker_trait_inner::<Trait>(input)

src/lib.rs

@@ -113,8 +113,8 @@ pub use transparent::*;
 
 #[cfg(feature = "derive")]
 pub use bytemuck_derive::{
-  AnyBitPattern, CheckedBitPattern, Contiguous, NoUninit, Pod,
-  TransparentWrapper, Zeroable,
+  AnyBitPattern, ByteEq, ByteHash, CheckedBitPattern, Contiguous, NoUninit,
+  Pod, TransparentWrapper, Zeroable,
 };
 
 /// The things that can go wrong when casting between [`Pod`] data forms.

tests/derive.rs

@@ -1,9 +1,9 @@
 #![cfg(feature = "derive")]
 #![allow(dead_code)]
 
-use bytemuck::{Pod, TransparentWrapper, Zeroable};
+use bytemuck::{ByteEq, ByteHash, Pod, TransparentWrapper, Zeroable};
 
-#[derive(Copy, Clone, Pod, Zeroable)]
+#[derive(Copy, Clone, Pod, Zeroable, ByteEq, ByteHash)]
 #[repr(C)]
 struct Test {
   a: u16,