1 files changed, 83 insertions, 0 deletions
diff --git a/library/core/src/cmp/bytewise.rs b/library/core/src/cmp/bytewise.rs
new file mode 100644
index 000000000..2548d9e24
--- /dev/null
+++ b/library/core/src/cmp/bytewise.rs
@@ -0,0 +1,83 @@
+use crate::num::{NonZeroI128, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI8, NonZeroIsize};
+use crate::num::{NonZeroU128, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};
+
+/// Types where `==` & `!=` are equivalent to comparing their underlying bytes.
+///
+/// Importantly, this means no floating-point types, as those have different
+/// byte representations (like `-0` and `+0`) which compare as the same.
+/// Since byte arrays are `Eq`, that implies that these types are probably also
+/// `Eq`, but that's not technically required to use this trait.
+///
+/// `Rhs` is *de facto* always `Self`, but the separate parameter is important
+/// to avoid the `specializing impl repeats parameter` error when consuming this.
+///
+/// # Safety
+///
+/// - `Self` and `Rhs` have no padding.
+/// - `Self` and `Rhs` have the same layout (size and alignment).
+/// - Neither `Self` nor `Rhs` have provenance, so integer comparisons are correct.
+/// - `<Self as PartialEq<Rhs>>::{eq,ne}` are equivalent to comparing the bytes.
+#[rustc_specialization_trait]
+pub(crate) unsafe trait BytewiseEq<Rhs = Self>: PartialEq<Rhs> + Sized {}
+
+macro_rules! is_bytewise_comparable {
+    ($($t:ty),+ $(,)?) => {$(
+        unsafe impl BytewiseEq for $t {}
+    )+};
+}
+
+// SAFETY: All the ordinary integer types have no padding, and are not pointers.
+is_bytewise_comparable!(u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, isize);
+
+// SAFETY: These have *niches*, but no *padding* and no *provenance*,
+// so we can compare them directly.
+is_bytewise_comparable!(bool, char, super::Ordering);
+
+// SAFETY: Similarly, the non-zero types have a niche, but no undef and no pointers,
+// and they compare like their underlying numeric type.
+is_bytewise_comparable!(
+    NonZeroU8,
+    NonZeroU16,
+    NonZeroU32,
+    NonZeroU64,
+    NonZeroU128,
+    NonZeroUsize,
+    NonZeroI8,
+    NonZeroI16,
+    NonZeroI32,
+    NonZeroI64,
+    NonZeroI128,
+    NonZeroIsize,
+);
+
+// SAFETY: The NonZero types have the "null" optimization guaranteed, and thus
+// are also safe to equality-compare bitwise inside an `Option`.
+// The way `PartialOrd` is defined for `Option` means that this wouldn't work
+// for `<` or `>` on the signed types, but since we only do `==` it's fine.
+is_bytewise_comparable!(
+    Option<NonZeroU8>,
+    Option<NonZeroU16>,
+    Option<NonZeroU32>,
+    Option<NonZeroU64>,
+    Option<NonZeroU128>,
+    Option<NonZeroUsize>,
+    Option<NonZeroI8>,
+    Option<NonZeroI16>,
+    Option<NonZeroI32>,
+    Option<NonZeroI64>,
+    Option<NonZeroI128>,
+    Option<NonZeroIsize>,
+);
+
+macro_rules! is_bytewise_comparable_array_length {
+    ($($n:literal),+ $(,)?) => {$(
+        // SAFETY: Arrays have no padding between elements, so if the elements are
+        // `BytewiseEq`, then the whole array can be too.
+        unsafe impl<T: BytewiseEq<U>, U> BytewiseEq<[U; $n]> for [T; $n] {}
+    )+};
+}
+
+// Frustratingly, this can't be made const-generic as it gets
+//    error: specializing impl repeats parameter `N`
+// so just do it for a couple of plausibly-common ones.
+is_bytewise_comparable_array_length!(0, 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, 64);