Do array-slice equality via arrays, rather than always via slices

This'll still go via slices eventually for large arrays, but this way slice comparisons to short arrays can use the same memcmp-avoidance tricks.

Added some tests for all the combinations to make sure I didn't accidentally infinitely-recurse something.

library/core/src/array/equality.rs

@@ -1,3 +1,4 @@
+use crate::convert::TryInto;
 use crate::num::{NonZeroI128, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI8, NonZeroIsize};
 use crate::num::{NonZeroU128, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize};
 
@@ -23,11 +24,19 @@ where
 {
     #[inline]
     fn eq(&self, other: &[B]) -> bool {
-        self[..] == other[..]
+        let b: Result<&[B; N], _> = other.try_into();
+        match b {
+            Ok(b) => *self == *b,
+            Err(_) => false,
+        }
     }
     #[inline]
     fn ne(&self, other: &[B]) -> bool {
-        self[..] != other[..]
+        let b: Result<&[B; N], _> = other.try_into();
+        match b {
+            Ok(b) => *self != *b,
+            Err(_) => true,
+        }
     }
 }
 
@@ -38,11 +47,19 @@ where
 {
     #[inline]
     fn eq(&self, other: &[A; N]) -> bool {
-        self[..] == other[..]
+        let b: Result<&[B; N], _> = self.try_into();
+        match b {
+            Ok(b) => *b == *other,
+            Err(_) => false,
+        }
     }
     #[inline]
     fn ne(&self, other: &[A; N]) -> bool {
-        self[..] != other[..]
+        let b: Result<&[B; N], _> = self.try_into();
+        match b {
+            Ok(b) => *b != *other,
+            Err(_) => true,
+        }
     }
 }
 
@@ -53,11 +70,11 @@ where
 {
     #[inline]
     fn eq(&self, other: &&[B]) -> bool {
-        self[..] == other[..]
+        *self == **other
     }
     #[inline]
     fn ne(&self, other: &&[B]) -> bool {
-        self[..] != other[..]
+        *self != **other
     }
 }
 
@@ -68,11 +85,11 @@ where
 {
     #[inline]
     fn eq(&self, other: &[A; N]) -> bool {
-        self[..] == other[..]
+        **self == *other
     }
     #[inline]
     fn ne(&self, other: &[A; N]) -> bool {
-        self[..] != other[..]
+        **self != *other
     }
 }
 
@@ -83,11 +100,11 @@ where
 {
     #[inline]
     fn eq(&self, other: &&mut [B]) -> bool {
-        self[..] == other[..]
+        *self == **other
     }
     #[inline]
     fn ne(&self, other: &&mut [B]) -> bool {
-        self[..] != other[..]
+        *self != **other
     }
 }
 
@@ -98,11 +115,11 @@ where
 {
     #[inline]
     fn eq(&self, other: &[A; N]) -> bool {
-        self[..] == other[..]
+        **self == *other
     }
     #[inline]
     fn ne(&self, other: &[A; N]) -> bool {
-        self[..] != other[..]
+        **self != *other
     }
 }