Auto merge of rust-lang#76610 - hch12907:master, r=LukasKalbertodt

Implement as_ne_bytes() for integers and floats

This is related to issue rust-lang#64464.

I am pretty sure that these functions are actually const-ify-able, and technically as_bits() can also be implemented for floats, but I might need some comments on both.

library/core/src/num/f32.rs

@@ -773,6 +773,35 @@ impl f32 {
         self.to_bits().to_ne_bytes()
     }
 
+    /// Return the memory representation of this floating point number as a byte array in
+    /// native byte order.
+    ///
+    /// [`to_ne_bytes`] should be preferred over this whenever possible.
+    ///
+    /// [`to_ne_bytes`]: #method.to_ne_bytes
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(num_as_ne_bytes)]
+    /// let num = 12.5f32;
+    /// let bytes = num.as_ne_bytes();
+    /// assert_eq!(
+    ///     bytes,
+    ///     if cfg!(target_endian = "big") {
+    ///         &[0x41, 0x48, 0x00, 0x00]
+    ///     } else {
+    ///         &[0x00, 0x00, 0x48, 0x41]
+    ///     }
+    /// );
+    /// ```
+    #[unstable(feature = "num_as_ne_bytes", issue = "76976")]
+    #[inline]
+    pub fn as_ne_bytes(&self) -> &[u8; 4] {
+        // SAFETY: `f32` is a plain old datatype so we can always transmute to it
+        unsafe { &*(self as *const Self as *const _) }
+    }
+
     /// Create a floating point value from its representation as a byte array in big endian.
     ///
     /// # Examples

library/core/src/num/f64.rs

@@ -787,6 +787,35 @@ impl f64 {
         self.to_bits().to_ne_bytes()
     }
 
+    /// Return the memory representation of this floating point number as a byte array in
+    /// native byte order.
+    ///
+    /// [`to_ne_bytes`] should be preferred over this whenever possible.
+    ///
+    /// [`to_ne_bytes`]: #method.to_ne_bytes
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(num_as_ne_bytes)]
+    /// let num = 12.5f64;
+    /// let bytes = num.as_ne_bytes();
+    /// assert_eq!(
+    ///     bytes,
+    ///     if cfg!(target_endian = "big") {
+    ///         &[0x40, 0x29, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]
+    ///     } else {
+    ///         &[0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x29, 0x40]
+    ///     }
+    /// );
+    /// ```
+    #[unstable(feature = "num_as_ne_bytes", issue = "76976")]
+    #[inline]
+    pub fn as_ne_bytes(&self) -> &[u8; 8] {
+        // SAFETY: `f64` is a plain old datatype so we can always transmute to it
+        unsafe { &*(self as *const Self as *const _) }
+    }
+
     /// Create a floating point value from its representation as a byte array in big endian.
     ///
     /// # Examples

library/core/src/num/int_macros.rs

@@ -2054,6 +2054,41 @@ assert_eq!(
             }
         }
 
+        doc_comment! {
+            concat!("
+Return the memory representation of this integer as a byte array in
+native byte order.
+
+[`to_ne_bytes`] should be preferred over this whenever possible.
+
+[`to_ne_bytes`]: #method.to_ne_bytes
+",
+
+"
+# Examples
+
+```
+#![feature(num_as_ne_bytes)]
+let num = ", $swap_op, stringify!($SelfT), ";
+let bytes = num.as_ne_bytes();
+assert_eq!(
+    bytes,
+    if cfg!(target_endian = \"big\") {
+        &", $be_bytes, "
+    } else {
+        &", $le_bytes, "
+    }
+);
+```"),
+            #[unstable(feature = "num_as_ne_bytes", issue = "76976")]
+            #[inline]
+            pub fn as_ne_bytes(&self) -> &[u8; mem::size_of::<Self>()] {
+                // SAFETY: integers are plain old datatypes so we can always transmute them to
+                // arrays of bytes
+                unsafe { &*(self as *const Self as *const _) }
+            }
+        }
+
 doc_comment! {
             concat!("Create an integer value from its representation as a byte array in
 big endian.

library/core/src/num/uint_macros.rs

@@ -1812,6 +1812,41 @@ assert_eq!(
             }
         }
 
+        doc_comment! {
+            concat!("
+Return the memory representation of this integer as a byte array in
+native byte order.
+
+[`to_ne_bytes`] should be preferred over this whenever possible.
+
+[`to_ne_bytes`]: #method.to_ne_bytes
+",
+
+"
+# Examples
+
+```
+#![feature(num_as_ne_bytes)]
+let num = ", $swap_op, stringify!($SelfT), ";
+let bytes = num.as_ne_bytes();
+assert_eq!(
+    bytes,
+    if cfg!(target_endian = \"big\") {
+        &", $be_bytes, "
+    } else {
+        &", $le_bytes, "
+    }
+);
+```"),
+            #[unstable(feature = "num_as_ne_bytes", issue = "76976")]
+            #[inline]
+            pub fn as_ne_bytes(&self) -> &[u8; mem::size_of::<Self>()] {
+                // SAFETY: integers are plain old datatypes so we can always transmute them to
+                // arrays of bytes
+                unsafe { &*(self as *const Self as *const _) }
+            }
+        }
+
         doc_comment! {
             concat!("Create a native endian integer value from its representation
 as a byte array in big endian.