Make all remaining methods of std::net::Ipv4Addr const

Makes the following methods of `std::net::Ipv4Addr` unstable const under the `const_ipv4` feature:
 - `is_global`
 - `is_reserved`
 - `is_broadcast`
 - `to_ipv6_compatible`
 - `to_ipv6_mapped`

This results in all methods of `Ipv4Addr` being const.

Also adds tests for these methods in a const context.

Author: CDirkx

std/src/net/ip.rs

@@ -542,10 +542,13 @@ impl Ipv4Addr {
     /// assert_eq!(Ipv4Addr::new(1, 1, 1, 1).is_global(), true);
     /// assert_eq!(Ipv4Addr::new(80, 9, 12, 3).is_global(), true);
     /// ```
-    pub fn is_global(&self) -> bool {
+    #[rustc_const_unstable(feature = "const_ipv4", issue = "76205")]
+    pub const fn is_global(&self) -> bool {
         // check if this address is 192.0.0.9 or 192.0.0.10. These addresses are the only two
         // globally routable addresses in the 192.0.0.0/24 range.
-        if u32::from(*self) == 0xc0000009 || u32::from(*self) == 0xc000000a {
+        if u32::from_be_bytes(self.octets()) == 0xc0000009
+            || u32::from_be_bytes(self.octets()) == 0xc000000a
+        {
             return true;
         }
         !self.is_private()
@@ -667,7 +670,8 @@ impl Ipv4Addr {
     /// // The broadcast address is not considered as reserved for future use by this implementation
     /// assert_eq!(Ipv4Addr::new(255, 255, 255, 255).is_reserved(), false);
     /// ```
-    pub fn is_reserved(&self) -> bool {
+    #[rustc_const_unstable(feature = "const_ipv4", issue = "76205")]
+    pub const fn is_reserved(&self) -> bool {
         self.octets()[0] & 240 == 240 && !self.is_broadcast()
     }
 
@@ -709,9 +713,10 @@ impl Ipv4Addr {
     /// assert_eq!(Ipv4Addr::new(255, 255, 255, 255).is_broadcast(), true);
     /// assert_eq!(Ipv4Addr::new(236, 168, 10, 65).is_broadcast(), false);
     /// ```
+    #[rustc_const_unstable(feature = "const_ipv4", issue = "76205")]
     #[stable(since = "1.7.0", feature = "ip_17")]
-    pub fn is_broadcast(&self) -> bool {
-        self == &Self::BROADCAST
+    pub const fn is_broadcast(&self) -> bool {
+        u32::from_be_bytes(self.octets()) == u32::from_be_bytes(Self::BROADCAST.octets())
     }
 
     /// Returns [`true`] if this address is in a range designated for documentation.
@@ -762,10 +767,13 @@ impl Ipv4Addr {
     ///     Ipv6Addr::new(0, 0, 0, 0, 0, 0, 49152, 767)
     /// );
     /// ```
+    #[rustc_const_unstable(feature = "const_ipv4", issue = "76205")]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn to_ipv6_compatible(&self) -> Ipv6Addr {
+    pub const fn to_ipv6_compatible(&self) -> Ipv6Addr {
         let [a, b, c, d] = self.octets();
-        Ipv6Addr::from([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, a, b, c, d])
+        Ipv6Addr {
+            inner: c::in6_addr { s6_addr: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, a, b, c, d] },
+        }
     }
 
     /// Converts this address to an IPv4-mapped [`IPv6` address].
@@ -782,10 +790,13 @@ impl Ipv4Addr {
     /// assert_eq!(Ipv4Addr::new(192, 0, 2, 255).to_ipv6_mapped(),
     ///            Ipv6Addr::new(0, 0, 0, 0, 0, 65535, 49152, 767));
     /// ```
+    #[rustc_const_unstable(feature = "const_ipv4", issue = "76205")]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn to_ipv6_mapped(&self) -> Ipv6Addr {
+    pub const fn to_ipv6_mapped(&self) -> Ipv6Addr {
         let [a, b, c, d] = self.octets();
-        Ipv6Addr::from([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF, a, b, c, d])
+        Ipv6Addr {
+            inner: c::in6_addr { s6_addr: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0xFF, a, b, c, d] },
+        }
     }
 }