Add Ipv6Addr::is_ipv4_mapped

This change consists of cherry-picking the content from the original
PR[1], which got closed due to inactivity, and applying the following
changes:

* Resolving merge conflicts (obviously)
* Linked to to_ipv4_mapped instead of to_ipv4 in the documentation (seems
  more appropriate)
* Added the must_use and rustc_const_unstable attributes the original
  didn't have

I think it's a reasonably useful method.

[1] rust-lang#86490

library/core/src/net/ip_addr.rs

@@ -1777,6 +1777,31 @@ impl Ipv6Addr {
         (self.segments()[0] & 0xff00) == 0xff00
     }
 
+    /// Returns [`true`] if the address is an IPv4-mapped address (`::ffff:0:0/96`).
+    ///
+    /// IPv4-mapped addresses can be converted to their canonical IPv4 address with
+    /// [`to_ipv4_mapped`](Ipv6Addr::to_ipv4_mapped).
+    ///
+    /// # Examples
+    /// ```
+    /// #![feature(ip)]
+    ///
+    /// use std::net::{Ipv4Addr, Ipv6Addr};
+    ///
+    /// let ipv4_mapped = Ipv4Addr::new(192, 0, 2, 255).to_ipv6_mapped();
+    /// assert_eq!(ipv4_mapped.is_ipv4_mapped(), true);
+    /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc000, 0x2ff).is_ipv4_mapped(), true);
+    ///
+    /// assert_eq!(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 0).is_ipv4_mapped(), false);
+    /// ```
+    #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
+    #[unstable(feature = "ip", issue = "27709")]
+    #[must_use]
+    #[inline]
+    pub const fn is_ipv4_mapped(&self) -> bool {
+        matches!(self.segments(), [0, 0, 0, 0, 0, 0xffff, _, _])
+    }
+
     /// Converts this address to an [`IPv4` address] if it's an [IPv4-mapped] address,
     /// as defined in [IETF RFC 4291 section 2.5.5.2], otherwise returns [`None`].
     ///

library/core/tests/net/ip_addr.rs

@@ -508,6 +508,7 @@ fn ipv6_properties() {
                 | multicast_realm_local
                 | multicast_site_local
                 | multicast_organization_local;
+            let ipv4_mapped: u32 = 1 << 17;
 
             if ($mask & unspecified) == unspecified {
                 assert!(ip!($s).is_unspecified());
@@ -584,6 +585,11 @@ fn ipv6_properties() {
                 assert_eq!(ip!($s).multicast_scope().unwrap(),
                            Ipv6MulticastScope::Global);
             }
+            if ($mask & ipv4_mapped) == ipv4_mapped {
+                assert!(ip!($s).is_ipv4_mapped());
+            } else {
+                assert!(!ip!($s).is_ipv4_mapped());
+            }
         }
     }
 
@@ -602,6 +608,7 @@ fn ipv6_properties() {
     let multicast_site_local: u32 = 1 << 13;
     let multicast_organization_local: u32 = 1 << 14;
     let multicast_global: u32 = 1 << 15;
+    let ipv4_mapped: u32 = 1 << 17;
 
     check!("::", &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], unspecified);
 
@@ -614,7 +621,7 @@ fn ipv6_properties() {
     check!(
         "::ffff:127.0.0.1",
         &[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, 0x7f, 0, 0, 1],
-        unicast_global
+        unicast_global | ipv4_mapped
     );
 
     check!(
@@ -982,6 +989,9 @@ fn ipv6_const() {
     const IS_MULTICAST: bool = IP_ADDRESS.is_multicast();
     assert!(!IS_MULTICAST);
 
+    const IS_IPV4_MAPPED: bool = IP_ADDRESS.is_ipv4_mapped();
+    assert!(!IS_IPV4_MAPPED);
+
     const IP_V4: Option<Ipv4Addr> = IP_ADDRESS.to_ipv4();
     assert_eq!(IP_V4.unwrap(), Ipv4Addr::new(0, 0, 0, 1));
 }