Stabilize IpAddr::is_ipv4 and is_ipv6 as const

Insta-stabilize the methods `is_ipv4` and `is_ipv6` of `IpAddr`.

Possible because of the recent stabilization of const control flow.

Also adds a test for these methods in a const context.

Author: CDirkx

std/src/net/ip.rs

@@ -263,8 +263,9 @@ impl IpAddr {
     /// assert_eq!(IpAddr::V4(Ipv4Addr::new(203, 0, 113, 6)).is_ipv4(), true);
     /// assert_eq!(IpAddr::V6(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 0)).is_ipv4(), false);
     /// ```
+    #[rustc_const_stable(feature = "const_ip", since = "1.48.0")]
     #[stable(feature = "ipaddr_checker", since = "1.16.0")]
-    pub fn is_ipv4(&self) -> bool {
+    pub const fn is_ipv4(&self) -> bool {
         matches!(self, IpAddr::V4(_))
     }
 
@@ -281,8 +282,9 @@ impl IpAddr {
     /// assert_eq!(IpAddr::V4(Ipv4Addr::new(203, 0, 113, 6)).is_ipv6(), false);
     /// assert_eq!(IpAddr::V6(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 0)).is_ipv6(), true);
     /// ```
+    #[rustc_const_stable(feature = "const_ip", since = "1.48.0")]
     #[stable(feature = "ipaddr_checker", since = "1.16.0")]
-    pub fn is_ipv6(&self) -> bool {
+    pub const fn is_ipv6(&self) -> bool {
         matches!(self, IpAddr::V6(_))
     }
 }