Rollup merge of #76206 - CDirkx:const-ipv6, r=ecstatic-morse

Make all methods of `std::net::Ipv6Addr` const

Make the following methods of `std::net::Ipv6Addr` unstable const under the `const_ipv6` feature:
- `segments`
- `is_unspecified`
- `is_loopback`
- `is_global` (unstable)
- `is_unique_local`
- `is_unicast_link_local_strict`
- `is_documentation`
- `multicast_scope`
- `is_multicast`
- `to_ipv4_mapped`
- `to_ipv4`

This would make all methods of `Ipv6Addr` const.

Changed the implementation of `is_unspecified` and `is_loopback` to use a `match` instead of `==`, all other methods did not require a change.

All these methods are dependent on `segments`, the current implementation of which requires unstable `const_fn_transmute` ([PR#75085](https://github.com/rust-lang/rust/pull/75085)).

Part of #76205

src/test/ui/consts/std/net/ipv6.rs

@@ -0,0 +1,53 @@
+// run-pass
+
+#![feature(ip)]
+#![feature(const_ipv6)]
+
+use std::net::{Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
+
+fn main() {
+    const IP_ADDRESS : Ipv6Addr = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1);
+    assert_eq!(IP_ADDRESS, Ipv6Addr::LOCALHOST);
+
+    const SEGMENTS : [u16; 8] = IP_ADDRESS.segments();
+    assert_eq!(SEGMENTS, [0 ,0 ,0 ,0 ,0 ,0 ,0, 1]);
+
+    const OCTETS : [u8; 16] = IP_ADDRESS.octets();
+    assert_eq!(OCTETS, [0 ,0 ,0 ,0 ,0 ,0 ,0, 0 ,0 ,0 ,0 ,0 ,0 ,0, 0, 1]);
+
+    const IS_UNSPECIFIED : bool = IP_ADDRESS.is_unspecified();
+    assert!(!IS_UNSPECIFIED);
+
+    const IS_LOOPBACK : bool = IP_ADDRESS.is_loopback();
+    assert!(IS_LOOPBACK);
+
+    const IS_GLOBAL : bool = IP_ADDRESS.is_global();
+    assert!(!IS_GLOBAL);
+
+    const IS_UNIQUE_LOCAL : bool = IP_ADDRESS.is_unique_local();
+    assert!(!IS_UNIQUE_LOCAL);
+
+    const IS_UNICAST_LINK_LOCAL_STRICT : bool = IP_ADDRESS.is_unicast_link_local_strict();
+    assert!(!IS_UNICAST_LINK_LOCAL_STRICT);
+
+    const IS_UNICAST_LINK_LOCAL : bool = IP_ADDRESS.is_unicast_link_local();
+    assert!(!IS_UNICAST_LINK_LOCAL);
+
+    const IS_UNICAST_SITE_LOCAL : bool = IP_ADDRESS.is_unicast_site_local();
+    assert!(!IS_UNICAST_SITE_LOCAL);
+
+    const IS_DOCUMENTATION : bool = IP_ADDRESS.is_documentation();
+    assert!(!IS_DOCUMENTATION);
+
+    const IS_UNICAST_GLOBAL : bool = IP_ADDRESS.is_unicast_global();
+    assert!(!IS_UNICAST_GLOBAL);
+
+    const MULTICAST_SCOPE : Option<Ipv6MulticastScope> = IP_ADDRESS.multicast_scope();
+    assert_eq!(MULTICAST_SCOPE, None);
+
+    const IS_MULTICAST : bool = IP_ADDRESS.is_multicast();
+    assert!(!IS_MULTICAST);
+
+    const IP_V4 : Option<Ipv4Addr> = IP_ADDRESS.to_ipv4();
+    assert_eq!(IP_V4.unwrap(), Ipv4Addr::new(0, 0, 0, 1));
+}