Auto merge of rust-lang#85820 - CDirkx:is_unicast_site_local, r=joshtriplett

Remove `Ipv6Addr::is_unicast_site_local`

Removes the unstable method `Ipv6Addr::is_unicast_site_local`, see also rust-lang#85604 where I have tried to summarize related discussion so far.

Unicast site-local addresses (`fec0::/10`) were deprecated in [IETF RFC rust-lang#3879](https://datatracker.ietf.org/doc/html/rfc3879), see also [RFC rust-lang#4291 Section 2.5.7](https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.7). Any new implementation must no longer support the special behaviour of site-local addresses. This is mentioned in the docs of `is_unicast_site_local` and already implemented in `is_unicast_global`, which considers addresses in `fec0::/10` to have global scope, thus overlapping with `is_unicast_site_local`.

Given that RFC rust-lang#3879 was published in 2004, long before Rust existed, and it is specified that any new implementation must no longer support the special behaviour of site-local addresses, I don't see how a user would ever have a need for `is_unicast_site_local`. It is also confusing that currently both `is_unicast_site_local` and `is_unicast_global` can be `true` for an address, but an address can actually only have a single scope. The deprecating RFC mentions that Site-Local scope was confusing to work with and that the classification of an address as either Link-Local or Global better matches the mental model of users.

There has been earlier discussion of removing `is_unicast_site_local` (rust-lang#60145 (comment)) which decided against it, but that had the incorrect assumption that the method was already stable; it is not. (This confusion arose from the placement of the unstable attribute on the entire module, instead of on individual methods, resolved in rust-lang#85672)

r? `@joshtriplett` as reviewer of all the related PRs

Author: bors

std/src/net/ip.rs

@@ -1346,47 +1346,6 @@ impl Ipv6Addr {
         (self.segments()[0] & 0xffc0) == 0xfe80
     }
 
-    /// Returns [`true`] if this is a deprecated unicast site-local address (`fec0::/10`). The
-    /// unicast site-local address format is defined in [RFC 4291 section 2.5.7] as:
-    ///
-    /// ```no_rust
-    /// |   10     |
-    /// |  bits    |         54 bits         |         64 bits            |
-    /// +----------+-------------------------+----------------------------+
-    /// |1111111011|        subnet ID        |       interface ID         |
-    /// +----------+-------------------------+----------------------------+
-    /// ```
-    ///
-    /// [RFC 4291 section 2.5.7]: https://tools.ietf.org/html/rfc4291#section-2.5.7
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(ip)]
-    ///
-    /// use std::net::Ipv6Addr;
-    ///
-    /// assert_eq!(
-    ///     Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff).is_unicast_site_local(),
-    ///     false
-    /// );
-    /// assert_eq!(Ipv6Addr::new(0xfec2, 0, 0, 0, 0, 0, 0, 0).is_unicast_site_local(), true);
-    /// ```
-    ///
-    /// # Warning
-    ///
-    /// As per [RFC 3879], the whole `fec0::/10` prefix is
-    /// deprecated. New software must not support site-local
-    /// addresses.
-    ///
-    /// [RFC 3879]: https://tools.ietf.org/html/rfc3879
-    #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
-    #[unstable(feature = "ip", issue = "27709")]
-    #[inline]
-    pub const fn is_unicast_site_local(&self) -> bool {
-        (self.segments()[0] & 0xffc0) == 0xfec0
-    }
-
     /// Returns [`true`] if this is an address reserved for documentation
     /// (`2001:db8::/32`).
     ///

std/src/net/ip/tests.rs

@@ -480,7 +480,6 @@ fn ipv6_properties() {
             let unique_local: u16 = 1 << 2;
             let global: u16 = 1 << 3;
             let unicast_link_local: u16 = 1 << 4;
-            let unicast_site_local: u16 = 1 << 6;
             let unicast_global: u16 = 1 << 7;
             let documentation: u16 = 1 << 8;
             let multicast_interface_local: u16 = 1 << 9;
@@ -523,11 +522,6 @@ fn ipv6_properties() {
             } else {
                 assert!(!ip!($s).is_unicast_link_local());
             }
-            if ($mask & unicast_site_local) == unicast_site_local {
-                assert!(ip!($s).is_unicast_site_local());
-            } else {
-                assert!(!ip!($s).is_unicast_site_local());
-            }
             if ($mask & unicast_global) == unicast_global {
                 assert!(ip!($s).is_unicast_global());
             } else {
@@ -581,7 +575,6 @@ fn ipv6_properties() {
     let unique_local: u16 = 1 << 2;
     let global: u16 = 1 << 3;
     let unicast_link_local: u16 = 1 << 4;
-    let unicast_site_local: u16 = 1 << 6;
     let unicast_global: u16 = 1 << 7;
     let documentation: u16 = 1 << 8;
     let multicast_interface_local: u16 = 1 << 9;
@@ -651,7 +644,7 @@ fn ipv6_properties() {
     check!(
         "fec0::",
         &[0xfe, 0xc0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
-        unicast_site_local | unicast_global | global
+        unicast_global | global
     );
 
     check!(
@@ -889,9 +882,6 @@ fn ipv6_const() {
     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);