For invalid IP address conversions with future Rust versions

Rust's standard library no longer guarantees that Ipv4Addr and Ipv6Addr
are wrappers around the C types (though for now at least, they are
identical on all platforms I'm aware of).  So do the conversions
explicitly instead of transmuting.

Fixes #2053

Author: asomers

CHANGELOG.md

@@ -40,6 +40,11 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   ([#1964](https://github.com/nix-rust/nix/pull/1964))
 - Fix: send ETH_P_ALL in htons format 
   ([#1925](https://github.com/nix-rust/nix/pull/1925))
+- Fix potentially invalid conversions in
+  `SockaddrIn::from<std::net::SocketAddrV4>`,
+  `SockaddrIn6::from<std::net::SockaddrV6>`, `IpMembershipRequest::new`, and
+  `Ipv6MembershipRequest::new` with future Rust versions.
+  ([#TODO](https://github.com/nix-rust/nix/pull/TODO))
 
 ### Removed
 

Cargo.toml

@@ -31,7 +31,6 @@ libc = { version = "0.2.141", features = ["extra_traits"] }
 bitflags = "1.1"
 cfg-if = "1.0"
 pin-utils = { version = "0.1.0", optional = true }
-static_assertions = "1"
 memoffset = { version = "0.9", optional = true }
 
 [features]

src/sys/socket/addr.rs

@@ -39,18 +39,17 @@ use std::{fmt, mem, net, ptr, slice};
 /// Convert a std::net::Ipv4Addr into the libc form.
 #[cfg(feature = "net")]
 pub(crate) const fn ipv4addr_to_libc(addr: net::Ipv4Addr) -> libc::in_addr {
-    static_assertions::assert_eq_size!(net::Ipv4Addr, libc::in_addr);
-    // Safe because both types have the same memory layout, and no fancy Drop
-    // impls.
-    unsafe { mem::transmute(addr) }
+    libc::in_addr {
+        s_addr: u32::from_ne_bytes(addr.octets())
+    }
 }
 
 /// Convert a std::net::Ipv6Addr into the libc form.
 #[cfg(feature = "net")]
 pub(crate) const fn ipv6addr_to_libc(addr: &net::Ipv6Addr) -> libc::in6_addr {
-    static_assertions::assert_eq_size!(net::Ipv6Addr, libc::in6_addr);
-    // Safe because both are Newtype wrappers around the same libc type
-    unsafe { mem::transmute(*addr) }
+    libc::in6_addr {
+        s6_addr: addr.octets()
+    }
 }
 
 /// These constants specify the protocol family to be used
@@ -949,9 +948,6 @@ impl SockaddrLike for () {
 }
 
 /// An IPv4 socket address
-// This is identical to net::SocketAddrV4.  But the standard library
-// doesn't allow direct access to the libc fields, which we need.  So we
-// reimplement it here.
 #[cfg(feature = "net")]
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]