Misc internal optimizations

* Make ipv4addr_to_libc const
* Use mem::transmute in ipv4addr_to_libc and ipv6addr_to_libc

Fixes nix-rust#1687
Fixes nix-rust#1688

Cargo.toml

@@ -31,6 +31,7 @@ libc = { git = "https://github.com/rust-lang/libc", rev = "cc19b6f0801", feature
 bitflags = "1.1"
 cfg-if = "1.0"
 pin-utils = { version = "0.1.0", optional = true }
+static_assertions = "1"
 
 [target.'cfg(not(target_os = "redox"))'.dependencies]
 memoffset = { version = "0.6.3", optional = true }

src/sys/socket/addr.rs

@@ -40,23 +40,22 @@ use std::{fmt, mem, net, ptr, slice};
 
 /// Convert a std::net::Ipv4Addr into the libc form.
 #[cfg(feature = "net")]
-pub(crate) fn ipv4addr_to_libc(addr: net::Ipv4Addr) -> libc::in_addr {
-    let octets = addr.octets();
-    libc::in_addr {
-        s_addr: u32::to_be(
-            ((octets[0] as u32) << 24)
-                | ((octets[1] as u32) << 16)
-                | ((octets[2] as u32) << 8)
-                | (octets[3] as u32),
-        ),
+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)
     }
 }
 
 /// 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 {
-    libc::in6_addr {
-        s6_addr: addr.octets(),
+    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)
     }
 }