Add bindings for iconv calls

FreeBSD-likes all implement iconv:
- DragonflyBSD: https://github.com/DragonFlyBSD/DragonFlyBSD/blob/bbb35c81f71fe2a0880a1f8bb77876ee98b63338/include/iconv.h
- FreeBSD: https://github.com/freebsd/freebsd-src/blob/a6dc68c0e0f8a24ffaf0b4e78e58141ef7897047/include/iconv.h

NetBSD-likes:
- NetBSD: https://github.com/NetBSD/src/blob/81a39f60870b617d7fca299c126de6553d78cc5a/include/iconv.h
- OpenBSD doesn't implement it

macOS: apparently ships a conforming implementation as a separate
library:
https://stackoverflow.com/questions/57734434/libiconv-or-iconv-undefined-symbol-on-mac-osx/57734435#57734435

Linux:
- glibc: https://sourceware.org/git/?p=glibc.git;a=blob;f=iconv/iconv.h;h=fdddf53d99c3046ef9c280db01a425c2f499043e;hb=HEAD
- musl: https://git.musl-libc.org/cgit/musl/tree/include/iconv.h?id=455f96857f91d14e193219ca00969354a981c09c

Author: Minoru

build.rs

@@ -10,6 +10,7 @@ fn main() {
     let const_extern_fn_cargo_feature =
         env::var("CARGO_FEATURE_CONST_EXTERN_FN").is_ok();
     let libc_ci = env::var("LIBC_CI").is_ok();
+    let target = env::var("TARGET").unwrap();
 
     if env::var("CARGO_FEATURE_USE_STD").is_ok() {
         println!(
@@ -82,6 +83,12 @@ fn main() {
         }
         println!("cargo:rustc-cfg=libc_const_extern_fn");
     }
+
+    // For unknown reason, libiconv can't be linked by adding #[link(name = iconv)] to
+    // a macOS-specific struct, so we do the linking here.
+    if target.contains("-apple-") {
+        println!("cargo:rustc-link-lib=iconv");
+    }
 }
 
 fn rustc_minor_nightly() -> Option<(u32, bool)> {

libc-test/build.rs

@@ -107,6 +107,7 @@ fn test_apple(target: &str) {
         "fcntl.h",
         "glob.h",
         "grp.h",
+        "iconv.h",
         "ifaddrs.h",
         "langinfo.h",
         "limits.h",
@@ -360,6 +361,7 @@ fn test_openbsd(target: &str) {
         "pthread_np.h",
         "sys/syscall.h",
         "sys/shm.h",
+        "iconv.h",
     }
 
     cfg.skip_struct(move |ty| {
@@ -558,6 +560,7 @@ fn test_redox(target: &str) {
         "errno.h",
         "fcntl.h",
         "grp.h",
+        "iconv.h",
         "limits.h",
         "locale.h",
         "netdb.h",
@@ -618,6 +621,7 @@ fn test_solarish(target: &str) {
         "fcntl.h",
         "glob.h",
         "grp.h",
+        "iconv.h",
         "ifaddrs.h",
         "langinfo.h",
         "limits.h",
@@ -893,6 +897,7 @@ fn test_netbsd(target: &str) {
         "sys/event.h",
         "sys/quota.h",
         "sys/shm.h",
+        "iconv.h",
     }
 
     cfg.type_name(move |ty, is_struct, is_union| {
@@ -1100,6 +1105,7 @@ fn test_dragonflybsd(target: &str) {
         "utime.h",
         "utmpx.h",
         "wchar.h",
+        "iconv.h",
     }
 
     cfg.type_name(move |ty, is_struct, is_union| {
@@ -1329,6 +1335,7 @@ fn test_android(target: &str) {
                "errno.h",
                "fcntl.h",
                "grp.h",
+               "iconv.h",
                "ifaddrs.h",
                "limits.h",
                "locale.h",
@@ -1381,8 +1388,8 @@ fn test_android(target: &str) {
                "sys/syscall.h",
                "sys/sysinfo.h",
                "sys/time.h",
-               "sys/times.h",
                "sys/timerfd.h",
+               "sys/times.h",
                "sys/types.h",
                "sys/ucontext.h",
                "sys/uio.h",
@@ -1609,6 +1616,7 @@ fn test_freebsd(target: &str) {
                 "fcntl.h",
                 "glob.h",
                 "grp.h",
+                "iconv.h",
                 "ifaddrs.h",
                 "langinfo.h",
                 "libutil.h",
@@ -1915,6 +1923,7 @@ fn test_emscripten(target: &str) {
                "fcntl.h",
                "glob.h",
                "grp.h",
+               "iconv.h",
                "ifaddrs.h",
                "langinfo.h",
                "limits.h",
@@ -2279,6 +2288,7 @@ fn test_linux(target: &str) {
                "fcntl.h",
                "glob.h",
                "grp.h",
+               "iconv.h",
                "ifaddrs.h",
                "langinfo.h",
                "limits.h",

src/unix/bsd/apple/mod.rs

@@ -37,6 +37,8 @@ pub type sae_connid_t = u32;
 
 pub type mach_port_t = ::c_uint;
 
+pub type iconv_t = *mut ::c_void;
+
 deprecated_mach! {
     pub type vm_prot_t = ::c_int;
     pub type vm_size_t = ::uintptr_t;
@@ -3764,6 +3766,19 @@ extern "C" {
         bufsize: ::c_int,
         flags: ::c_int,
     ) -> ::c_int;
+
+    pub fn iconv_open(
+        tocode: *const ::c_char,
+        fromcode: *const ::c_char,
+    ) -> iconv_t;
+    pub fn iconv(
+        cd: iconv_t,
+        inbuf: *mut *mut ::c_char,
+        inbytesleft: *mut ::size_t,
+        outbuf: *mut *mut ::c_char,
+        outbytesleft: *mut ::size_t,
+    ) -> ::size_t;
+    pub fn iconv_close(cd: iconv_t) -> ::c_int;
 }
 
 cfg_if! {

src/unix/bsd/freebsdlike/mod.rs

@@ -32,6 +32,8 @@ pub type Elf64_Sxword = i64;
 pub type Elf64_Word = u32;
 pub type Elf64_Xword = u64;
 
+pub type iconv_t = *mut ::c_void;
+
 cfg_if! {
     if #[cfg(target_pointer_width = "64")] {
         type Elf_Addr = Elf64_Addr;
@@ -1593,6 +1595,19 @@ extern "C" {
         >,
         data: *mut ::c_void,
     ) -> ::c_int;
+
+    pub fn iconv_open(
+        tocode: *const ::c_char,
+        fromcode: *const ::c_char,
+    ) -> iconv_t;
+    pub fn iconv(
+        cd: iconv_t,
+        inbuf: *mut *mut ::c_char,
+        inbytesleft: *mut ::size_t,
+        outbuf: *mut *mut ::c_char,
+        outbytesleft: *mut ::size_t,
+    ) -> ::size_t;
+    pub fn iconv_close(cd: iconv_t) -> ::c_int;
 }
 
 #[link(name = "rt")]

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -29,6 +29,8 @@ pub type Elf64_Sxword = i64;
 pub type Elf64_Word = u32;
 pub type Elf64_Xword = u64;
 
+pub type iconv_t = *mut ::c_void;
+
 cfg_if! {
     if #[cfg(target_pointer_width = "64")] {
         type Elf_Addr = Elf64_Addr;
@@ -2081,6 +2083,19 @@ extern "C" {
         >,
         data: *mut ::c_void,
     ) -> ::c_int;
+
+    pub fn iconv_open(
+        tocode: *const ::c_char,
+        fromcode: *const ::c_char,
+    ) -> iconv_t;
+    pub fn iconv(
+        cd: iconv_t,
+        inbuf: *mut *mut ::c_char,
+        inbytesleft: *mut ::size_t,
+        outbuf: *mut *mut ::c_char,
+        outbytesleft: *mut ::size_t,
+    ) -> ::size_t;
+    pub fn iconv_close(cd: iconv_t) -> ::c_int;
 }
 
 #[link(name = "util")]

src/unix/linux_like/linux/mod.rs

@@ -40,6 +40,8 @@ pub type Elf64_Section = u16;
 pub type canid_t = u32;
 pub type can_err_mask_t = u32;
 
+pub type iconv_t = *mut ::c_void;
+
 #[cfg_attr(feature = "extra_traits", derive(Debug))]
 pub enum fpos64_t {} // FIXME: fill this out with a struct
 impl ::Copy for fpos64_t {}
@@ -3576,6 +3578,19 @@ extern "C" {
     ) -> ::size_t;
 
     pub fn regfree(preg: *mut ::regex_t);
+
+    pub fn iconv_open(
+        tocode: *const ::c_char,
+        fromcode: *const ::c_char,
+    ) -> iconv_t;
+    pub fn iconv(
+        cd: iconv_t,
+        inbuf: *mut *mut ::c_char,
+        inbytesleft: *mut ::size_t,
+        outbuf: *mut *mut ::c_char,
+        outbytesleft: *mut ::size_t,
+    ) -> ::size_t;
+    pub fn iconv_close(cd: iconv_t) -> ::c_int;
 }
 
 cfg_if! {