Refactor uses of objc_msgSend to no longer have clashing definitions

This is very similar to what Apple's own headers encourage you to do (cast the function pointer before use instead of making new declarations).

Additionally, I'm documenting a few of the memory management rules we're following, ensuring that the `args` function doesn't leak memory (if you wrap it in an autorelease pool).

library/std/src/sys/unix/args.rs

@@ -201,9 +201,9 @@ mod imp {
 
     // As _NSGetArgc and _NSGetArgv aren't mentioned in iOS docs
     // and use underscores in their names - they're most probably
-    // are considered private and therefore should be avoided
-    // Here is another way to get arguments using Objective C
-    // runtime
+    // are considered private and therefore should be avoided.
+    // Here is another way to get arguments using the Objective-C
+    // runtime.
     //
     // In general it looks like:
     // res = Vec::new()
@@ -213,51 +213,60 @@ mod imp {
     // res
     #[cfg(any(target_os = "ios", target_os = "tvos", target_os = "watchos"))]
     pub fn args() -> Args {
-        use crate::ffi::OsString;
+        use crate::ffi::{c_char, c_uchar, c_void, OsString};
         use crate::mem;
         use crate::str;
 
-        extern "C" {
-            fn sel_registerName(name: *const libc::c_uchar) -> Sel;
-            fn objc_getClass(class_name: *const libc::c_uchar) -> NsId;
-        }
+        type Sel = *const c_void;
+        type NsId = *const c_void;
+        type NSUInteger = usize;
 
-        #[cfg(target_arch = "aarch64")]
         extern "C" {
-            fn objc_msgSend(obj: NsId, sel: Sel) -> NsId;
-            #[allow(clashing_extern_declarations)]
-            #[link_name = "objc_msgSend"]
-            fn objc_msgSend_ul(obj: NsId, sel: Sel, i: libc::c_ulong) -> NsId;
-        }
+            fn sel_registerName(name: *const c_uchar) -> Sel;
+            fn objc_getClass(class_name: *const c_uchar) -> NsId;
 
-        #[cfg(not(target_arch = "aarch64"))]
-        extern "C" {
-            fn objc_msgSend(obj: NsId, sel: Sel, ...) -> NsId;
-            #[allow(clashing_extern_declarations)]
-            #[link_name = "objc_msgSend"]
-            fn objc_msgSend_ul(obj: NsId, sel: Sel, ...) -> NsId;
+            // This must be transmuted to an appropriate function pointer type before being called.
+            fn objc_msgSend();
         }
 
-        type Sel = *const libc::c_void;
-        type NsId = *const libc::c_void;
+        const MSG_SEND_PTR: unsafe extern "C" fn() = objc_msgSend;
+        const MSG_SEND_NO_ARGUMENTS_RETURN_PTR: unsafe extern "C" fn(NsId, Sel) -> *const c_void =
+            unsafe { mem::transmute(MSG_SEND_PTR) };
+        const MSG_SEND_NO_ARGUMENTS_RETURN_NSUINTEGER: unsafe extern "C" fn(
+            NsId,
+            Sel,
+        ) -> NSUInteger = unsafe { mem::transmute(MSG_SEND_PTR) };
+        const MSG_SEND_NSINTEGER_ARGUMENT_RETURN_PTR: unsafe extern "C" fn(
+            NsId,
+            Sel,
+            NSUInteger,
+        )
+            -> *const c_void = unsafe { mem::transmute(MSG_SEND_PTR) };
 
         let mut res = Vec::new();
 
         unsafe {
             let process_info_sel = sel_registerName("processInfo\0".as_ptr());
             let arguments_sel = sel_registerName("arguments\0".as_ptr());
-            let utf8_sel = sel_registerName("UTF8String\0".as_ptr());
             let count_sel = sel_registerName("count\0".as_ptr());
-            let object_at_sel = sel_registerName("objectAtIndex:\0".as_ptr());
+            let object_at_index_sel = sel_registerName("objectAtIndex:\0".as_ptr());
+            let utf8string_sel = sel_registerName("UTF8String\0".as_ptr());
 
             let klass = objc_getClass("NSProcessInfo\0".as_ptr());
-            let info = objc_msgSend(klass, process_info_sel);
-            let args = objc_msgSend(info, arguments_sel);
+            // `+[NSProcessInfo processInfo]` returns an object with +0 retain count, so no need to manually `retain/release`.
+            let info = MSG_SEND_NO_ARGUMENTS_RETURN_PTR(klass, process_info_sel);
+
+            // `-[NSProcessInfo arguments]` returns an object with +0 retain count, so no need to manually `retain/release`.
+            let args = MSG_SEND_NO_ARGUMENTS_RETURN_PTR(info, arguments_sel);
 
-            let cnt: usize = mem::transmute(objc_msgSend(args, count_sel));
+            let cnt = MSG_SEND_NO_ARGUMENTS_RETURN_NSUINTEGER(args, count_sel);
             for i in 0..cnt {
-                let tmp = objc_msgSend_ul(args, object_at_sel, i as libc::c_ulong);
-                let utf_c_str: *const libc::c_char = mem::transmute(objc_msgSend(tmp, utf8_sel));
+                // `-[NSArray objectAtIndex:]` returns an object whose lifetime is tied to the array, so no need to manually `retain/release`.
+                let ns_string =
+                    MSG_SEND_NSINTEGER_ARGUMENT_RETURN_PTR(args, object_at_index_sel, i);
+                // The lifetime of this pointer is tied to the NSString, as well as the current autorelease pool, which is why we heap-allocate the string below.
+                let utf_c_str: *const c_char =
+                    MSG_SEND_NO_ARGUMENTS_RETURN_PTR(ns_string, utf8string_sel).cast();
                 let bytes = CStr::from_ptr(utf_c_str).to_bytes();
                 res.push(OsString::from(str::from_utf8(bytes).unwrap()))
             }