unistd: groups: Update comment about casting Gid pointer to gid_t

Author: JayH5

src/unistd.rs

@@ -1002,8 +1002,9 @@ pub fn getgroups() -> Result<Vec<Gid>> {
     // now too small
     let mut groups = Vec::<Gid>::with_capacity(size as usize);
     loop {
-        // We can coerce a pointer to some `Gid`s as a pointer to some `gid_t`s
-        // as they have the same representation in memory.
+        // FIXME: On the platforms we currently support, the `Gid` struct has
+        // the same representation in memory as a bare `gid_t`. This is not
+        // necessarily the case on all Rust platforms, though. See RFC 1785.
         let ret = unsafe { libc::getgroups(size, groups.as_mut_ptr() as *mut gid_t) };
 
         match Errno::result(ret) {
@@ -1043,8 +1044,9 @@ pub fn setgroups(groups: &[Gid]) -> Result<()> {
             type setgroups_ngroups_t = size_t;
         }
     }
-    // We can coerce a pointer to some `Gid`s as a pointer to some `gid_t`s as
-    // they have the same representation in memory.
+    // FIXME: On the platforms we currently support, the `Gid` struct has the
+    // same representation in memory as a bare `gid_t`. This is not necessarily
+    // the case on all Rust platforms, though. See RFC 1785.
     let res = unsafe {
         libc::setgroups(groups.len() as setgroups_ngroups_t, groups.as_ptr() as *const gid_t)
     };