Fix a panic collecting cpu data on OSX

There's very little documentation on `host_processor_info` from what I
can tell, so I'm just cribbing examples I've found elsewhere on the
internet. It appears that I've misinterpreted one of the out parameters
of `host_processor_info` as the number of elements of an array, but it
actually is just something forwarded to `vm_deallocate`.

src/cargo/util/cpu.rs

@@ -90,7 +90,6 @@ mod imp {
 mod imp {
     use std::io;
     use std::ptr;
-    use std::slice;
 
     type host_t = u32;
     type mach_port_t = u32;
@@ -109,6 +108,7 @@ mod imp {
     const CPU_STATE_SYSTEM: usize = 1;
     const CPU_STATE_IDLE: usize = 2;
     const CPU_STATE_NICE: usize = 3;
+    const CPU_STATE_MAX: usize = 4;
 
     extern "C" {
         static mut mach_task_self_: mach_port_t;
@@ -135,39 +135,45 @@ mod imp {
         nice: u64,
     }
 
+    #[repr(C)]
+    struct processor_cpu_load_info_data_t {
+        cpu_ticks: [u32; CPU_STATE_MAX],
+    }
+
     pub fn current() -> io::Result<State> {
+        // There's scant little documentation on `host_processor_info`
+        // throughout the internet, so this is just modeled after what everyone
+        // else is doing. For now this is modeled largely after libuv.
+
         unsafe {
             let mut num_cpus_u = 0;
             let mut cpu_info = ptr::null_mut();
-            let mut cpu_info_cnt = 0;
+            let mut msg_type = 0;
             let err = host_processor_info(
                 mach_host_self(),
                 PROESSOR_CPU_LOAD_INFO,
                 &mut num_cpus_u,
                 &mut cpu_info,
-                &mut cpu_info_cnt,
+                &mut msg_type,
             );
             if err != 0 {
                 return Err(io::Error::last_os_error());
             }
-            let cpu_info_slice = slice::from_raw_parts(cpu_info, cpu_info_cnt as usize);
             let mut ret = State {
                 user: 0,
                 system: 0,
                 idle: 0,
                 nice: 0,
             };
-            for chunk in cpu_info_slice.chunks(num_cpus_u as usize) {
-                ret.user += chunk[CPU_STATE_USER] as u64;
-                ret.system += chunk[CPU_STATE_SYSTEM] as u64;
-                ret.idle += chunk[CPU_STATE_IDLE] as u64;
-                ret.nice += chunk[CPU_STATE_NICE] as u64;
+            let mut current = cpu_info as *const processor_cpu_load_info_data_t;
+            for _ in 0..num_cpus_u {
+                ret.user += (*current).cpu_ticks[CPU_STATE_USER] as u64;
+                ret.system += (*current).cpu_ticks[CPU_STATE_SYSTEM] as u64;
+                ret.idle += (*current).cpu_ticks[CPU_STATE_IDLE] as u64;
+                ret.nice += (*current).cpu_ticks[CPU_STATE_NICE] as u64;
+                current = current.offset(1);
             }
-            vm_deallocate(
-                mach_task_self_,
-                cpu_info as vm_address_t,
-                cpu_info_cnt as usize,
-            );
+            vm_deallocate(mach_task_self_, cpu_info as vm_address_t, msg_type as usize);
             Ok(ret)
         }
     }