auto merge of #8404 : stepancheg/rust/zero-unit-inline, r=alexcrichton

Follow-up to #8155

src/libstd/nil.rs

@@ -49,7 +49,9 @@ fn equals(&self, _other: &()) -> bool { true }
 
 #[cfg(not(test))]
 impl Zero for () {
+    #[inline]
     fn zero() -> () { () }
+    #[inline]
     fn is_zero(&self) -> bool { true }
 }