avoid underflow during computation of uint::MAX.

Author: pnkfelix

src/libcore/num/uint_macros.rs

@@ -20,6 +20,6 @@ pub const BYTES : uint = ($bits / 8);
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const MIN: $T = 0 as $T;
 #[stable(feature = "rust1", since = "1.0.0")]
-pub const MAX: $T = 0 as $T - 1 as $T;
+pub const MAX: $T = !0 as $T;
 
 ) }