std: add Primitive.is_signed

src/libstd/num/f32.rs

@@ -590,6 +590,9 @@ fn bits(_: Option<f32>) -> uint { 32 }
 
     #[inline]
     fn bytes(_: Option<f32>) -> uint { Primitive::bits(Some(0f32)) / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<f32>) -> bool { true }
 }
 
 impl Float for f32 {

src/libstd/num/f64.rs

@@ -638,6 +638,9 @@ fn bits(_: Option<f64>) -> uint { 64 }
 
     #[inline]
     fn bytes(_: Option<f64>) -> uint { Primitive::bits(Some(0f64)) / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<f64>) -> bool { true }
 }
 
 impl Float for f64 {

src/libstd/num/int_macros.rs

@@ -380,6 +380,9 @@ fn bits(_: Option<$T>) -> uint { bits }
 
     #[inline]
     fn bytes(_: Option<$T>) -> uint { bits / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<$T>) -> bool { true }
 }
 
 // String conversion functions and impl str -> num

src/libstd/num/num.rs

@@ -286,6 +286,7 @@ pub trait Primitive: Clone
     // FIXME (#8888): Removing `unused_self` requires #8888 to be fixed.
     fn bits(unused_self: Option<Self>) -> uint;
     fn bytes(unused_self: Option<Self>) -> uint;
+    fn is_signed(unused_self: Option<Self>) -> bool;
 }
 
 /// A collection of traits relevant to primitive signed and unsigned integers

src/libstd/num/uint_macros.rs

@@ -306,6 +306,9 @@ fn bits(_: Option<$T>) -> uint { bits }
 
     #[inline]
     fn bytes(_: Option<$T>) -> uint { bits / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<$T>) -> bool { false }
 }
 
 impl BitCount for $T {