diff --git a/src/libstd/num/f32.rs b/src/libstd/num/f32.rs
index 41a3d1933795..2b4a636e1ad9 100644
--- a/src/libstd/num/f32.rs
+++ b/src/libstd/num/f32.rs
@@ -590,6 +590,9 @@ impl Primitive for f32 {
 
     #[inline]
     fn bytes(_: Option<f32>) -> uint { Primitive::bits(Some(0f32)) / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<f32>) -> bool { true }
 }
 
 impl Float for f32 {
diff --git a/src/libstd/num/f64.rs b/src/libstd/num/f64.rs
index 772596d15fba..d4442e5b34fa 100644
--- a/src/libstd/num/f64.rs
+++ b/src/libstd/num/f64.rs
@@ -638,6 +638,9 @@ impl Primitive for f64 {
 
     #[inline]
     fn bytes(_: Option<f64>) -> uint { Primitive::bits(Some(0f64)) / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<f64>) -> bool { true }
 }
 
 impl Float for f64 {
diff --git a/src/libstd/num/int_macros.rs b/src/libstd/num/int_macros.rs
index 1070e8e592f0..7fae567809b0 100644
--- a/src/libstd/num/int_macros.rs
+++ b/src/libstd/num/int_macros.rs
@@ -380,6 +380,9 @@ impl Primitive for $T {
 
     #[inline]
     fn bytes(_: Option<$T>) -> uint { bits / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<$T>) -> bool { true }
 }
 
 // String conversion functions and impl str -> num
diff --git a/src/libstd/num/num.rs b/src/libstd/num/num.rs
index bccb20de4585..fde1928f4a32 100644
--- a/src/libstd/num/num.rs
+++ b/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
diff --git a/src/libstd/num/uint_macros.rs b/src/libstd/num/uint_macros.rs
index 45280482b87b..f52feced67c7 100644
--- a/src/libstd/num/uint_macros.rs
+++ b/src/libstd/num/uint_macros.rs
@@ -306,6 +306,9 @@ impl Primitive for $T {
 
     #[inline]
     fn bytes(_: Option<$T>) -> uint { bits / 8 }
+
+    #[inline]
+    fn is_signed(_: Option<$T>) -> bool { false }
 }
 
 impl BitCount for $T {