Automatically #[derive] the AddressSpace default implementation

As @jfrimmel points out, the standard integer types implement `Default`.

Author: dylanmckay

src/librustc_target/abi/mod.rs

@@ -752,15 +752,9 @@ impl FieldsShape {
 /// An identifier that specifies the address space that some operation
 /// should operate on. Special address spaces have an effect on code generation,
 /// depending on the target and the address spaces it implements.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Default)]
 pub struct AddressSpace(pub u32);
 
-impl Default for AddressSpace {
-    fn default() -> Self {
-        AddressSpace(0)
-    }
-}
-
 /// Describes how values of the type are passed by target ABIs,
 /// in terms of categories of C types there are ABI rules for.
 #[derive(Clone, PartialEq, Eq, Hash, Debug, HashStable_Generic)]