Derive Zero and One for newtypes

Author: asayers

src/lib.rs

@@ -496,3 +496,45 @@ pub fn num_cast(input: TokenStream) -> TokenStream {
         }
     }).into()
 }
+
+/// Derives [`num_traits::Zero`][zero] for newtypes.  The inner type must already implement `Zero`.
+///
+/// [zero]: https://docs.rs/num-traits/0.2/num_traits/cast/trait.Zero.html
+#[proc_macro_derive(Zero)]
+pub fn zero(input: TokenStream) -> TokenStream {
+    let ast: syn::DeriveInput = syn::parse(input).unwrap();
+    let name = &ast.ident;
+    let inner_ty = newtype_inner(&ast.data).expect(NEWTYPE_ONLY);
+    dummy_const_trick("Zero", &name, quote! {
+        extern crate num_traits as _num_traits;
+        impl _num_traits::Zero for #name {
+            fn zero() -> Self {
+                #name(<#inner_ty as _num_traits::Zero>::zero())
+            }
+            fn is_zero(&self) -> bool {
+                <#inner_ty as _num_traits::Zero>::is_zero(&self.0)
+            }
+        }
+    }).into()
+}
+
+/// Derives [`num_traits::One`][one] for newtypes.  The inner type must already implement `One`.
+///
+/// [one]: https://docs.rs/num-traits/0.2/num_traits/cast/trait.One.html
+#[proc_macro_derive(One)]
+pub fn one(input: TokenStream) -> TokenStream {
+    let ast: syn::DeriveInput = syn::parse(input).unwrap();
+    let name = &ast.ident;
+    let inner_ty = newtype_inner(&ast.data).expect(NEWTYPE_ONLY);
+    dummy_const_trick("One", &name, quote! {
+        extern crate num_traits as _num_traits;
+        impl _num_traits::One for #name {
+            fn one() -> Self {
+                #name(<#inner_ty as _num_traits::One>::one())
+            }
+            fn is_one(&self) -> bool {
+                <#inner_ty as _num_traits::One>::is_one(&self.0)
+            }
+        }
+    }).into()
+}

tests/newtype.rs

@@ -2,7 +2,7 @@ extern crate num as num_renamed;
 #[macro_use]
 extern crate num_derive;
 
-use num_renamed::{FromPrimitive, ToPrimitive, NumCast};
+use num_renamed::{FromPrimitive, ToPrimitive, NumCast, One, Zero};
 
 #[derive(
     Debug,
@@ -14,6 +14,8 @@ use num_renamed::{FromPrimitive, ToPrimitive, NumCast};
     FromPrimitive,
     NumOps,
     NumCast,
+    One,
+    Zero,
 )]
 struct MyFloat(f64);
 
@@ -40,3 +42,13 @@ fn test_num_ops() {
 fn test_num_cast() {
     assert_eq!(<MyFloat as NumCast>::from(25u8), Some(MyFloat(25.0)));
 }
+
+#[test]
+fn test_zero() {
+    assert_eq!(MyFloat::zero(), MyFloat(0.0));
+}
+
+#[test]
+fn test_one() {
+    assert_eq!(MyFloat::one(), MyFloat(1.0));
+}