Add derive macro for core::ops::Neg

src/lib.rs

@@ -696,6 +696,26 @@ pub fn num(input: TokenStream) -> TokenStream {
     import.wrap(impl_).into()
 }
 
+/// Derives [`core::ops::Neg`][neg] for newtypes.  The inner type must already implement `Neg`.
+///
+/// [neg]: https://doc.rust-lang.org/stable/core/ops/trait.Neg.html
+#[proc_macro_derive(Neg, attributes(num_traits))]
+pub fn neg(input: TokenStream) -> TokenStream {
+    let ast = parse!(input as syn::DeriveInput);
+    let name = &ast.ident;
+    let inner_ty = newtype_inner(&ast.data).expect(NEWTYPE_ONLY);
+    let impl_ = quote! {
+        impl ::core::ops::Neg for #name {
+            type Output = Self;
+            #[inline]
+            fn neg(self) -> Self {
+                #name(<#inner_ty as ::core::ops::Neg>::neg(self.0))
+            }
+        }
+    };
+    impl_.into()
+}
+
 /// Derives [`num_traits::Float`][float] for newtypes.  The inner type must already implement
 /// `Float`.
 ///

tests/newtype.rs

@@ -31,18 +31,12 @@ fn test_derive_unsigned_works() {
     One,
     Zero,
     Num,
+    Neg,
     Float,
     Signed,
 )]
 struct MyFloat(f64);
 
-impl Neg for MyFloat {
-    type Output = MyFloat;
-    fn neg(self) -> Self {
-        MyFloat(self.0.neg())
-    }
-}
-
 #[test]
 fn test_from_primitive() {
     assert_eq!(MyFloat::from_u32(25), Some(MyFloat(25.0)));
@@ -97,6 +91,11 @@ fn test_num() {
     assert_eq!(MyFloat::from_str_radix("25", 10).ok(), Some(MyFloat(25.0)));
 }
 
+#[test]
+fn test_neg() {
+    assert_eq!(MyFloat(4.0).neg(), MyFloat(-4.0));
+}
+
 #[test]
 fn test_float() {
     assert_eq!(MyFloat(4.0).log(MyFloat(2.0)), MyFloat(2.0));