Derive NumCast for newtypes

Author: asayers

src/lib.rs

@@ -496,3 +496,22 @@ pub fn num_ops(input: TokenStream) -> TokenStream {
         }
     }).into()
 }
+
+/// Derives [`num_traits::NumCast`][num_cast] for newtypes.  The inner type must already implement
+/// `NumCast`.
+///
+/// [num_cast]: https://docs.rs/num-traits/0.2/num_traits/cast/trait.NumCast.html
+#[proc_macro_derive(NumCast)]
+pub fn num_cast(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("NumCast", &name, quote! {
+        extern crate num_traits as _num_traits;
+        impl _num_traits::NumCast for #name {
+            fn from<T: _num_traits::ToPrimitive>(n: T) -> Option<Self> {
+                <#inner_ty as _num_traits::NumCast>::from(n).map(#name)
+            }
+        }
+    }).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};
+use num_renamed::{FromPrimitive, ToPrimitive, NumCast};
 
 #[derive(
     Debug,
@@ -13,6 +13,7 @@ use num_renamed::{FromPrimitive, ToPrimitive};
     ToPrimitive,
     FromPrimitive,
     NumOps,
+    NumCast,
 )]
 struct MyFloat(f64);
 
@@ -34,3 +35,8 @@ fn test_num_ops() {
     assert_eq!(MyFloat(25.0) / MyFloat(10.0), MyFloat(2.5));
     assert_eq!(MyFloat(25.0) % MyFloat(10.0), MyFloat(5.0));
 }
+
+#[test]
+fn test_num_cast() {
+    assert_eq!(<MyFloat as NumCast>::from(25u8), Some(MyFloat(25.0)));
+}