From 9d32e5fa070bf70c4e332230c51ce827396342ec Mon Sep 17 00:00:00 2001
From: Alex Sayers <alex.sayers@gmail.com>
Date: Sat, 29 Sep 2018 21:28:07 +0900
Subject: [PATCH] Derive Num for newtypes

---
 src/lib.rs       | 19 +++++++++++++++++++
 tests/newtype.rs |  8 +++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index 2c30849..4a510d2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -538,3 +538,22 @@ pub fn one(input: TokenStream) -> TokenStream {
         }
     }).into()
 }
+
+/// Derives [`num_traits::Num`][num] for newtypes.  The inner type must already implement `Num`.
+///
+/// [num]: https://docs.rs/num-traits/0.2/num_traits/trait.Num.html
+#[proc_macro_derive(Num)]
+pub fn num(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("Num", &name, quote! {
+        extern crate num_traits as _num_traits;
+        impl _num_traits::Num for #name {
+            type FromStrRadixErr = <#inner_ty as _num_traits::Num>::FromStrRadixErr;
+            fn from_str_radix(s: &str, radix: u32) -> Result<Self, Self::FromStrRadixErr> {
+                <#inner_ty as _num_traits::Num>::from_str_radix(s, radix).map(#name)
+            }
+        }
+    }).into()
+}
diff --git a/tests/newtype.rs b/tests/newtype.rs
index cbdff4d..7136357 100644
--- a/tests/newtype.rs
+++ b/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, One, Zero};
+use num_renamed::{FromPrimitive, ToPrimitive, NumCast, One, Zero, Num};
 
 #[derive(
     Debug,
@@ -16,6 +16,7 @@ use num_renamed::{FromPrimitive, ToPrimitive, NumCast, One, Zero};
     NumCast,
     One,
     Zero,
+    Num,
 )]
 struct MyFloat(f64);
 
@@ -52,3 +53,8 @@ fn test_zero() {
 fn test_one() {
     assert_eq!(MyFloat::one(), MyFloat(1.0));
 }
+
+#[test]
+fn test_num() {
+    assert_eq!(MyFloat::from_str_radix("25", 10).ok(), Some(MyFloat(25.0)));
+}