add ConstDefault

num_enum/src/lib.rs

@@ -8,8 +8,8 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
 pub use ::num_enum_derive::{
-    ConstFromPrimitive, ConstIntoPrimitive, ConstTryFromPrimitive, Default, FromPrimitive,
-    IntoPrimitive, TryFromPrimitive, UnsafeFromPrimitive,
+    ConstDefault, ConstFromPrimitive, ConstIntoPrimitive, ConstTryFromPrimitive, Default,
+    FromPrimitive, IntoPrimitive, TryFromPrimitive, UnsafeFromPrimitive,
 };
 
 use ::core::fmt;

num_enum/tests/const_default.rs

@@ -0,0 +1,37 @@
+#![allow(non_upper_case_globals)]
+
+// Guard against https://github.com/illicitonion/num_enum/issues/27
+mod alloc {}
+mod core {}
+mod num_enum {}
+mod std {}
+
+#[test]
+fn default() {
+    #[derive(Debug, Eq, PartialEq, ::num_enum::ConstDefault)]
+    #[repr(u8)]
+    enum Enum {
+        #[allow(unused)]
+        Zero = 0,
+        #[num_enum(default)]
+        NonZero = 1,
+    }
+
+    const nz: Enum = Enum::const_default();
+    assert_eq!(Enum::NonZero, nz);
+}
+
+#[test]
+fn default_standard_default_attribute() {
+    #[derive(Debug, Eq, PartialEq, ::num_enum::ConstDefault)]
+    #[repr(u8)]
+    enum Enum {
+        #[allow(unused)]
+        Zero = 0,
+        #[default]
+        NonZero = 1,
+    }
+
+    const nz: Enum = Enum::const_default();
+    assert_eq!(Enum::NonZero, nz);
+}

num_enum_derive/src/lib.rs

@@ -588,3 +588,47 @@ pub fn derive_default(stream: TokenStream) -> TokenStream {
         }
     })
 }
+
+/// Generates a `const_default() -> Self` method to obtain the default enum value in const contexts.
+///
+/// Whichever variant has the `#[default]` or `#[num_enum(default)]` attribute will be returned.
+/// ----------------------------------------------
+///
+/// ```rust
+/// #[derive(Debug, Eq, PartialEq, num_enum::ConstDefault)]
+/// #[repr(u8)]
+/// enum Number {
+///     Zero,
+///     #[default]
+///     One,
+/// }
+///
+/// const one: Number = Number::const_default();
+/// assert_eq!(one, Number::One);
+/// assert_eq!(Number::One, Number::const_default());
+/// ```
+#[proc_macro_derive(ConstDefault, attributes(num_enum, default))]
+pub fn derive_const_default(stream: TokenStream) -> TokenStream {
+    let enum_info = parse_macro_input!(stream as EnumInfo);
+
+    let default_ident = match enum_info.default() {
+        Some(ident) => ident,
+        None => {
+            let span = Span::call_site();
+            let message =
+                "#[derive(num_enum::ConstDefault)] requires enum to be exhaustive, or a variant marked with `#[default]` or `#[num_enum(default)]`";
+            return syn::Error::new(span, message).to_compile_error().into();
+        }
+    };
+
+    let EnumInfo { ref name, .. } = enum_info;
+
+    TokenStream::from(quote! {
+        impl #name {
+            #[inline]
+            pub const fn const_default() -> Self {
+                Self::#default_ident
+            }
+        }
+    })
+}