Use from_none in codegen

This removes the need to use #[darling(default)] on options
in deriving structs.

Fixes #161

core/src/codegen/field.rs

@@ -191,11 +191,19 @@ impl<'a> ToTokens for CheckMissing<'a> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         if !self.0.multiple && self.0.default_expression.is_none() {
             let ident = self.0.ident;
+            let ty = self.0.ty;
             let name_in_attr = &self.0.name_in_attr;
 
             tokens.append_all(quote! {
                 if !#ident.0 {
-                    __errors.push(::darling::Error::missing_field(#name_in_attr));
+                    match <#ty as ::darling::FromMeta>::from_none() {
+                        ::darling::export::Some(__type_fallback) => {
+                            #ident.1 = ::darling::export::Some(__type_fallback);
+                        }
+                        ::darling::export::None => {
+                            __errors.push(::darling::Error::missing_field(#name_in_attr))
+                        }
+                    }
                 }
             })
         }

tests/defaults.rs

@@ -121,3 +121,69 @@ mod stacked_defaults {
         assert_eq!(person.name.last, "Doe");
     }
 }
+
+mod implicit_default {
+    use darling::{util::Flag, FromDeriveInput};
+    use syn::parse_quote;
+
+    // No use of `darling(default)` here at all!
+    // This struct will fill in missing fields using FromMeta::from_none.
+    #[derive(FromDeriveInput)]
+    #[darling(attributes(person))]
+    struct Person {
+        first_name: String,
+        last_name: Option<String>,
+        lefty: Flag,
+    }
+
+    #[test]
+    fn missing_fields_fill() {
+        let person = Person::from_derive_input(&parse_quote! {
+            #[person(first_name = "James")]
+            struct Foo;
+        })
+        .unwrap();
+
+        assert_eq!(person.first_name, "James");
+        assert_eq!(person.last_name, None);
+        assert_eq!(person.lefty, false);
+    }
+}
+
+/// Test that a field-level implicit default using FromMeta::from_none is superseded
+/// by the parent declaring `#[darling(default)]`.
+mod overridden_implicit_default {
+    use darling::{util::Flag, FromDeriveInput};
+    use syn::parse_quote;
+
+    #[derive(FromDeriveInput)]
+    #[darling(default, attributes(person))]
+    struct Person {
+        first_name: String,
+        last_name: Option<String>,
+        lefty: Flag,
+    }
+
+    impl Default for Person {
+        fn default() -> Self {
+            Self {
+                first_name: "Jane".into(),
+                last_name: Some("Doe".into()),
+                lefty: Flag::default(),
+            }
+        }
+    }
+
+    #[test]
+    fn fill_missing() {
+        let person = Person::from_derive_input(&parse_quote!(
+            #[person(last_name = "Archer")]
+            struct Foo;
+        ))
+        .unwrap();
+
+        assert_eq!(person.first_name, "Jane");
+        assert_eq!(person.last_name, Some("Archer".into()));
+        assert_eq!(person.lefty, false);
+    }
+}