Add test for flattening in FromField

tests/flatten_from_field.rs

@@ -0,0 +1,46 @@
+use darling::{ast, util::Ignored, FromDeriveInput, FromField, FromMeta};
+use proc_macro2::{Ident, Span};
+use syn::parse_quote;
+
+#[derive(FromMeta)]
+struct Vis {
+    public: bool,
+    #[darling(default)]
+    private: bool,
+}
+
+#[derive(FromField)]
+#[darling(attributes(v))]
+struct Field {
+    ident: Option<Ident>,
+    example: Option<String>,
+    #[darling(flatten)]
+    visibility: Vis,
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(v))]
+struct Input {
+    data: ast::Data<Ignored, Field>,
+}
+
+#[test]
+fn field_flattens() {
+    let di = Input::from_derive_input(&parse_quote! {
+        struct Demo {
+            #[v(public, example = "world")]
+            hello: String
+        }
+    })
+    .unwrap();
+
+    let fields = di.data.take_struct().unwrap();
+    let first_field = fields.into_iter().next().unwrap();
+    assert_eq!(
+        first_field.ident,
+        Some(Ident::new("hello", Span::call_site()))
+    );
+    assert!(first_field.visibility.public);
+    assert!(!first_field.visibility.private);
+    assert_eq!(first_field.example.unwrap(), "world");
+}