Emit Error when parsing FromMeta containing NestedMeta::Lit (#193)

darling silently ignored literals in NestedMeta, which could indirectly cause confusing error messages if the user provided something that wasn't of the intended format.

This update makes that error more explicit, and includes tests to showcase the behavior.

Fixes #192

core/src/codegen/variant_data.rs

@@ -58,11 +58,17 @@ impl<'a> FieldsGen<'a> {
 
         quote!(
             for __item in __items {
-                if let ::syn::NestedMeta::Meta(ref __inner) = *__item {
-                    let __name = ::darling::util::path_to_string(__inner.path());
-                    match __name.as_str() {
-                        #(#arms)*
-                        __other => { #handle_unknown }
+                match *__item {
+                    ::syn::NestedMeta::Meta(ref __inner) => {
+                        let __name = ::darling::util::path_to_string(__inner.path());
+                        match __name.as_str() {
+                            #(#arms)*
+                            __other => { #handle_unknown }
+                        }
+                    }
+                    ::syn::NestedMeta::Lit(ref __inner) => {
+                        __errors.push(::darling::Error::unsupported_format("literal")
+                            .with_span(__inner));
                     }
                 }
             }

tests/from_meta.rs

@@ -0,0 +1,66 @@
+use darling::{Error, FromMeta};
+use syn::parse_quote;
+
+#[derive(Debug, FromMeta)]
+struct Meta {
+    #[darling(default)]
+    meta1: Option<String>,
+    #[darling(default)]
+    meta2: bool,
+}
+
+#[test]
+fn nested_meta_meta_value() {
+    let meta = Meta::from_list(&vec![parse_quote! {
+        meta1 = "thefeature"
+    }])
+    .unwrap();
+    assert_eq!(meta.meta1, Some("thefeature".to_string()));
+    assert_eq!(meta.meta2, false);
+}
+
+#[test]
+fn nested_meta_meta_bool() {
+    let meta = Meta::from_list(&vec![parse_quote! {
+        meta2
+    }])
+    .unwrap();
+    assert_eq!(meta.meta1, None);
+    assert_eq!(meta.meta2, true);
+}
+
+#[test]
+fn nested_meta_lit_string_errors() {
+    let err = Meta::from_list(&vec![parse_quote! {
+        "meta2"
+    }])
+    .unwrap_err();
+    assert_eq!(
+        err.to_string(),
+        Error::unsupported_format("literal").to_string()
+    );
+}
+
+#[test]
+fn nested_meta_lit_integer_errors() {
+    let err = Meta::from_list(&vec![parse_quote! {
+        2
+    }])
+    .unwrap_err();
+    assert_eq!(
+        err.to_string(),
+        Error::unsupported_format("literal").to_string()
+    );
+}
+
+#[test]
+fn nested_meta_lit_bool_errors() {
+    let err = Meta::from_list(&vec![parse_quote! {
+        true
+    }])
+    .unwrap_err();
+    assert_eq!(
+        err.to_string(),
+        Error::unsupported_format("literal").to_string()
+    );
+}