Don't check variant exhaustiveness if a fallback variant is provided.

cynic-codegen/src/enum_derive/mod.rs

@@ -191,9 +191,11 @@ fn join_variants<'a>(
     rename_all: RenameAll,
     enum_span: &Span,
 ) -> Result<Vec<(&'a EnumDeriveVariant, &'a EnumValue<'a>)>, TokenStream> {
+    let mut has_fallback = false;
     let mut map = BTreeMap::new();
     for variant in variants {
         if *variant.fallback {
+            has_fallback = true;
             // We can't join up a fallback as it has no corresponding GQL value.
             // We handle them separately.
             continue;
@@ -237,7 +239,7 @@ fn join_variants<'a>(
             _ => (),
         }
     }
-    if !missing_variants.is_empty() {
+    if !has_fallback && !missing_variants.is_empty() {
         let missing_variants_string = missing_variants.join(", ");
         errors.extend(
             syn::Error::new(
@@ -253,7 +255,7 @@ fn join_variants<'a>(
 
     Ok(map
         .into_iter()
-        .map(|(_, (a, b))| (a.unwrap(), b.unwrap()))
+        .filter_map(|(_, (a, b))| Some((a?, b.unwrap())))
         .collect())
 }
 

cynic/tests/enum-fallbacks.rs

@@ -20,6 +20,14 @@ enum PostStateStringFallback {
     Unknown(String),
 }
 
+#[derive(Enum, Debug, PartialEq)]
+#[cynic(graphql_type = "PostState", schema_path = "tests/test-schema.graphql")]
+enum PostStateNonexhaustiveFallback {
+    Posted,
+    #[cynic(fallback)]
+    Unknown(String),
+}
+
 #[allow(non_snake_case, non_camel_case_types)]
 mod schema {
     cynic::use_schema!("tests/test-schema.graphql");
@@ -46,3 +54,16 @@ fn test_string_fallback() {
 
     assert_eq!(val, serde_json::Value::String("BLAH".into()));
 }
+
+#[test]
+fn test_nonexhaustive_fallback() {
+    assert_eq!(
+        PostStateNonexhaustiveFallback::deserialize(json!("DRAFT")).unwrap(),
+        PostStateNonexhaustiveFallback::Unknown("DRAFT".to_string())
+    );
+
+    let val =
+        serde_json::to_value(PostStateNonexhaustiveFallback::Unknown("DRAFT".to_string())).unwrap();
+
+    assert_eq!(val, serde_json::Value::String("DRAFT".into()));
+}