Add deriving toEncoder for opaques

crates/compiler/test_gen/src/gen_abilities.rs

@@ -374,6 +374,30 @@ fn encode_use_stdlib_without_wrapping_custom() {
     )
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn encode_derive_to_encoder_for_opaque() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test"
+                imports [Json]
+                provides [main] to "./platform"
+
+            HelloWorld := { a: Str } has [Encoding]
+
+            main =
+                result = Str.fromUtf8 (Encode.toBytes (@HelloWorld { a: "Hello, World!" }) Json.toUtf8)
+                when result is
+                    Ok s -> s
+                    _ -> "<bad>"
+            "#
+        ),
+        RocStr::from(r#"{"a":"Hello, World!"}"#),
+        RocStr
+    )
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn to_encoder_encode_custom_has_capture() {