Add deriving toEncoder for opaques

crates/compiler/can/src/def.rs

@@ -639,6 +639,45 @@ fn separate_implemented_and_required_members(
     }
 }
 
+fn synthesize_derived_to_encoder<'a>(
+    env: &mut Env<'a>,
+    at_opaque: &'a str,
+    region: Region,
+) -> ast::Expr<'a> {
+    let alloc_pat = |it| env.arena.alloc(Loc::at(region, it));
+    let alloc_expr = |it| env.arena.alloc(Loc::at(region, it));
+
+    let payload = env.arena.alloc_str("#payload");
+
+    // \@Opaq payload
+    let opaque_ref = alloc_pat(ast::Pattern::OpaqueRef(at_opaque));
+    let opaque_apply_pattern = ast::Pattern::Apply(
+        opaque_ref,
+        &*env
+            .arena
+            .alloc([Loc::at(region, ast::Pattern::Identifier(payload))]),
+    );
+
+    // Encode.toEncoder payload
+    let call_member = alloc_expr(ast::Expr::Apply(
+        alloc_expr(ast::Expr::Var {
+            module_name: "Encode",
+            ident: "toEncoder",
+        }),
+        &*env.arena.alloc([&*alloc_expr(ast::Expr::Var {
+            module_name: "",
+            ident: payload,
+        })]),
+        roc_module::called_via::CalledVia::Space,
+    ));
+
+    // \@Opaq payload -> Encode.toEncoder payload
+    ast::Expr::Closure(
+        env.arena.alloc([Loc::at(region, opaque_apply_pattern)]),
+        call_member,
+    )
+}
+
 fn synthesize_derived_hash<'a>(
     env: &mut Env<'a>,
     at_opaque: &'a str,
@@ -756,14 +795,9 @@ fn synthesize_derived_member_impl<'a>(
     let (impl_name, def_body): (String, ast::Expr<'a>) = match ability_member {
         Symbol::ENCODE_TO_ENCODER => (
             format!("#{}_toEncoder", opaque_name),
-            todo!(),
+            synthesize_derived_to_encoder(env, at_opaque, region),
-            //synthesize_derived_to_encoder(env, scope, at_opaque, region),
-        ),
-        Symbol::DECODE_DECODER => (
-            format!("#{}_decoder", opaque_name),
-            todo!(),
-            //synthesize_derived_decoder(env, scope, at_opaque, region),
         ),
+        Symbol::DECODE_DECODER => (format!("#{}_decoder", opaque_name), todo!()),
         Symbol::HASH_HASH => (
             format!("#{}_hash", opaque_name),
             synthesize_derived_hash(env, at_opaque, region),

crates/compiler/solve/tests/solve_expr.rs

@@ -8045,6 +8045,23 @@ mod solve_expr {
         );
     }
 
+    #[test]
+    fn derive_to_encoder_for_opaque() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test" provides [main] to "./platform"
+
+                N := U8 has [Encoding]
+
+                main = Encode.toEncoder (@N 15)
+                #      ^^^^^^^^^^^^^^^^
+                "#
+            ),
+            @"N#Encode.toEncoder(3) : N -[[#N_toEncoder(3)]]-> Encoder fmt | fmt has EncoderFormatting"
+        );
+    }
+
     #[test]
     fn derive_hash_for_opaque() {
         infer_queries!(

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() {