wit/bindgen: use custom GC shape for tuples

Monotypic tuples may not be dense, as they can include a bool, or a struct with a bool.

Author: ydnar

testdata/issues/issue352.wit

@@ -0,0 +1,9 @@
+package issues:issue352;
+
+world w {
+	import i;
+}
+
+interface i {
+	type a = result<tuple<bool, bool, bool>, s16>;
+}

testdata/issues/issue352.wit.json

@@ -0,0 +1,64 @@
+{
+  "worlds": [
+    {
+      "name": "w",
+      "imports": {
+        "interface-0": {
+          "interface": {
+            "id": 0
+          }
+        }
+      },
+      "exports": {},
+      "package": 0
+    }
+  ],
+  "interfaces": [
+    {
+      "name": "i",
+      "types": {
+        "a": 1
+      },
+      "functions": {},
+      "package": 0
+    }
+  ],
+  "types": [
+    {
+      "name": null,
+      "kind": {
+        "tuple": {
+          "types": [
+            "bool",
+            "bool",
+            "bool"
+          ]
+        }
+      },
+      "owner": null
+    },
+    {
+      "name": "a",
+      "kind": {
+        "result": {
+          "ok": 0,
+          "err": "s16"
+        }
+      },
+      "owner": {
+        "interface": 0
+      }
+    }
+  ],
+  "packages": [
+    {
+      "name": "issues:issue352",
+      "interfaces": {
+        "i": 0
+      },
+      "worlds": {
+        "w": 0
+      }
+    }
+  ]
+}

testdata/issues/issue352.wit.json.golden.wit

@@ -0,0 +1,9 @@
+package issues:issue352;
+
+interface i {
+	type a = result<tuple<bool, bool, bool>, s16>;
+}
+
+world w {
+	import i;
+}

wit/bindgen/generator.go

@@ -1032,20 +1032,15 @@ func (g *generator) typeDefShape(file *gen.File, dir wit.Direction, t *wit.TypeD
 	case wit.Type:
 		return g.typeShape(file, dir, kind)
 	case *wit.Variant:
-		if kind.Enum() != nil {
-			// Variants that can be represented as an enum do not need a custom shape.
+		if len(kind.Types()) == 0 {
+			// Variants without associated types do not need a custom shape.
 			return g.typeRep(file, dir, t)
 		}
 	case *wit.Result:
 		if len(kind.Types()) == 0 {
 			// Results without associated types do not need a custom shape.
 			return g.typeRep(file, dir, t)
 		}
-	case *wit.Tuple:
-		if kind.Type() != nil {
-			// Monotypic tuples have a packed memory layout.
-			return g.typeRep(file, dir, t)
-		}
 	case *wit.Enum, *wit.Flags, *wit.List,
 		*wit.Resource, *wit.Own, *wit.Borrow, *wit.Stream, *wit.Future:
 		// Certain types do not need custom type shapes: