Attribute slot causes to the owner only for types without their own slot

packages/compiler/src/frontend/lowering/dynamic-tier.ts

@@ -223,8 +223,10 @@ function slotCause(lowerer: Lowerer, n: ts.Node): UntypedCause | null {
       ? lowerer.checker.getTypeOfSymbol(symbol)
       : lowerer.checker.getTypeAtLocation(n);
   const id = (type as { id?: number }).id ?? -1;
-  // A whole degraded type, or a dynamic piece of a pushed-down one.
-  const owner = lowerer.lenientPieces.get(id);
+  // A whole degraded type, or a dynamic piece of a pushed-down one. A type
+  // that is also a slot of its own belongs to that slot, not to a container
+  // it is a piece of.
+  const owner = lowerer.degradedTypes.has(id) ? undefined : lowerer.lenientPieces.get(id);
   if (!lowerer.degradedTypes.has(id) && owner === undefined) return null;
   const text = ts.getTextOfNode(n);
   const declaredType = lowerer.checker.typeToString(type);

tests/coverage-fixtures/dynamic-slot-shared.ts

@@ -0,0 +1,23 @@
+// A type that degrades as a piece of a container (`Plugin.hook`) and as a
+// slot of its own (`current`): the operations on `current` belong to
+// `current`, not to the container.
+type Hook = (() => number) | AbortSignal | Map<string, (x: number) => string>;
+
+interface Plugin {
+  name: string;
+  hook: Hook;
+}
+
+const plugins: Plugin[] = [{ name: "one", hook: () => 1 }];
+
+let current: Hook = () => 2;
+
+function run(rounds: number): number {
+  let total = 0;
+  for (let i = 0; i < rounds; i++) {
+    if (typeof current === "function") total += current();
+  }
+  return total;
+}
+
+console.log(run(10), plugins.length);

tests/harness/coverage.test.ts

@@ -496,6 +496,15 @@ test("dynamic slots rank by their dynamic operations and name themselves in SC61
   expect(hot?.message).toContain("`hook` is a dynamic piece of `Plugin[]` (declared `Hook`");
 });
 
+test("a type that is a dynamic piece and a slot of its own counts toward its own slot", () => {
+  const { coverage } = analyze(fixture("dynamic-slot-shared.ts"));
+  const slots = coverage.dynamicSlots ?? [];
+  expect(slots.find((s) => s.name === "current")?.operations).toBeGreaterThan(0);
+  expect(slots.find((s) => s.name === "plugins")?.operations ?? 0).toBe(0);
+  const hot = coverage.performance?.find((d) => d.code === "SC6101");
+  expect(hot?.message).toContain("`current` is a dynamic slot (declared `Hook`");
+});
+
 test("CLI: --tier and --explain-dynamic", () => {
   const scriptcCli = join(repoRoot, "packages/cli/src/main.ts");
   const outDir = mkdtempSync(join(tmpdir(), "scriptc-tier-"));