fix(reducers): support requiring array receiver evidence

README.md

@@ -109,7 +109,7 @@ export default defineConfig({
 ### Generic rules
 
 - `no-array-filter-map` — rejects adjacent eager array filter/map passes while allowing lazy iterator pipelines.
-- `no-reduce-accumulator-copy` — rejects non-spread accumulator copies inside reducers; complements native `oxc/no-accumulating-spread`.
+- `no-reduce-accumulator-copy` — rejects non-spread accumulator copies inside reducers; complements native `oxc/no-accumulating-spread`. By default, this uses the `reduce`/`reduceRight` method name as a heuristic, including unknown and custom receivers. Configure `["error", { "requireArrayReceiver": true }]` to report only receivers with local array evidence (literals, array annotations, or recognized array chains), avoiding warnings on custom one-call APIs.
 - `no-chained-type-assertions` — rejects nested `as` and angle-bracket assertions that fabricate evidence; chains made only of `as const` remain valid.
 - `no-conditional-empty-object-spread` — reports object spreads that use a conditional `{}` branch to omit fields. It intentionally has no autofix because omission is not equivalent to assigning `undefined`.
 - `no-known-value-widening` — rejects known expressions flowing into explicit `unknown`, `object`, anonymous-object, or open-dictionary targets, including known arguments passed to local `unknown` type predicates. Empty dictionary accumulators and finite-key `Record` targets remain valid.

skills/install-anti-slop/assets/anti-slop/rules/no-reduce-accumulator-copy.ts

@@ -27,7 +27,7 @@ function enclosingReducer(node: ESTree.Node) {
       const firstParameter = callback.params[0];
       const accumulator = firstParameter?.type === "AssignmentPattern" ? firstParameter.left : firstParameter;
       if (accumulator?.type !== "Identifier") return null;
-      return { callback, accumulator, initialValue: owner.arguments[1] };
+      return { callback, accumulator, receiver: method.object, initialValue: owner.arguments[1] };
     }
     parent = parent.parent;
   }
@@ -72,6 +72,12 @@ export const noReduceAccumulatorCopyRule = defineRule({
     messages: {
       accumulatorCopy: "Do not copy the reducer accumulator on every iteration; growing copies can cause quadratic work. Mutate a fresh, locally owned accumulator and return it, or use an iterator pipeline/flatMap.",
     },
+    schema: [{
+      type: "object",
+      properties: { requireArrayReceiver: { type: "boolean" } },
+      additionalProperties: false,
+    }],
+    defaultOptions: [{ requireArrayReceiver: false }],
   },
   createOnce(context) {
     return {
@@ -80,6 +86,12 @@ export const noReduceAccumulatorCopyRule = defineRule({
         if (method === null) return;
         const reducer = enclosingReducer(node);
         if (reducer === null) return;
+        const option = context.options?.[0];
+        if (
+          typeof option === "object" && option !== null && !Array.isArray(option) &&
+          option.requireArrayReceiver === true &&
+          !isKnownArrayExpression(context.sourceCode, reducer.receiver)
+        ) return;
         const accumulator = context.sourceCode.getDeclaredVariables(reducer.callback).find(variable =>
           variable.identifiers.some(identifier => identifier.start === reducer.accumulator.start),
         );

src/rules/no-reduce-accumulator-copy.test.ts

@@ -26,6 +26,22 @@ tester.run("anti-slop/no-reduce-accumulator-copy", noReduceAccumulatorCopyRule,
     "items.reduce((acc, item) => { let alias = acc; alias = item; return Object.assign({}, alias); }, {});",
     "items.reduce((acc, item) => [...acc, item], []);", // Owned by the native rule.
     "items.reduce((acc, item) => ({ ...acc, [item.id]: item }), {});",
+    {
+      code: "const once = { reduce(fn, initial) { return fn(initial, { id: 1 }); } }; once.reduce((acc, item) => Object.assign({}, acc, item), {});",
+      options: [{ requireArrayReceiver: true }],
+    },
+    {
+      code: "items.reduce((acc, item) => Object.assign({}, acc, item), {});",
+      options: [{ requireArrayReceiver: true }],
+    },
+    {
+      code: "const custom = { reduceRight(fn, initial) { return fn(initial); } }; custom.reduceRight(acc => Array.from(acc), []);",
+      options: [{ requireArrayReceiver: true }],
+    },
+    {
+      code: "const items = [1, 2]; items.reduce((acc, item) => { acc.push(item); return acc; }, []);",
+      options: [{ requireArrayReceiver: true }],
+    },
   ],
   invalid: [
     { code: "items.reduce((acc, item) => Object.assign({}, acc, { [item.id]: item }), {});", errors: [error] },
@@ -48,5 +64,17 @@ tester.run("anti-slop/no-reduce-accumulator-copy", noReduceAccumulatorCopyRule,
     { code: "items.reduce((acc, item) => acc.toSorted(), []);", errors: [error] },
     { code: "items.reduce((acc, item) => acc.toReversed(), []);", errors: [error] },
     { code: "items.reduce((acc, item) => acc.with(0, item), []);", errors: [error] },
+    {
+      code: "[1, 2].reduce((acc, item) => Object.assign({}, acc, { [item]: item }), {});",
+      options: [{ requireArrayReceiver: true }], errors: [error],
+    },
+    {
+      code: "const source = [1, 2]; const alias = source; alias.reduceRight((acc, item) => Array.from(acc), []);",
+      options: [{ requireArrayReceiver: true }], errors: [error],
+    },
+    {
+      code: "function run(items: readonly string[]) { return items.filter(Boolean).reduce((acc, item) => acc.concat(item), []); }",
+      options: [{ requireArrayReceiver: true }], errors: [error],
+    },
   ],
 });

src/rules/no-reduce-accumulator-copy.ts

@@ -27,7 +27,7 @@ function enclosingReducer(node: ESTree.Node) {
       const firstParameter = callback.params[0];
       const accumulator = firstParameter?.type === "AssignmentPattern" ? firstParameter.left : firstParameter;
       if (accumulator?.type !== "Identifier") return null;
-      return { callback, accumulator, initialValue: owner.arguments[1] };
+      return { callback, accumulator, receiver: method.object, initialValue: owner.arguments[1] };
     }
     parent = parent.parent;
   }
@@ -72,6 +72,12 @@ export const noReduceAccumulatorCopyRule = defineRule({
     messages: {
       accumulatorCopy: "Do not copy the reducer accumulator on every iteration; growing copies can cause quadratic work. Mutate a fresh, locally owned accumulator and return it, or use an iterator pipeline/flatMap.",
     },
+    schema: [{
+      type: "object",
+      properties: { requireArrayReceiver: { type: "boolean" } },
+      additionalProperties: false,
+    }],
+    defaultOptions: [{ requireArrayReceiver: false }],
   },
   createOnce(context) {
     return {
@@ -80,6 +86,12 @@ export const noReduceAccumulatorCopyRule = defineRule({
         if (method === null) return;
         const reducer = enclosingReducer(node);
         if (reducer === null) return;
+        const option = context.options?.[0];
+        if (
+          typeof option === "object" && option !== null && !Array.isArray(option) &&
+          option.requireArrayReceiver === true &&
+          !isKnownArrayExpression(context.sourceCode, reducer.receiver)
+        ) return;
         const accumulator = context.sourceCode.getDeclaredVariables(reducer.callback).find(variable =>
           variable.identifiers.some(identifier => identifier.start === reducer.accumulator.start),
         );