chore(pass-style): make CopyArray compatible with ReadonlyArray

packages/pass-style/NEWS.md

@@ -2,11 +2,16 @@ User-visible changes in `@endo/pass-style`:
 
 # Next release
 
-- deprecates `assertChecker`. Use `Fail` in the confirm/reject pattern instead, as supported by `@endo/errors/rejector.js`.
+- Deprecates `assertChecker`. Use `Fail` in the confirm/reject pattern instead,
+  as supported by `@endo/errors/rejector.js`.
 - Enables `passStyleOf` to make errors passable as a side-effect when SES locks
   down with `hardenTaming` set to `unsafe`, which impacts errors on V8 starting
   with Node.js 21, and similar engines, that own a `stack` getter and setter
   that would otherwise be repaired as a side-effect of `harden`.
+- Updates `CopyArray<T>` to be a `ReadonlyArray<Passable>`.  We know dynamically
+  that the CopyArray is hardened, so this typing statically reveals where
+  mutation is attempted.  Use `Passable[]` if you want to have a mutable array
+  that contains only passable values.
 
 # 1.6.3 (2025-07-11)
 

packages/pass-style/src/types.d.ts

@@ -116,11 +116,11 @@ export type Container<PC extends PassableCap, E extends Error> =
   | CopyArrayInterface<PC, E>
   | CopyRecordInterface<PC, E>
   | CopyTaggedInterface<PC, E>;
-interface CopyArrayInterface<PC extends PassableCap, E extends Error>
+export interface CopyArrayInterface<PC extends PassableCap, E extends Error>
   extends CopyArray<Passable<PC, E>> {}
-interface CopyRecordInterface<PC extends PassableCap, E extends Error>
+export interface CopyRecordInterface<PC extends PassableCap, E extends Error>
   extends CopyRecord<Passable<PC, E>> {}
-interface CopyTaggedInterface<PC extends PassableCap, E extends Error>
+export interface CopyTaggedInterface<PC extends PassableCap, E extends Error>
   extends CopyTagged<string, Passable<PC, E>> {}
 
 export type PassStyleOf = {
@@ -134,7 +134,7 @@ export type PassStyleOf = {
   (p: Promise<any>): 'promise';
   (p: Error): 'error';
   (p: CopyTagged): 'tagged';
-  (p: any[]): 'copyArray';
+  (p: readonly any[]): 'copyArray';
   (p: Iterable<any>): 'remotable';
   (p: Iterator<any, any, undefined>): 'remotable';
   <T extends PassStyled<PassStyleMarker, any>>(p: T): ExtractStyle<T>;
@@ -199,10 +199,15 @@ export type PassableCap =
   | RemotableObject
   | RemotableBrand<any, any>;
 
+/**
+ * Types you would get from Awaited<PassableCap>
+ */
+export type AwaitedPassableCap = RemotableObject | RemotableBrand<any, any>;
+
 /**
  * A Passable sequence of Passable values.
  */
-export type CopyArray<T extends Passable = any> = Array<T>;
+export type CopyArray<T extends Passable = any> = ReadonlyArray<T>;
 
 /**
  * A hardened immutable ArrayBuffer.

packages/pass-style/src/types.test-d.ts

@@ -33,6 +33,28 @@ expectType<PassStyle>(passStyleOf(someUnknown));
 
 const expectPassable = (val: Passable) => {};
 
+() => {
+  const arr = ['hello'] as string[];
+  expectPassable(arr);
+  arr.push('world');
+  const slice = arr.slice();
+  expectPassable(slice);
+  arr.shift();
+  slice.shift();
+};
+
+() => {
+  const roArr = ['hello'] as Readonly<string[]>;
+  expectPassable(roArr);
+  // @ts-expect-error immutable
+  roArr.push('world');
+  const roSlice = roArr.slice();
+  expectPassable(roSlice);
+  // @ts-expect-error immutable
+  roArr.shift();
+  roSlice.shift();
+};
+
 const fn = () => {};
 
 expectPassable(1);
@@ -44,6 +66,12 @@ expectPassable(fn());
 
 expectPassable({});
 expectPassable({ a: {} });
+expectPassable({ a: { b: {} } });
+expectPassable(['car', 'cdr']);
+expectPassable(['car', 'cdr'] as string[]);
+expectPassable([['a'], ['b']] as const);
+expectPassable(['car', 'cdr'] as Readonly<string[]>);
+expectPassable(['car', 'cdr'] as Readonly<[string, string]>);
 // @ts-expect-error not passable
 expectPassable(fn);
 // FIXME promise for a non-Passable is not Passable