infer fields with no setter as readonly

Author: rotu

deno.json

@@ -1,6 +1,6 @@
 {
   "name": "@rotu/structview",
-  "version": "0.11.1",
+  "version": "0.12.0",
   "license": "MIT",
   "tasks": {
     "dev": "deno test --watch",

fields.ts

@@ -5,6 +5,7 @@
 
 import { structBytes, structDataView } from "./core.ts"
 import type {
+  ReadOnlyAccessorDescriptor,
   StructConstructor,
   StructPropertyDescriptor,
   TypedArraySpecies,
@@ -277,7 +278,7 @@ export function bool(fieldOffset: number): StructPropertyDescriptor<boolean> {
  */
 export function fromDataView<T>(
   fieldGetter: (dv: DataView) => T,
-): StructPropertyDescriptor<T> {
+): StructPropertyDescriptor<T> & ReadOnlyAccessorDescriptor<T> {
   return {
     get() {
       const dv = structDataView(this)
@@ -299,7 +300,7 @@ export function substruct<
   ctor: StructConstructor<T>,
   byteOffset?: number,
   bytelength?: number,
-): StructPropertyDescriptor<T> {
+): StructPropertyDescriptor<T> & ReadOnlyAccessorDescriptor<T> {
   return fromDataView(
     function (dv) {
       const offset2 = dv.byteOffset + (byteOffset ?? 0)
@@ -332,7 +333,7 @@ export function typedArray<T>(
     /** TypedArray constructor */
     readonly species: TypedArraySpecies<T>
   },
-): StructPropertyDescriptor<T> {
+): StructPropertyDescriptor<T> & ReadOnlyAccessorDescriptor<T> {
   const { length, species } = kwargs
   return {
     get() {

mod_test.ts

@@ -397,3 +397,30 @@ Deno.test("typedArray", () => {
   const f32s2 = new Float32Array([1 / 3, 1 / 6, 1 / 9])
   assertEquals(new Float32Array(buf.buffer.slice(4, 16)), f32s2)
 })
+
+Deno.test("getter-only properties inferred as readonly", () => {
+  class S extends defineStruct({
+    y: {
+      get() {
+        return 42
+      },
+    },
+    z: typedArray(9, { species: Uint8Array, length: 3 }),
+    s: substruct(Struct, 0),
+  }) {}
+  const obj = new S(new Uint8Array(10))
+
+  // note: this test is about the type assertions
+  assertThrows(() => {
+    // @ts-expect-error assigning to readonly property
+    obj.y = 1
+  })
+  assertThrows(() => {
+    // @ts-expect-error assigning to readonly property
+    obj.z = new Uint8Array()
+  })
+  assertThrows(() => {
+    // @ts-expect-error assigning to readonly property
+    obj.s = {}
+  })
+})

types.ts

@@ -32,20 +32,38 @@ export type TPropertyDescriptor<T> = {
 }
 
 /**
- * Object type that would result from Object.defineProperties(p:Props)
+ * Object type that would result from Object.defineProperties({}, p:Props)
  */
-export type MixinFromProps<Props extends object> = {
-  -readonly [K in keyof Props]: Props[K] extends TPropertyDescriptor<infer V>
-    ? V
-    : unknown
-}
+export type MixinFromProps<Props extends object> =
+  & {
+    +readonly [
+      K
+        in keyof Props as (Props[K] extends ReadOnlyAccessorDescriptor<unknown>
+          ? K
+          : never)
+    ]: Props[K] extends TPropertyDescriptor<infer V> ? V : unknown
+  }
+  & {
+    -readonly [
+      K
+        in keyof Props as (Props[K] extends ReadOnlyAccessorDescriptor<unknown>
+          ? never
+          : K)
+    ]: Props[K] extends TPropertyDescriptor<infer V> ? V : unknown
+  }
 
 /**
  * Type of a property descriptor for a struct
  */
 export type StructPropertyDescriptor<T> =
   & ThisType<AnyStruct>
   & TPropertyDescriptor<T>
+
+export type ReadOnlyAccessorDescriptor<T> = {
+  get(): T
+  set?: undefined
+}
+
 export type StructConstructor<T extends object> = {
   new (arg: {
     readonly buffer: ArrayBufferLike