docs: infer attrs rfc

active-rfcs/0000-infer-attrs.md

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+- Start Date: 2023-01-01
+- Target Major Version: 3.x
+- Reference Issues: [vuejs/core#3452](https://github.com/vuejs/core/issues/3452), [vuejs/core#5423](https://github.com/vuejs/core/issues/5423), 
+- Implementation PR: [vuejs/core#7444](https://github.com/vuejs/core/pull/7444)
+
+# Summary
+Allow to infer `attrs` when passing type on the second param of `defineComponent` and `defineCustomElement`. 
+And in the `setup-script`,  passing generic type on the `useAttrs<T>` will infer `attrs` to `T`.
+
+I already published an npm package named [vue-ts-utils](https://github.com/rudy-xhd/vue-ts-utils), so that you can use `defineComponent` to infer attrs in advance.
+
+# Basic example
+
+## Using `defineComponent`
+[TS Playground](https://www.typescriptlang.org/play?jsx=1#code/JYWwDg9gTgLgBAbzgEwKYDNgDtUGELgQ5bwC+c6UBcA5AG4CuqAtDAM7MMzAA2bNAWABQwgMZE28fODgBeFBmx4CkYjAAUCYXDhgqYNgC5E2nRQgRjAZRhRsAcwA0p0s6E6oqLGijqAlIikwq6IcACGMLZGgeFsoQBGYVDGWAwg8ahQcOSkfsJiEvDiMvIAPNJg5hCyCDQAjDTkiVA1deQA9AB8wkA)
+```tsx
+const Comp = defineComponent({
+  props: {
+    foo: String
+  },
+  render() {
+    // number
+    console.log(this.$attrs.bar)
+  }
+}, { attrs: {} as { bar: number } })
+
+const comp = <Comp foo={'str'} bar={1} />
+```
+
+## Using `useAttrs<T>` in `script-setup`
+
+```vue
+<script setup lang="ts">
+const attrs = useAttrs<{bar: number}>()
+</script>
+```
+
+<details>
+<summary>Compiled Output</summary>
+
+```js
+export default /*#__PURE__*/_defineComponent({
+  setup(__props, { expose }) {
+  expose();
+
+      const attrs = useAttrs<{ foo: number }>()
+
+return { attrs, useAttrs, ref }
+}
+
+}, { attrs: {} as { foo: number }})"
+```
+
+</details>
+
+
+## Using `defineCustomElement`
+```tsx
+const Comp = defineCustomElement({
+  props: {
+    foo: String
+  },
+  render() {
+    // number
+    console.log(this.$attrs.bar)
+  }
+}, { attrs: {} as { bar: number } })
+
+```
+
+# Motivation
+This proposal is mainly to infer `attrs` using `defineComponent`.
+
+When using typescript in Vue3, the fallthrough attributes is unable to be used. It's not appropriate obviously that only one can be chosen from `typescript` and `the fallthrough attributes`. In most cases, we choose `typescript` and set attributes to `props` options instead of using the fallthrough attributes.
+
+
+# Detailed design
+
+## `defineComponent`
+Due to typescript limitation from [microsoft/TypeScript#10571](https://github.com/microsoft/TypeScript/issues/10571), it's not possible to skip generics up to now in the `defineComponent` like below.
+```tsx
+// it's not work
+const Comp = defineComponent<Props, Attrs>({})
+```
+
+There still has two ways to be chosen.
+
+1. Defining the first param that already existing, just like [vuejs/rfcs#192](https://github.com/vuejs/rfcs/pull/192) did.
+```tsx
+const Comp = defineComponent({
+  attrs: {} as { bar: number },
+  props: {
+    foo: String
+  },
+  render() {
+    // number
+    console.log(this.$attrs.bar)
+  }
+})
+
+const comp = <Comp foo={'str'} bar={1} />
+```
+2. Defining the second param as proposed.
+```tsx
+const Comp = defineComponent({
+  props: {
+    foo: String
+  },
+  render() {
+    // number
+    console.log(this.$attrs.bar)
+  }
+}, { attrs: {} as { bar: number } })
+
+const comp = <Comp foo={'str'} bar={1} />
+```
+
+At last i chosen the second way that pass `attrs` type to the second params of `defineComponent`, because I think the code of the component should not be involved just for type definition.
+
+
+The following below is the design details.
+- `attrs` is inferred to `{ class: unknown; style: unknown }` when the value of the second param is `undefined`
+- `attrs` is lower priority  than `props`.
+- [see for more detail cases](https://github.com/vuejs/core/pull/7444/files)
+
+## `useAttrs<T>`
+In the `setup-script`, the generic type of `useAttrs` will compile to the second param of `defineComponent`.
+```ts
+export default /*#__PURE__*/_defineComponent({
+  setup(__props, { expose }) {
+  expose();
+
+      const attrs = useAttrs<{ foo: number }>()
+
+return { attrs, useAttrs, ref }
+}
+
+}, { attrs: {} as { foo: number }})"
+```
+
+## `defineCustomElement`
+The type inferrence of `defineCustomElement` is the same as `defineComponent`.
+
+
+# Unresolved questions
+Naming suggestions or improvements on the API are welcome.
+