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#]()
+
+# Summary
+
+Allow to infer `attrs` when passing type on the second param of`defineComponent`. And In the `setup-script`,  passing generic type on the `useAttrs<T>` make `attrs` inferred.
+
+I already publish 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` directly
+[TS Playground](https://www.typescriptlang.org/play?jsx=1#code/JYWwDg9gTgLgBAbzgEwKYDNgDtUGELgQ5bwC+c6UBcA5AG4CuqAtDAM7MMzAA2bNAWABQwgMZE28fODgBeFBmx4CkYjAAUCYXDhgqYNgC5E2nRQgRjAZRhRsAcwA0p0s6E6oqLGijqAlIikwq6IcACGMLZGgeFsoQBGYVDGWAwg8ahQcOSkfsJiEvDiMvIAPNJg5hCyCDQAjDTkiVA1deQA9AB8wkA)
+```tsx
+const Comp = defineComponent({
+  props: {
+    foo: String
+  }
+}, { attrs: {} as { bar: number } })
+
+const comp = <Comp foo={'1'} 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>
+
+# 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 possable to allow skipping generics in the `defineComponent` like below.
+```tsx
+// it's not work
+const Comp = defineComponent<Props, Attrs>({})
+```
+So I propose to pass `attrs` type to the second params of `defineComponent`.
+
+The following blow 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`.
+
+
+## `useAttrs`
+In the `setup-script`, the generic type of `useAttrs` will compile to the second param of `defineComponent`
+
+
+# Unresolved questions
+Naming suggestions or improvements on the API are welcome.
+