Better support for multiple processors

The version of ESLint you are using. v5.8.0

The problem you want to solve. I’m currently using two plugins to lint vue codeblocks in markdown files – eslint-plugin-markdown and eslint-plugin-vue – and they both have processors of their own that only apply to certain file extensions: eslint-plugin-markdown and eslint-plugin-vue. The problem is that the vue processor doesn’t get applied to the Vue codeblocks in the markdown due to the file extension being md.

Your take on the correct solution to problem. Wildcard extension support or the ability to add/mutate the effective filename of each src outputted by the preprocessor so that an extension can be appended (eg. the codeblock type).

Are you willing to submit a pull request to implement this change? Yes

[mysticatea]

function preprocess(code: string, filename: string): string[]

This is the current interface. We cannot chain the preprocess because the input type and the output type are different. I think that the enhancement that changes the output type solves this issue.

type VirtualFile = { code: string, filename: string }
function preprocess(code: string, filename: string): VirtualFile[] | string[]

Then we can chain the preprocess. For example:

<!-- example.md -->
# Docs

```js
console.log("hello")
```

```vue
<template><div>Hello</div></template>
```

```c
int main() {}
```

1. eslint-plugin-markdown’s processor returns

   [
       { "code": "console.log(\"hello\")", "filename": "example.md.js" },
       { "code": "<template><div>Hello</div></template>", "filename": "example.md.vue" },
       { "code": "int main() {}", "filename": "example.md.c" },
   ]
   

2. eslint checks the virtual files whether it should lint or not by the filename. (by --ext option and input glob patterns?)

3. eslint gives proper processors the virtual files. So eslint-plugin-vue can handle the <template><div>Hello</div></template>.

4. Then, eslint gives rules the last code and filename.

5. eslint gives processors the result messages to postprocess those in the reversed order of the preprocessing.

[btmills]

🎉 After today’s TSC meeting, this is now accepted based on the RFC: https://github.com/eslint/rfcs/tree/master/designs/2018-processors-improvements