Build requires PostCSS @ 8 support

[andreasvirkus]

Bug report

The dev server (saber) fails to compile with the message:

ERROR in ./node_modules/prismjs/themes/prism-tomorrow.css (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-3-1!./node_modules/@egoist/postcss-loader/src??ref--5-oneOf-3-2!./node_modules/prismjs/themes/prism-tomorrow.css)
Module build failed (from ./node_modules/@egoist/postcss-loader/src/index.js):
Error: PostCSS plugin autoprefixer requires PostCSS 8.
Migration guide for end-users:
https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users
    at Processor.normalize (/Users/anc/code/cottage/node_modules/postcss/lib/processor.js:167:15)
    at new Processor (/Users/anc/code/cottage/node_modules/postcss/lib/processor.js:56:25)
    at postcss (/Users/anc/code/cottage/node_modules/postcss/lib/postcss.js:55:10)
    at /Users/anc/code/cottage/node_modules/@egoist/postcss-loader/src/index.js:145:12
 @ ./node_modules/prismjs/themes/prism-tomorrow.css 4:14-154 14:3-18:5 15:22-162
 @ ./saber-browser.js
 @ ./.saber/extend-browser-api.js
 @ ./node_modules/saber/vue-app/create-app.js
 @ ./node_modules/saber/vue-app/entry-client.js
 @ multi ./node_modules/saber/vue-app/entry-client.js

Steps to reproduce

1. Brand new saber project
2. npm run dev

What is expected?

Saber would compile

What is actually happening?

Post CSS complains about not having the minimum required version

Other relevant information

• Result of running saber -v: 0.11.7
• Browser version (optional): N/A
• Is Saber a global or local install? Local
• Which package manager did you use for the install? npm
• Does this issue occur when all plugins are disabled? Yup

[andreasvirkus]

Can we swap out @egoist/postcss-loader for the upstream package (https://github.com/webpack-contrib/postcss-loader)? There doesn't seem to be any diff - webpack-contrib/postcss-loader@master...egoist:master

Here's also Post CSS's official migration guide - https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users

[andreasvirkus]

Downgrading and fixing autoprefixer to version 9.8.6 allows to compile Saber currently as a workaround

[Carrie999]

[Carrie999]