Allow tooling authors to provide custom postcss factory through options

[phated]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Ref #510

I know @alexander-akait said it is out-of-scope, but I needed to write the code anyway. So I figured I'd make the code suitable to upstream and submit it anyway—under the assumption this will be closed.

I'll be publishing my fork as @phated/postcss-loader for our use in Storybook until we can make the 7.0 breaking release—at which time, we'll be able to require users to rely on peerDeps and switch back to this upstream project.

Cheers! 🚀

Breaking Changes

Additional Info

[codecov]

Codecov Report

Merging #511 (14b9cb5) into master (d7657e5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
+ Coverage   97.95%   97.96%   +0.01%     
==========================================
  Files           5        5              
  Lines         244      246       +2     
  Branches       80       81       +1     
==========================================
+ Hits          239      241       +2     
  Misses          5        5              

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)

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[alexander-akait]

Can you provide example of usage? Do you want to use postcss@7 or postcss@8? Why not validate it on boilerplate side? In theory I can add this as the migration options (and when we drop postcss@7 support we remove it)

[phated]

Currently, the way a user consumes Storybook looks (more-or-less) like:

/users-project-root
├─┬ node_modules
│ └─┬ storybook
│   ├── postcss-loader
│   └── postcss@^7
└── storybook-config-file

The internally-included postcss@^7 dependency cannot be removed before the 7.0 released, and users are using it implicitly.

However, a new option can be exposed that allows users to specify a custom postcss factory that has priority over the implicit dependency using this feature. So they would do:

/users-project-root
├─┬ node_modules
│ ├─┬ storybook
│ │ ├── postcss-loader
│ │ └── postcss@^7
│ └── postcss@^8 <-- add postcss 8 as top-level dep
└── storybook-config-file <-- add require("postcss")

[alexander-akait]

Okay, let's rename customPostcss to implementation, we have the same option in sass-loader (https://github.com/webpack-contrib/sass-loader/blob/master/src/options.json#L4)

[phated]

@alexander-akait great thought! I've updated to be more consistent with sass-loader. Do you have any recommendations for getting the snapshots to succeed consistently across node versions?

[alexander-akait]

I think it is global eslint problem, we don't have this rules

[alexander-akait]

Let's return to postcss

[phated]

The problem I ran into was naming the local variable postcss too. I can change that and put this back to postcss

[alexander-akait]

Let's remove Default from here, because we describe this below

[alexander-akait]

Also can you accept CLA?

[phated]

I think the CLA app is bugged

[phated]

@alexander-akait I addressed your comments and squashed the commits. I removed my package-lock updates and other things that made the tests pass locally for me, I think those should be fixed outside this feature.

[alexander-akait]

Thanks