Enable Sass comments and run prefixing before minification

[knowler]

I was looking into a fix for #2077 and here’s what I discovered:

• cssnano was not the issue with the above mentioned issue the desired functionality works perfectly when using normal CSS instead of Sass.
• Sass removes the source comments by default.
  • So even disabling cssnano completely still resulted with the same issue.
• Autoprefixer requires /*! autoprefixer: off */ in Sass and Less.

The problem:

• When processing Sass, comments are removed before the PostCSS plugins are processed.
  • PostCSS plugins (i.e. Autoprefixer, Purgecss) often use comments for processing so we need them to be there when that processing takes place.
  • We still want cssnano’s discardComments set to removeAll: true since we don’t want any comments in the production built code.

This pull request:

• Enables the source comments in Sass.
  • This is temporary since they will be removed by minification.
• Moves the autoprefixing task before minification.
  • I do not know the side effects of doing this. I am actively testing for anything undesirable. Everything seems to run fine from my tests. I do know that cssnano doesn’t remove prefixes as of 4.0.0-rc.0, so there should be no issues there.

Examples

Before as described in #2077

Input:

image {
  /* autoprefixer: off */
  clip-path: url(#imagemask);
}

Output:

image{-webkit-clip-path:url(#imagemask);clip-path:url(#imagemask)}

With the important comment and discardComments removed

Input:

image {
  /*! autoprefixer: off */
  clip-path: url(#imagemask);
}

Output:

image{
  /*! autoprefixer: off */clip-path:url(#imagemask)}

This works but we don’t want the comment in there.

With this PR

Input:

image {
  /*! autoprefixer: off */
  clip-path: url(#imagemask);
}

Output:

image{clip-path:url(#imagemask)}

Perfect!

Related

• Postcss's control comments not working on production #2077
• https://discourse.roots.io/t/removing-unused-css-with-purgecss-uncss/11586/13?u=knowler

[retlehs]

thank you!