Prerequisites

• As server admin, install SassC on your server
• In config.php, configure the path to SassC, for example:

  $CFG->pathtosassc = '/bin/sassc';

Problem 1: Core SCSS Variables are not passed to SassC

Steps to reproduce:

• Login as admin
• Go to Site Administration -> Apperance -> Themes -> Boost -> "Advanced settings" tab
• Set the theme_boost | scss setting to

  body {

  color: $gray !important;

  }

• Click "Save changes"

Expected result:

• The RAW SCSS is stored and compiled and used
• All body text is gray now

Actual result:

• The RAW SCSS is not accepted
• There is a Debug message saying:

  Coding error detected, it must be fixed by a programmer: Error: Undefined variable: "$gray". on line 2 of stdin >> color: $gray !important; -----------^

• The stack track is:

  Stack trace:

  line 138 of /lib/classes/scss.php: coding_exception thrown

  line 10934 of /lib/adminlib.php: call to core_scss->compile()

  line 2404 of /lib/adminlib.php: call to admin_setting_scsscode->validate()

  line 8499 of /lib/adminlib.php: call to admin_setting_configtext->write_setting()

  line 44 of /admin/settings.php: call to admin_write_settings()

Problem 2: Bootstrap includes are not possible with SassC

Steps to reproduce:

• Login as admin
• Go to Site Administration -> Apperance -> Themes -> Boost -> "Advanced settings" tab
• Set the theme_boost | scss setting to

  @include media-breakpoint-down(md) {

  body {

  color: red !important;

  }

  }

• Click "Save changes"

Expected result:

• The RAW SCSS is stored and compiled and used
• On small screens, all body text is red now

Actual result:

• The RAW SCSS is not accepted
• There is a Debug message saying:

  Coding error detected, it must be fixed by a programmer: Error: no mixin named media-breakpoint-down Backtrace: stdin:1 on line 1 of stdin >> @include media-breakpoint-down(md) { ---------^

• The stack track is the same as with problem 1.

Analysis

• Both problems happen only if you configure Moodle to use SassC. If you use the Built-in SaSS compiler, everything is fine.
• The problem only happens because the theme_boost | scss setting validates the configured RAW SCSS. This was introduced in MDL-56488. If you change the theme_boost | scss setting from admin_setting_scsscode to admin_setting_configtextarea in /theme/boost/settings.php, the RAW SCSS is accepted and properly processed anyway even if SassC is used. That's why I mark this issue as a regression of MDL-56488, knowing that this is not fully true.
• This issue is related to MDL-62773 where callumb described something similar for custom SCSS variables.