@nex3 feel free to word smith the deprecation warning.

Linking to a specific custom importer implementation/documentation is tricky because LibSass is mostly used embedded into other projects.

Back ported to 3.5-stable

Rebased this off of master since it only belongs in 3.5.
The deprecated raw css imports are being removed in 3.6. See #2615.

There needs to be something somewhere that tells you what to do about this warning. It cropped up all of a sudden, and I have no idea how to make it go away. Where is the documentation about what to do when you hit this? There appears to be zero documentation about sass_option_push_import_extension anywhere... did I miss something? Is that a function you call? Is that a configuration item? What is it?

There needs to be something somewhere that tells you what to do about this warning

The resolution is to stop importing .css files. This is a non-standard feature that goes against the Sass language spec.

There appears to be zero documentation about sass_option_push_import_extension anywhere

It's a temporary work around in the C API for integrators. It's use is highly discouraged! It is only offered a temporary polyfill to enable each implementation to migrate their users off of the invalid behaviour