Migrator treats @imports as nested if they are preceded by any rule, regardless of actual nesting

[ettmetal]

When using the migrator for modules, if there is any rule in the file preceding an @import directive for a partial which uses sass variables declared in that file, the tool fails. The following error message is produced:

Error: This stylesheet was loaded by a nested import in minimal.sass. The module system only supports loading nested CSS using the load-css() mixin, which doesn't allow access to variables from the outer stylesheet.

The error is not documented, and I couldn't find it anywhere in this repo outside of a test spec which didn't offer much more insight into the cause. In this failure case, it's a little confusing as output, because there are no nested imports.

Expected behaviour: The migrator does not treat non-nested imports as nested.

Minimal reproducable example:

minimal.sass:

$text-color: #fff

body
    background: #000

// Import partials.
@import "minimal/base"

_base.sass:

body
  color: $text-color

command:

$ sass-migrator module -dv ./minimal.sass

[jathak]

The migrator is correct that it can't migrate this code because @use and @forward rules must come before any other rules in a stylesheet. Because of this, the migrator treats @imports that come after other rules as equivalent to nested @imports, both of which can only be migrated to meta.load-css, not @use.

However, you're right here that the error message is confusing and we should either mention both possibilities or have separate error messages for nested vs late @imports.