[Rule idea] Forbid defining unused private members

[stof]

Private members (i.e. functions, mixins, variables and selector placeholders) are not exposed outside the module boundary. so if they are not used inside the module, they are unused.

In Sass, private members have a name that start with either - or _ (for variables, this comes after the $ as the $ is not considered to be part of the name. Same for the % prefix of selector placeholders).

This rule should check for usages in the same file. Projects that are still using @import rather than the module-aware @use and @forward are expected to keep this rule disabled (as all files loaded through @import are loaded inside the same module, which makes it impossible to ensure we see all usages of private members).

[kristerkari]

Sounds like a good idea!