Function collect_structure_errors 

pub fn collect_structure_errors(
    schema: &Schema,
    errors: &mut Vec<ValidationError>,
)

Structural / reference diagnostics: duplicate names (model/struct/enum, fields within a container, enum variants), relation targets resolve, struct/enum type references resolve, structs contain only fixed-size types, and composite-index / projection field references exist.

These are always run (the parser does not gate them behind use_validation).