Function struct_field_type_matches 

pub fn struct_field_type_matches(
    expected: &DataType,
    received: &DataType,
    generics: &Generics,
) -> bool

Whether a value of type received fits a field declared expected.

null fits every field, because it is what an absent value carries. Everything else is param_type_matches, the rule a parameter uses: a field declared any is a dynamic slot and holds a value of any type, and two instantiations of one generic declaration line up when their arguments do.

The one place a field is stricter than a parameter is a dynamic value put into a typed field. A field is read back at the type it declares and used without a run-time test, so a string stored in an int field would be reported from inside the arithmetic that later read it, or not at all. It is reported at the line that wrote it instead; name the type with as int or another downcast to store one.