Function target_allows_null 

pub fn target_allows_null(
    ctx: &CodegenCtx<'_>,
    ty: &Type,
) -> Result<bool, CompilerFailure>

Whether emit_cast_to lets a null reach the target rather than lifting to non-null first. True for the targets whose lowering is a nullable (ref null $Object): bare Null, nullable unions, and the no-op erased/dynamic types — unknown, never, an erased generic (which can hold null after instantiation, e.g. T = string | null), and a recursion back-edge whose body may list null. emit_cast_to’s Union arm uses RefCastNullable for the same reason. A union’s nullability is its lowering, not whether a member is spelled null: T | number lowers nullable because T does.

A caller that must not hand emit_cast_to a null has to ask this, not is_nullable_ref: the two disagree — an InterfaceRef lowers to a nullable slot and is still ref.as_non_nulled there, which traps.