Function codegen_tag_value 

pub(super) fn codegen_tag_value<'tcx, V>(
    cx: &impl CodegenMethods<'tcx, Value = V>,
    variant_index: VariantIdx,
    layout: TyAndLayout<'tcx>,
) -> Result<Option<(FieldIdx, V)>, UninhabitedVariantError>

Calculates the value that needs to be stored to mark the discriminant.

This might be None for a struct or a niched variant (like Some(&3)).

If it’s Some, it returns the value to store and the field in which to store it. Note that this value is not the same as the discriminant, in general, as it might be a niche value or have a different size.

It might also be an Err because the variant is uninhabited.