Function field_ty 

pub fn field_ty<'tcx>(
    tcx: TyCtxt<'tcx>,
    field: &FieldDef,
    args: GenericArgsRef<'tcx>,
) -> Ty<'tcx>

Resolve a struct field’s type, normalizing where the rustc version requires it.

On newer rustc field.ty(tcx, args) returns an Unnormalized<Ty> that must be .skip_norm_wip()-ed; on older toolchains the SkipNormWip shim makes the same call a no-op, so this is version-independent.