To call adt.Node.Source().Pos() safely with nil checks. We had two other "pos" funcs, in the cue and adt package, as well as various hand-rolled versions in many func bodies. Signed-off-by: Daniel Martí <mvdan@mvdan.cc> Change-Id: I7d02082fc5759867751f1babd0f9a163d0203ff9 Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1227393 Reviewed-by: Matthew Sackman <matthew@cue.works> TryBot-Result: CUEcueckoo <cueckoo@cuelang.org> Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>