The Circle of Spores druid's Halo of Spores feature (Guildmasters' Guide to Ravnica, p. 27) states the following:
When a creature you can see moves into a space within 10 feet of you or starts its turn there, you can use your reaction to deal 1d4 necrotic damage to that creature unless it succeeds on a Constitution saving throw against your spell save DC.
My initial understanding was that this translates to: First your reaction is used (and thus "consumed"), then the creature would roll the Con save. Hence, if the creature succeeded, the reaction would have been wasted with no result.
However, upon second inspection I realise that it could also translate to this: You first declare you would like to use the feature, then the creature makes the save, and only if it fails is the reaction consumed to deal damage.
I'd be grateful for any help on this. Which approach is correct here? Is your reaction used regardless of whether the other creature succeeds on the saving throw?
Thanks in advance!