I think that's the wrong question. String templates don't need to support laziness. It's a well established pattern in Java that laziness is done via Supplier's

I think that's the wrong question. String templates don't need to support laziness. It's a well established pattern in Java that laziness is done via Supplier's and it directly solves op's question about logging where the level isn't enabled. This is exactly how logback and other logging implementations handle it now.