[aw1621107]

> Having a keyword to force it then becomes a very useful thing, vs relying on hopes that future compiler versions and different arch targets will all discover the optimisation opportunity.

Having a way to guarantee TCO/TCE is essential for some cases, yes. GP's question, though, was why a keyword specifically and not a hypothetical attribute that effectively does the same thing.

[IshKebab]

A keyword seems nicer to me. I think the only reason to use attributes is to avoid the work of adding actual new syntax, but seeing as they've already done that...