alt Hacker NewsI am definitely not a Scala expert.
As I wrote in a sibling comment, the key benefit here is the extra work from the compiler to deliver what you wanted, on top of the diagnostic if it can't.
I don't know if Scala has the problem that `become` addresses (C++ calls this RAII, but I have no idea what Scala would call it if they have the same idea)
However in my brief attempt to validate what Scala does do here, I found discussion of "always" optimising to a loop which is a bad sign. Tail recursion is an elegant way to write some loops but that's not the only thing it's useful for, and it seems as though Scala just doesn't care about other cases, at least for @tailrec
One thing you want TCO for in a language like Rust with lots of monomorphisation is to avoid function call overhead for the deliberately out-of-line slow path in some code. So in this case there was never an implied loop and we're not averting a stack overflow, we wanted to do a single instruction pointer change instead of an expensive function call wrapper. Seems like @tailrec isn't for that.
I jsut did some digging and it seems you're right, it's only for methods which call themselves (which indeed get compiled into a loop, as an entirely local transformation). So not hugely useful.
Apologies, I've not written Scala for many years; I just recalled that there was a way to annotate tail calls which the compiler checks. I didn't realise it was so limited!