[anamexis]

Isn't that kinda how all TCO works?

[pjmlp]

First level TCO yes, however the annotation way doesn't work for mutually recursive calls.

[jwesleyharding]

to clarity just a tad, the Scala compiler does TCO out of the box and the annotation is added only to check that the method is in fact so optimizable

[pjmlp]

Unless something changed since 2.0.9 (last time I did anything relevant in Scala), only one level, it isn't clever enough to rewrite mutually recursive calls.

This is rather important, specially when coming from languages like Scheme that have TCO as part of the language specification compliance.