Maybe, it probably depends on how you're looking at it. The optimization is obvious, I expect any optimizing compiler will TCO all naive tail calls - but the trouble in Rust or C++ or a dozen other languages is that you can so easily write code which you think can be optimized but the compiler either can't see how or can see that it's not possible and (without this keyword) you don't find out about this because growing the stack is a valid implementation of what you wrote even though it's not what you meant.

The "become" keyword allows us to express our meaning, we want the tail call, and, duh, of course the compiler will optimize that if it can be a tail call but also now the compiler is authorized to say "Sorry Dave, that's not possible" rather than grow the stack. Most often you wrote something silly. "Oh, the debug logging happens after the call, that's never going to work, I will shuffle things around".