> I wonder why they went with a new keyword; I assumed the compiler would opportunistically do TCO when it thinks it's possible, and I figured that the simplest way to require TCO (or else fail compilation) could be done with an attribute.

The first RFC for guaranteed tail calls stated an attribute on `return` was a possible alternative "if and when it becomes possible to attach attributes to expressions" [0]. That was from pre-1.0, though; I believe Rust now supports attributes on at least some expressions, but I don't know when that was added.

The second RFC [1] doesn't seem to discuss keyword vs. attribute, but it does mention that the proof-of-concept implementation "parses `become` exactly how it parses the `return` keyword. The difference in semantics is handled later", so perhaps a keyword is actually simpler implementation-wise?

There's some more discussion on attribute vs. keyword starting here [2], though the attribute being discussed there is a function-level attribute rather than something that effectively replaces a `return`. The consensus seems to be that a function-level attribute is not expressive enough to support the desired semantics, at least. There's also a brief mention of `become` vs. `return` (i.e., new keyword because different semantics).

[0]: https://github.com/rust-lang/rfcs/pull/81/changes

[1]: https://github.com/DemiMarie/rfcs/blob/become/0000-proper-ta...

[2]: https://github.com/rust-lang/rfcs/pull/1888#issuecomment-278...