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Which makes me wonder :

1/ could you build the same unsafe behiavor in Rust if you wanted to by not specifying lifetime constraints ?

2/ If yes, shouldn't lifetime constraints be mandatory ?

The compiler will not compile code it cannot verify as safe (outside an `unsafe` block), meaning it will complain about lifetime constraints that are missing, e.g. it is a compile-time error to not propagate the lifetime information out of the enum.

http://play.rust-lang.org/?run=1&code=enum%20Token%20{%0A%20...

The lifetime constraint in the enum is mandatory, the code won't compile without it.

The tokenization function itself doesn't have to explicitly specify lifetimes like it does in the blog post. It would also work without them and Rust would still ensure that you don't end up with a use-after-free.

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