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# Lifetimes
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Lifetimes tell the compiler how to check whether references live long
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enough to be valid in any given situation. For example lifetimes say
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"make sure parameter 'a' lives as long as parameter 'b' so that the return
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value is valid".
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They are only necessary on borrows, i.e. references,
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since copied parameters or moves are owned in their scope and cannot
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be referenced outside. Lifetimes mean that calling code of e.g. functions
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can be checked to make sure their arguments are valid. Lifetimes are
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restrictive of their callers.
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If you'd like to learn more about lifetime annotations, the
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[lifetimekata](https://tfpk.github.io/lifetimekata/) project
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has a similar style of exercises to Rustlings, but is all about
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learning to write lifetime annotations.
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## Further information
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- [Lifetimes (in Rust By Example)](https://doc.rust-lang.org/stable/rust-by-example/scope/lifetime.html)
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- [Validating References with Lifetimes](https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html)
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