+//@ To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
+//@ you borrowed your friend a `Vec<i32>`, or a book, is not good enough, unless you also agree on *how long*
+//@ your friend can borrow it. After all, you need to know when you can rely on owning your data (or book) again.
+//@
+//@ Every borrow in Rust has an associated lifetime, written `&'a T` for a borrow of type `T` with lifetime `'a`. The full
+//@ type of `head` reads as follows: `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which
+//@ represents how long the vector has been borrowed. The function type expresses that argument and return value have *the same lifetime*.
+//@
+//@ When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope
+//@ where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its
+//@ argument and return value together, this means that `&v` also has to borrow `v` for the entire duration of
+//@ the function. So when we try to borrow `v` mutable for `push`, Rust complains that the two borrows (the one
+//@ for `head`, and the one for `push`) overlap. Lucky us! Rust caught our mistake and made sure we don't crash the program.
+//@
+//@ So, to sum this up: Lifetimes enable Rust to reason about *how long* a pointer has been borrowed. We can thus
+//@ safely write functions like `head`, that return pointers into data they got as argument, and make sure they
+//@ are used correctly, *while looking only at the function type*. At no point in our analysis of `rust_foo` did
+//@ we have to look *into* `head`. That's, of course, crucial if we want to separate library code from application code.
+//@ Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*,
+//@ where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](https://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
+
+//@ [index](main.html) | [previous](part05.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part06.rs) | [next](part07.html)