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Rollup merge of #36997 - KillTheMule:patch-1, r=steveklabnik

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Book: Be very explicit of lifetimes being descriptive

... not prescriptive. Pointed out in https://users.rust-lang.org/t/what-if-i-get-lifetimes-wrong/7535/4, which was a revelation to me and made me think this should be more clear in the book. I'm not sure if I got this entirely right or if the wording is good, but I figured a PR is more helpful than a simple issue.

r? @steveklabnik

Small Note: There's also https://github.com/rust-lang/book, should I have sent the PR there? It doesn't coincide with the online book though, so I figured it's better of here.
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Guillaume Gomez 2016-10-11 17:51:26 +02:00 committed by GitHub
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src/doc/book/lifetimes.md
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@ -56,8 +56,12 @@ To fix this, we have to make sure that step four never happens after step
three. The ownership system in Rust does this through a concept called
lifetimes, which describe the scope that a reference is valid for.
When we have a function that takes an argument by reference, we can be
implicit or explicit about the lifetime of the reference:
**Note** It's important to understand that lifetime annotations are
_descriptive_, not _prescriptive_. This means that how long a reference is valid
is determined by the code, not by the annotations. The annotations, however,
give information about lifetimes to the compiler that uses them to check the
validity of references. The compiler can do so without annotations in simple
cases, but needs the programmers support in complex scenarios.
```rust
// implicit