nomicon/unbounded-lifetimes.md

% Unbounded Lifetimes

Unsafe code can often end up producing references or lifetimes out of thin air.
Such lifetimes come into the world as *unbounded*. The most common source of this
is derefencing a raw pointer, which produces a reference with an unbounded lifetime.
Such a lifetime becomes as big as context demands. This is in fact more powerful
than simply becoming `'static`, because for instance `&'static &'a T`
will fail to typecheck, but the unbound lifetime will perfectly mold into
`&'a &'a T` as needed. However for most intents and purposes, such an unbounded
lifetime can be regarded as `'static`.

Almost no reference is `'static`, so this is probably wrong. `transmute` and
`transmute_copy` are the two other primary offenders. One should endeavour to
bound an unbounded lifetime as quick as possible, especially across function
boundaries.

Given a function, any output lifetimes that don't derive from inputs are
unbounded. For instance:

```rust,ignore
fn get_str<'a>() -> &'a str;
```

will produce an `&str` with an unbounded lifetime. The easiest way to avoid
unbounded lifetimes is to use lifetime elision at the function boundary.
If an output lifetime is elided, then it *must* be bounded by an input lifetime.
Of course it might be bounded by the *wrong* lifetime, but this will usually
just cause a compiler error, rather than allow memory safety to be trivially
violated.

Within a function, bounding lifetimes is more error-prone. The safest and easiest
way to bound a lifetime is to return it from a function with a bound lifetime.
However if this is unacceptable, the reference can be placed in a location with
a specific lifetime. Unfortunately it's impossible to name all lifetimes involved
in a function. To get around this, you can in principle use `copy_lifetime`, though
these are unstable due to their awkward nature and questionable utility.