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Explain default trait object liftime bounds. #31353

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Explain default trait object liftime bounds.
steveklabnik Feb 2, 2016
0fd9bc9
address some nits
steveklabnik Apr 29, 2016
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39 changes: 39 additions & 0 deletions src/doc/reference.md
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Expand Up @@ -3603,6 +3603,45 @@ fn main() {
In this example, the trait `Printable` occurs as a trait object in both the
type signature of `print`, and the cast expression in `main`.

Trait objects may contain references, and so those references will need a
lifetime:

```rust,ignore
// Some trait like this...
trait Bar<'a>: 'a { }

// ... means these are the same:
Box<Bar>
Box<Bar + 'a>
```

If there’s no bound, then the default lifetime is the same as the pointer that
the trait object is behind:

```rust,ignore
// Some trait like this...
trait Bar { }

// ... means these are the same:
&'a Bar
&'a (Bar + 'a)
```

In any other case, the default is `'static`:

```rust,ignore
// Some trait like this...
trait Bar { }

// ... means these are the same:
Box<Bar>
Box<Bar + 'static>
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one case you may want to clarify is that &Box<Bar> is same as &Box<Bar+'static> -- i.e., it's the innermost case that matters

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there is technically I think one other rule, not sure how much detail we want to go into though. I think if you have struct Ref<'a, T: ?Sized> and you do Ref<'a, Trait> you get Ref<'a, Trait+'a> by default. Unless we changed that in the follow-up RFC. :P I guess I should go double check.


// and so are these:
&'a Box<Bar>
&'a Box<Bar + 'static>
```

### Type parameters

Within the body of an item that has type parameter declarations, the names of
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