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Trait object syntax + trailing plus #255

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Mar 3, 2018
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Trait object syntax + trailing plus #255

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Trait object syntax + trailing plus
Havvy Feb 26, 2018
2f16bdf
More restrictions on trait objects
Havvy Mar 3, 2018
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20 changes: 15 additions & 5 deletions src/types.md
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Expand Up @@ -498,18 +498,26 @@ Because captures are often by reference, the following general rules arise:

## Trait objects

> **<sup>Syntax</sup>**
> _TraitObjectType_ :
> &nbsp;&nbsp; _LifetimeOrPath_ ( `+` _LifetimeOrPath_ )<sup>\*</sup> `+`<sup>?</sup>
>
> _LifetimeOrPath_ :
> &nbsp;&nbsp; [_Path_] | [_LIFETIME_OR_LABEL_]

A *trait object* is an opaque value of another type that implements a set of
traits. The set of traits is made up of an [object safe] *base trait* plus any
number of [auto traits].

Trait objects implement the base trait, its auto traits, and any super traits
of the base trait.

Trait objects are written as the path to the base trait followed by the list
of auto traits followed optionally by a lifetime bound all separated by `+`. For
example, given a trait `Trait`, the following are all trait objects: `Trait`,
`Trait + Send`, `Trait + Send + Sync`, `Trait + 'static`,
`Trait + Send + 'static`.
Trait objects are written the same as trait bounds, but with the following
restrictions. All traits except the first trait must be auto traits, there may
not be more than one lifetime, and opt-out bounds (e.g. `?sized`) are not
allowed. For example, given a trait `Trait`, the following are all trait
objects: `Trait`, `Trait + Send`, `Trait + Send + Sync`, `Trait + 'static`,
`Trait + Send + 'static`, `Trait +`, `'static + Trait`.

Two trait object types alias each other if the base traits alias each other and
if the sets of auto traits are the same and the lifetime bounds are the same.
Expand Down Expand Up @@ -701,3 +709,5 @@ impl Printable for String {
[object safe]: items/traits.html#object-safety
[issue 47010]: https://github.com/rust-lang/rust/issues/47010
[issue 33140]: https://github.com/rust-lang/rust/issues/33140
[_PATH_]: paths.html
[_LIFETIME_OR_LABEL_]: tokens.html#lifetimes-and-loop-labels