Rollup merge of #45166 - tinaun:more_unstable_docs, r=steveklabnik

Documented a few more unstable feature gates.

unboxed closures, fn_traits, and OIBIT. hope these are decent!
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kennytm 2017-10-12 16:14:28 +08:00
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# `optin_builtin_traits`
The tracking issue for this feature is [#13231]
[#13231]: https://github.com/rust-lang/rust/issues/13231
----
The `optin_builtin_traits` feature gate allows you to define auto traits.
Auto traits, like [`Send`] or [`Sync`] in the standard library, are marker traits
that are automatically implemented for every type, unless the type, or a type it contains,
has explictly opted out via a negative impl.
[`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html
[`Sync`]: https://doc.rust-lang.org/std/marker/trait.Sync.html
```rust,ignore
impl !Type for Trait
```
Example:
```rust
#![feature(optin_builtin_traits)]
trait Valid {}
impl Valid for .. {}
struct True;
struct False;
impl !Valid for False {}
struct MaybeValid<T>(T);
fn must_be_valid<T: Valid>(_t: T) { }
fn main() {
// works
must_be_valid( MaybeValid(True) );
// compiler error - trait bound not satisfied
// must_be_valid( MaybeValid(False) );
}
```

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# `unboxed_closures`
The tracking issue for this feature is [#29625]
See Also: [`fn_traits`](library-features/fn-traits.html)
[#29625]: https://github.com/rust-lang/rust/issues/29625
----
The `unboxed_closures` feature allows you to write functions using the `"rust-call"` ABI,
required for implmenting the [`Fn*`] family of traits. `"rust-call"` functions must have
exactly one (non self) argument, a tuple representing the argument list.
[`Fn*`]: https://doc.rust-lang.org/std/ops/trait.Fn.html
```rust
#![feature(unboxed_closures)]
extern "rust-call" fn add_args(args: (u32, u32)) -> u32 {
args.0 + args.1
}
fn main() {}
```

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# `fn_traits`
The tracking issue for this feature is [#29625]
See Also: [`unboxed_closures`](language-features/unboxed-closures.html)
[#29625]: https://github.com/rust-lang/rust/issues/29625
----
The `fn_traits` feature allows for implementation of the [`Fn*`] traits
for creating custom closure-like types.
[`Fn*`]: https://doc.rust-lang.org/std/ops/trait.Fn.html
```rust
#![feature(unboxed_closures)]
#![feature(fn_traits)]
struct Adder {
a: u32
}
impl FnOnce<(u32, )> for Adder {
type Output = u32;
extern "rust-call" fn call_once(self, b: (u32, )) -> Self::Output {
self.a + b.0
}
}
fn main() {
let adder = Adder { a: 3 };
assert_eq!(adder(2), 5);
}
```