unstable book: OIBIT

src/doc/unstable-book/src/language-features/optin-builtin-traits.md

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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
+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) );
+    }
+```