unstable book: unboxed_closures

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+# `unboxed_closures`
+
+The tracking issue for this feature is [#29625]
+
+See Also: [`fn_traits`](library-features/fn-traits.md)
+
+[#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)) {
+    args.0 + args.1
+}
+
+fn main() {}
+```