unstable book: default_free_fn

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diff --git a/src/doc/unstable-book/src/library-features/default-free-fn.md b/src/doc/unstable-book/src/library-features/default-free-fn.md
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+# `default_free_fn`
+
+The tracking issue for this feature is: [#73014]
+
+[#73014]: https://github.com/rust-lang/rust/issues/73014
+
+------------------------
+
+Adds a free `default()` function to the `std::default` module.  This function
+just forwards to [`Default::default()`], but may remove repetition of the word
+"default" from the call site.
+
+Here is an example:
+
+```rust
+#![feature(default_free_fn)]
+use std::default::default;
+
+#[derive(Default)]
+struct AppConfig {
+    foo: FooConfig,
+    bar: BarConfig,
+}
+
+#[derive(Default)]
+struct FooConfig {
+    foo: i32,
+}
+
+#[derive(Default)]
+struct BarConfig {
+    bar: f32,
+    baz: u8,
+}
+
+fn main() {
+    let options = AppConfig {
+        foo: default(),
+        bar: BarConfig {
+            bar: 10.1,
+            ..default()
+        },
+    };
+}
+```