add an unstable book chapter

Author: dianne

src/doc/unstable-book/src/language-features/deref-patterns.md

@@ -0,0 +1,48 @@
+# `deref_patterns`
+
+The tracking issue for this feature is: [#87121]
+
+[#87121]: https://github.com/rust-lang/rust/issues/87121
+
+------------------------
+
+This feature permits pattern matching on [library-defined smart pointers] through their `Deref`
+target types, either implicitly or with the placeholder `deref!(_)` syntax.
+
+```rust
+#![feature(deref_patterns)]
+#![allow(incomplete_features)]
+
+pub fn main() {
+    let mut v = vec![Box::new(Some(0))];
+    if let [Some(ref mut x)] = v {
+        *x += 1;
+    }
+    if let deref!([deref!(Some(ref mut x))]) = v {
+        *x += 1;
+    }
+    assert_eq!(v, [Box::new(Some(2))]);
+}
+```
+
+Without this feature, it may be necessary to introduce temporaries to represent dereferenced places
+when matching on nested structures:
+
+```rust
+pub fn main() {
+    let mut v = vec![Box::new(Some(0))];
+    if let [ref mut b] = *v {
+        if let Some(ref mut x) = **b {
+            *x += 1;
+        }
+    }
+    if let [ref mut b] = *v {
+        if let Some(ref mut x) = **b {
+            *x += 1;
+        }
+    }
+    assert_eq!(v, [Box::new(Some(2))]);
+}
+```
+
+[library-defined smart pointers]: https://doc.rust-lang.org/std/ops/trait.DerefPure.html#implementors