Document adt_const_params feature in Unstable Book

src/doc/unstable-book/src/language-features/adt-const-params.md

@@ -0,0 +1,35 @@
+# `adt_const_params`
+
+The tracking issue for this feature is: [#95174]
+
+[#95174]: https://github.com/rust-lang/rust/issues/95174
+
+------------------------
+
+Allows for using more complex types for const parameters, such as structs or enums.
+
+```rust
+#![feature(adt_const_params)]
+#![allow(incomplete_features)]
+
+use std::marker::ConstParamTy;
+
+#[derive(ConstParamTy, PartialEq, Eq)]
+enum Foo {
+    A,
+    B,
+    C,
+}
+
+#[derive(ConstParamTy, PartialEq, Eq)]
+struct Bar {
+    flag: bool,
+}
+
+fn is_foo_a_and_bar_true<const F: Foo, const B: Bar>() -> bool {
+    match (F, B.flag) {
+        (Foo::A, true) => true,
+        _ => false,
+    }
+}
+```