Update opaque-types-type-alias-impl-trait.md

1. Clarify that there is a single concrete type for an opaque type
2. Clarify that defining a type alias to an opaque type is an unstable feature
3. Add complete example
4. Clarify that defining an associate type as opaque is an unstable
   feature
5. Add another complete example

Author: smanilov

src/opaque-types-type-alias-impl-trait.md

@@ -12,22 +12,44 @@ type Foo = impl Bar;
 
 This declares an opaque type named `Foo`, of which the only information is that
 it implements `Bar`. Therefore, any of `Bar`'s interface can be used on a `Foo`,
-but nothing else (regardless of whether it implements any other traits).
+but nothing else (regardless of whether the concrete type implements any other traits).
 
 Since there needs to be a concrete background type,
-you can (as of <!-- date-check --> January 2021) express that type
+you can (as of <!-- date-check --> May 2025) express that type
 by using the opaque type in a "defining use site".
 
 ```rust,ignore
 struct Struct;
 impl Bar for Struct { /* stuff */ }
+#[define_opaque(Foo)]
 fn foo() -> Foo {
     Struct
 }
 ```
 
 Any other "defining use site" needs to produce the exact same type.
 
+Note that defining a type alias to an opaque type is an unstable feature.
+To use it, you need `nightly` and the annotations `#![feature(type_alias_impl_trait)]` on the file and `#[define_opaque(Foo)]` on the method that links the opaque type to the concrete type.
+Complete example:
+
+```rust
+#![feature(type_alias_impl_trait)]
+
+trait Bar { /* stuff */ }
+
+type Foo = impl Bar;
+
+struct Struct;
+
+impl Bar for Struct { /* stuff */ }
+
+#[define_opaque(Foo)]
+fn foo() -> Foo {
+    Struct
+}
+```
+
 ## Defining use site(s)
 
 Currently only the return value of a function can be a defining use site
@@ -61,3 +83,28 @@ impl Baz for Quux {
     fn foo() -> Self::Foo { ... }
 }
 ```
+
+For this you would also need to use `nightly` and the (different) `#![feature(impl_trait_in_assoc_type)]` annotation.
+Note that you don't need a `#[define_opaque(Foo)]` on the method anymore.
+Complete example:
+
+```
+#![feature(impl_trait_in_assoc_type)]
+
+trait Bar {}
+struct Zap;
+
+impl Bar for Zap {}
+
+trait Baz {
+    type Foo;
+    fn foo() -> Self::Foo;
+}
+
+struct Quux;
+
+impl Baz for Quux {
+    type Foo = impl Bar;
+    fn foo() -> Self::Foo { Zap }
+}
+```