Updates release notes for PR #3752

.release-notes/next-release.md

@@ -113,3 +113,59 @@ use @printf[I32](fmt: Pointer[U8] tag, ...)
 
 FFI declarations are visible to an entire package, so you don't need to add type signatures to all Pony files.
 
+## Change the return type of String.add to String iso^ (RFC 69)
+
+This release introduces a breaking change by changing the return type of `String.add` from `String val` to `String iso^`.
+
+Where you previously had code like:
+
+```pony
+let c = Circle
+let str = "Radius: " + c.get_radius().string() + "\n"
+env.out.print(str)
+```
+
+you now need:
+
+```pony
+let c = Circle
+let str = recover val "Radius: " + c.get_radius().string() + "\n" end
+env.out.print(str)
+```
+
+or you can also let the compiler do the work for you by using explicit type declarations:
+
+```pony
+let c = Circle
+let str: String = "Radius: " + c.get_radius().string() + "\n"
+env.out.print(str)
+```
+
+The above code works since `val` is the default reference capability of the `String` type.
+
+The new type makes it simpler to implement the `Stringable` interface by using `String.add`. Where before you had code like:
+
+```pony
+class MyClass is Stringable
+  let var1: String = "hello"
+  let var2: String = " world"
+
+  fun string(): String iso^ =>
+    recover
+      String.create(var1.size() + var1.size())
+        .>append(var1)
+        .>append(var2)
+    end
+```
+
+you can now implement the `string` method as such:
+
+```pony
+class MyClass is Stringable
+  let var1: String = "hello"
+  let var2: String = " world"
+
+  fun string(): String iso^ =>
+    var1 + var2
+```
+