To maximize the run-time performance, programmers specialize their code by hand, replacing library collections and containers by custom objects in which the data is restructured for efficient access. However, this is a tedious and error-prone process that makes it hard to test, maintain and evolve the source code.

We present an automated and composable approach that allows programmers to safely change the data representation in delimited scopes containing anything from expressions to entire class definitions. The transformation itself is defined by programmers and can cover a wide range of use cases.

Our technique leverages the type system in order to infer where the data representation needs to be converted, while offering a strong correctness guarantee on the interaction with other object-oriented language features, such as dynamic dispatch, inheritance and generics.

We have embedded this technique in a Scala compiler plugin and used it in four very different transformations, ranging from improving the data layout and encoding, to retrofitting specialization and value class status, and to collection deforestation. These transformations obtain speedups between 1.9 and 13x on user programs.