We present a new approach for supporting user-defined type refinements, which augment existing types to specify and check additional invariants of interest to programmers. We provide an expressive language in which users define new refinements and associated type rules. These rules are automatically incorporated by an extensible typechecker during static typechecking of programs. Separately, a soundness checkerautomatically proves that each refinement's type rules ensure the intended invariant, for all possible programs. We have formalized our approach and have instantiated it as a framework for adding new type qualifiers to C programs. We have used this framework to define and automatically prove sound a host of type qualifiers of different sorts, including pos and neg for integers, tainted and untainted for strings, and nonnull and unique for pointers, and we have applied our qualifiers to ensure important invariants on open-source C programs.
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