This paper reformulates type qualifiers as language extensions that can be automatically and reliably composed. Type qualifiers annotate type expressions to introduce new subtyping relations and are powerful enough to detect many kinds of errors. Type qualifiers, as illustrated in our ableC extensible language framework for C, can introduce rich forms of concrete syntax, can generate dynamic checks on data when static checks are infeasible or not appropriate, and inject code that affects the program's behavior, for example for conversions of data or logging.

ableC language extensions to C are implemented as attribute grammar fragments and provide an expressive mechanism for type qualifier implementations to check for additional errors, e.g. dereferences to pointers not qualified by a "nonnull" qualifier, and report custom error messages. Our approach distinguishes language extension users from developers and provides modular analyses to developers to ensure that when users select a set of extensions to use, they will automatically compose to form a working compiler.