Stable syntax extensions are part of the plan for Rust over the next year (see the “Plugins” section). @nrc is doing most of the work here, and the first steps are the “HIR” (RFC/design doc, tracking issue):
Currently the AST is used by libsyntax for syntactic operations, by the compiler
for pretty much everything, and in syntax extensions. I propose splitting the
AST into a libsyntax version that is specialised for syntactic operation and
will eventually be stabilised for use by syntax extensions and tools, and the
HIR which is entirely internal to the compiler.
I guess RFC issue 440 somewhat serves as a tracking issue for syntax extensions themselves.
