This is a simple library that wraps the "jrubyparser" jar created by the JRuby team and used by several Java-based Ruby IDEs. It adds #to_ast to String and the resulting AST (a tree of org.jrubyparser.ast.Node subclasses) has a #to_source method.
This library works from RJB as well, though I have not done much testing there. RJB does not appear to be able to decorate an individual Java class (and cascade to its subclasses) so instead we just open the one RJB proxy superclass.
jrubyparser is at https://github.com/jruby/jruby-parser
Because jrubyparser was created for use by IDEs, it has the unique benefit of being able to round-trip code without losing information. This means constructs normally collapsed by other parsing libraries (like single vs double quotes, HERE docs, whitespace) are preserved and available.
jrubyparser also contains an AST rewriting subsystem that was originally created for experimental refactoring tools in the Eclipse-based Ruby Development Tools project. The rewriter is used in to_source to simply output the source version of a given AST.
require 'to_source' src = "a = 123" ast = src.to_ast puts ast # (RootNode, (NewlineNode, (LocalAsgnNode:a, (FixnumNode)))) src2 = ast.to_source p src2 # "a = 123"