I just got the error failed to find an implementation of interface middle::typeck::ast_conv for middle::typeck::crate_ctxt. It took me a second to figure out that my mistake was leaving off the @ on crate_ctxt. It would be nice if, in this case, the typechecker gave a hint like "did you mean @crate_ctxt?", in the case where there is an implementation of ast_conv for @crate_ctxt.