From the article: It’s pointing out that we are pushing into this vector which needs references into “the AST”, but we haven’t declared in our sign

From the article: It’s pointing out that we are pushing into this vector which needs references into “the AST”, but we haven’t declared in our signature that the ast::Ty must actually from “the AST”. I think someone needs to write a typechecker for English to help out the author of this sentence.