Looking at the Go AST documentation, I don't see any node type to represent a nil in the code. BasicLit doesn't cover it in the list of token types it accepts, and in fact when I look at the source to go/token I don't see a nil token at all!
If I want to create an AST of the form if x != nil, how do I do the nil?
As per the Go specification, nil is a predeclared identifier. Therefore, it is represented by *ast.Ident in an AST:
node := ast.NewIdent("nil")