The parsing error returned from pest are not very useful without understanding the grammar details

[mlaveaux]

The pest parse error actually prints the rules that could not be matched at the current location. This means that it contains for example DataExprConj, instead of actually talking about the symbol that must be matched (in this case "&&"). For a user these rules have no meaning and as such that parse errors should become more useful to the end user.

[mlaveaux]

It seems that PEG generally has the limitation of only resolving at which point one of the rules does not apply, which is not always the most helpful. However, we can improve the error messages by actually replacing the rule names by the first terminal of that rule. For example printing && instead of DataExprConj, and this will already significantly help.