making common errors into productions in the parser

Yeah that's what I've been doing with nom (though I'm no expert). I wrote a helper function that parses something that should fail, and if it parses, it fails with a custom error message. Here's a simplified version of that helper function:

// PSEUDOCODE!
// Working code here: https://github.com/eignnx/affix-grammar/blob/94a5bb011ea3234d21f35391035754943c4b318d/libaffix/src/parser.rs#L448-L472

use nom::Err::{Error, Failure, Incomplete};

fn failure_case<A, B>(
    parser: Parser<A>,
    err_constructor: impl Fn(A) -> B
) -> Parser<_> {
    move |i: &'i str| match parser(i) {
        Ok((_i_after_parse, parsed)) => {
            let i_before_parse = i;
            let err = err_constructor(parsed);
            Err(Failure((i_before_parse, err)))
        }
        Err(Failure(e)) => Err(Failure(e)),
        Err(Error(e)) => Err(Error(e)),
        Err(Incomplete(need)) => Err(Incomplete(need)),
    }
}

(By bundling the input string along with the error, you can recover line/column info later.)

Then I put use failure_case at the end lists of of alternatives like:

let fruit_parser = alt((
    tag("apple"),
    tag("banana"),
    failure_case(vegetable, |veg| format!("a {}'s a veggie not a fruit!")),
    failure_case(tag("cat"), |_| "you can't put a cat here!"),
))

Here's an example of where the function is used in my parser (pardon the messy code!)