Right-recursive productions Fail to Match

[amascolo]

Looks related to nullable productions: #108 #112 #117

This grammar fails to match when we change a production from left-recursive to right-recursive.

The left-recursive grammar failed on 0.4.0 ≤ bnf ≤ 0.4.1 – the right-recursive still fails on 0.4.0 ≤ bnf ≤ 0.4.3

So this issue must have been partially addressed by @CrockAgile in #109 (0.4.2)

Happy to help battle-testing any proposed fixes ⚔️

use bnf::Grammar;

#[test]
fn test() {
    let input = "aa a";

    let left_recursive: &str = "
    <conjunction> ::= <conjunction> <ws> <predicate> | <predicate>
    <predicate> ::= <string_null_one> | <special-string> '.'

    <string_null_one> ::= <string_null_two>
    <string_null_two> ::= <string_null_three>
    <string_null_three> ::= <string>

    <string> ::= <char_null> | <string> <char_null>
    <special-string> ::= <special-char> | <special-string> <special-char>

    <char_null> ::= <char>
    <char> ::= 'a'

    <special-char> ::= <char_null> | <whitespace>
    <whitespace> ::= ' '

    <ws> ::= ' ' | ' ' <ws>
    ";
    assert!(left_recursive.parse::<Grammar>().unwrap().parse_input(input).next().is_some());

    let right_recursive = left_recursive.replace(
        // rewrite production from left- to right- recursive
        "<string> ::= <char_null> | <string> <char_null>",
        "<string> ::= <char_null> | <char_null> <string>",
    );
    assert!(
        right_recursive.parse::<Grammar>().unwrap().parse_input(input).next().is_some(),
        "right-recursive grammar failed to parse: {input}"
    );
}