Why is my code returning an empty Rule?

[luisvgs]

Hi, I'm currently playing around with the library, and for testing purposes my grammar only consists of a String.

program = _{ SOI ~ "\n"* ~ (Str ~ "\n"+) * ~ Str ? ~ EOI }
Str = @{ "'" ~ ( "''" | (!"'" ~ ANY) )* ~ "'" }

With the following code:

extern crate pest;
#[macro_use]
extern crate pest_derive;

use pest::error::Error;
use pest::iterators::Pair;
use pest::Parser;

#[derive(Parser)]
#[grammar = "conway.pest"]
struct ConwayParser;

#[derive(PartialEq, Debug, Clone)]
pub enum AstNode {
    Str(String),
}

pub fn string_parser(string_node: Pair<Rule>) -> AstNode {
    let mut string = String::new();

    for node in string_node.clone().into_inner() {
        match node.as_rule() {
            Rule::Str => string = String::from(node.to_string()),
            _ => unreachable!()
        }
    }
    AstNode::Str(string)
}
pub fn build_ast_from_expr(pair: pest::iterators::Pair<Rule>) -> AstNode {
    match pair.as_rule() {
        Rule::Str => string_parser(pair),
        unknown => panic!("Unknown expr: {:?}", unknown),
    }
}

pub fn parse(source: &str) -> Result<Vec<AstNode>, Error<Rule>> {
    let mut ast = vec![];

    let pairs = ConwayParser::parse(Rule::program, source)?;
    for pair in pairs {
        match pair.as_rule() {
            Rule::Str => {
                ast.push(build_ast_from_expr(pair));
            }
            _ => {}
        }
    }

    Ok(ast)
}

fn main() {
    let unparsed_file = std::fs::read_to_string("conway.cy")
        .expect("cannot read conway file");
    let astnode = parse(&unparsed_file).expect("unsuccessful parse");
    println!("{:?}", &astnode);
}

However, instead of returning [Str("Hello world")] when I run my code it returns just an empty Rule:

[Str("")]

Also, although the code runs, rust-analyzer complains that the proc-macro server crashed, not allowing the Parser macro to expand. Whether or not I enable such feature, the warning as well as the code output is the same. I suspect this might be the root of the problem? Any advice would be deeply appreciated.

[rob-mur]

I've created a repo which replicates your issue and then gives a hint of where to go. https://github.com/rob-mur/pest-521

If you run this code you'll see that it prints out the string node you are trying to parse, which you will see has no inners. This means that string_node.clone().into_inner() is empty and so line 21-26 are essentially not called, leading to the empty string being returned.

After you work it out, I recommend using https://crates.io/crates/pest_consume to avoid this kinds of errors in future. Also, reading https://pest.rs/book/grammars/syntax.html to try and simplify your grammar as there is built in functionality to deal with whitespace.

Shout if you are still stuck.