xml_borrowing_problem.rs

extern crate sxd_document;
extern crate sxd_xpath;

use std::error::Error;
use sxd_document::parser;
use sxd_xpath::evaluate_xpath;
use sxd_xpath::Value;
use sxd_document::dom::Document;

pub struct XmlFixDictionary<'a> {
    length_tags: Vec<&'a [u8]>,
}

impl <'a> XmlFixDictionary<'a> {
    pub fn read(document: &'a Document<'a>) -> Result<XmlFixDictionary<'a>, Box<Error>> {
        let length_tags = Self::read_length_tags(document);
        Ok(XmlFixDictionary { length_tags })
    }

    pub fn read_length_tags(document: &'a Document<'a>) -> Vec<&'a [u8]> {
        let mut length_tags = vec![];

        let fields = evaluate_xpath(document, "/fix/fields/field");
        if let Ok(Value::Nodeset(nodes)) = fields {
            for node in nodes {
                let el = node.element().unwrap();
                let field_type = el.attribute_value("type").unwrap();
                if field_type.to_lowercase() == "length" {
                    let field_name = el.attribute_value("name").unwrap();
                    let field_number = el.attribute_value("number").unwrap();
                    println!("{} : {} = {}", field_name, field_type, field_number);
                    length_tags.push(field_number.as_bytes());
                }
            }
        }
        length_tags
    }

    pub fn is_length_tag(&self, tag: &[u8]) -> bool {
        self.length_tags.contains(&tag)
    }
}

fn main() -> Result<(), Box<Error>> {
    let xml = r#"<fix><fields><field name="blah" number="1" type="LENGTH"></field><field name="blahblah" number="12" type="STRING"></field></fields></fix>"#;
    let package = parser::parse(xml)?;
    let document = package.as_document();
    let fix_dict = XmlFixDictionary::read(&document)?;
    assert!(fix_dict.is_length_tag(b"1"));
    assert!(!fix_dict.is_length_tag(b"12"));
    assert!(!fix_dict.is_length_tag(b"120"));
    Ok(())
}

this doesn't compile

    error[E0597]: `package` does not live long enough
    --> examples\xml_example.rs:15:20
    |
    15 |     let document = package.as_document();
    |                    ^^^^^^^ borrowed value does not live long enough
    ...
    33 | }
    | - borrowed value only lives until here
    |
    note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 13:1...
    --> examples\xml_example.rs:13:1
    |
    13 | / pub fn parse1(xml: &str) -> Result<LengthTags, Box<Error>> {
    14 | |     let package = parser::parse(xml)?;
    15 | |     let document = package.as_document();
    16 | | 
    ...  |
    32 | |     Ok(length_tags)
    33 | | }
    | |_^

    error[E0597]: `document` does not live long enough
    --> examples\xml_example.rs:19:34
    |
    19 |     let fields = evaluate_xpath(&document, "/fix/fields/field")?;
    |                                  ^^^^^^^^ borrowed value does not live long enough
    ...
    33 | }
    | - borrowed value only lives until here
    |
    note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 13:1...
    --> examples\xml_example.rs:13:1
    |
    13 | / pub fn parse1(xml: &str) -> Result<LengthTags, Box<Error>> {
    14 | |     let package = parser::parse(xml)?;
    15 | |     let document = package.as_document();
    16 | | 
    ...  |
    32 | |     Ok(length_tags)
    33 | | }
    | |_^

    error: aborting due to 2 previous errors

    For more information about this error, try `rustc --explain E0597`.