make reference maps extensible

This commit makes it possible to add custom handling for unknown reference labels:

use markdown_it::parser::block::builtin::BlockParserRule;
use markdown_it::parser::core::{CoreRule, Root};
use markdown_it::plugins::cmark::block::reference::{ReferenceMap, DefaultReferenceMap, CustomReferenceMap};
use markdown_it::{MarkdownIt, Node};

let md = &mut MarkdownIt::new();
markdown_it::plugins::cmark::add(md);

#[derive(Debug, Default)]
struct RefMapOverride(DefaultReferenceMap);
impl CustomReferenceMap for RefMapOverride {
    fn get(&self, label: &str) -> Option<(&str, Option<&str>)> {
        // override a specific link
        if label == "rust" {
            return Some((
                "https://www.rust-lang.org/",
                Some("The Rust Language"),
            ));
        }

        self.0.get(label)
    }

    fn insert(&mut self, label: String, destination: String, title: Option<String>) -> bool {
        self.0.insert(label, destination, title)
    }
}

struct AddCustomReferences;
impl CoreRule for AddCustomReferences {
    fn run(root: &mut Node, _: &MarkdownIt) {
        let data = root.cast_mut::<Root>().unwrap();
        data.ext.insert(ReferenceMap::new(RefMapOverride::default()));
    }
}

md.add_rule::<AddCustomReferences>()
    .before::<BlockParserRule>();

let html = md.parse("[rust]").render();
assert_eq!(
    html.trim(),
    r#"<p><a href="https://www.rust-lang.org/" title="The Rust Language">rust</a></p>"#
);

Codecov Report

Patch coverage: 93.75% and project coverage change: -0.06 :warning:

Comparison is base (e781c6e) 94.60% compared to head (d0e4314) 94.55%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #17 +/- ## ========================================== - Coverage 94.60% 94.55% -0.06% ========================================== Files 56 56 Lines 2411 2425 +14 ========================================== + Hits 2281 2293 +12 - Misses 130 132 +2 ``` | [Impacted Files](https://app.codecov.io/gh/rlidwka/markdown-it.rs/pull/17?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Kocharin) | Coverage Δ | | |---|---|---| | [src/plugins/cmark/block/reference.rs](https://app.codecov.io/gh/rlidwka/markdown-it.rs/pull/17?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Kocharin#diff-c3JjL3BsdWdpbnMvY21hcmsvYmxvY2svcmVmZXJlbmNlLnJz) | `94.89% <91.30%> (-1.40%)` | :arrow_down: | | [src/generics/inline/full\_link.rs](https://app.codecov.io/gh/rlidwka/markdown-it.rs/pull/17?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Kocharin#diff-c3JjL2dlbmVyaWNzL2lubGluZS9mdWxsX2xpbmsucnM=) | `98.42% <100.00%> (-0.03%)` | :arrow_down: |

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markdown-it-rust / markdown-it

make reference maps extensible #17

Codecov Report