Feature request: get versions of dependencies

[BatmanAoD]

First, thank you for building this; it was very easy to get working and has lots of nice features.

It would be nice to have a way to emit the versions of specific Cargo dependencies; I don't know if that's possible without actually parsing the Cargo lockfile, though.

[BatmanAoD]

Okay, this is reasonably easy with cargo_metadata.

    let resolved_crates = MetadataCommand::new().exec()?.resolve.ok_or_else(|| anyhow!("could not load Cargo metadata"))?;
    let root_id = resolved_crates.root.ok_or_else(|| anyhow!("cargo metadata missing '.resolved.root'"))?;
    let root = resolved_crates
        .nodes
        .into_iter()
        .find(|node| node.id == root_id)
        .ok_or_else(|| anyhow!("cargo metadata missing root node in '.resolved.nodes'"))?;
    let results = root.dependencies.into_iter().filter_map(|dep| {
        let id = dep.to_string();
        if <matches> {
            Some(id)
        } else {
            None
        }
    });
    let mut deps_writer = BufWriter::new(Vec::new());
    serde_json::Serializer::new(&mut deps_writer)
        .collect_seq(results)?;
    println!(
        "cargo:rustc-env=QCS_DEPENDENCIES={}",
        String::from_utf8(deps_writer.into_inner()?)?
    );

The Vergen API could take, as an argument, a filter predicate to determine which dependencies to include.

[BatmanAoD]

This is great! Thanks!

One minor documentation note: it wasn't immediately clear to me that cargo_dependencies_name_filter doesn't do anything unless you also call cargo_dependencies(), so that may be worth mentioning in the doc comment.