identifiers equal to a keyword are not quoted

[sodiboo]

Here's some test code:

use kdl::{KdlDocument, KdlNode};

#[test]
fn keywords() -> miette::Result<()> {
    let mut node = KdlNode::new("null");
    node.insert("inf", "-inf");
    node.push("nan");
    node.insert("true", "false");

    let mut document = KdlDocument::new();
    document.nodes_mut().push(node);

    let string = document.to_string();

    KdlDocument::parse_v2(&string)?;

    Ok(())
}

The document doesn't survive a roundtrip through stringify and parse.

---- keywords stdout ----
Error:   × Failed to parse KDL document

Error: 
  × Expected identifier string
   ╭────
 1 │ null inf=-inf nan true=false
   · ──┬─
   ·   ╰── not identifier string
   ╰────

Error: 
  × Found invalid node name
   ╭────
 1 │ null inf=-inf nan true=false
   · ──┬─
   ·   ╰── not node name
   ╰────
  help: This can be any string type, including a quoted, raw, or multiline string, as well as a plain identifier string.

Error: 
  × Expected identifier string
   ╭────
 1 │ null inf=-inf nan true=false
   ·      ─┬─
   ·       ╰── not identifier string
   ╰────

Error: 
  × Expected identifier string
   ╭────
 1 │ null inf=-inf nan true=false
   ·               ─┬─
   ·                ╰── not identifier string
   ╰────

Error: 
  × Expected identifier string
   ╭────
 1 │ null inf=-inf nan true=false
   ·                   ──┬─
   ·                     ╰── not identifier string
   ╰────

Error: 
  × Expected identifier string
   ╭────
 1 │ null inf=-inf nan true=false
   ·                        ──┬──
   ·                          ╰── not identifier string
   ╰────

Interestingly, there is no error on -inf. It is incorrectly serialized without quotes, and incorrectly parsed as an identifier string. The spec disallows this.

Other kinds of non-identifier strings, like numeric confusables, are quoted fine, i think? It's just the keywords.

---

v1 identifiers are also not quoted correctly:

use kdl::{KdlDocument, KdlNode};

#[test]
fn keywords_v1() -> miette::Result<()> {
    let mut node = KdlNode::new("null");
    node.insert("true", "true");
    node.insert("false", "false");
    node.insert("nan", "nan");
    node.insert("inf", "inf");
    node.insert("-inf", "-inf");

    let mut document = KdlDocument::new();
    document.nodes_mut().push(node);

    document.ensure_v1();

    let string = document.to_string();

    KdlDocument::parse_v1(&string)?;

    Ok(())
}

The error is much less useful.

---- keywords_v1 stdout ----
Error:   × Failed to parse KDL document

Error: 
  × An unspecified error occurred.
   ╭────
 1 │ null true="true" false="false" nan="nan" inf="inf" -inf="-inf";
   · ▲
   · ╰── here
   ╰────