feat: Add support for Key-Value data in log records

See rust-lang/log#328

Cargo.toml

@@ -52,6 +52,7 @@ color = ["dep:anstream", "dep:anstyle"]
 auto-color = ["color", "anstream/auto"]
 humantime = ["dep:humantime"]
 regex = ["env_filter/regex"]
+unstable-kv = ["log/kv_unstable"]
 
 [dependencies]
 log = { version = "0.4.8", features = ["std"] }

src/fmt/kv.rs

@@ -0,0 +1,47 @@
+use std::io::{self, Write};
+
+use super::Formatter;
+use log::kv::{source::Source, Error, Key, Value, Visitor};
+
+/// Format function for serializing key/value pairs
+///
+/// This function determines how key/value pairs for structured logs are serialized within the default
+/// format.
+pub(crate) type KvFormatFn = dyn Fn(&mut Formatter, &dyn Source) -> io::Result<()> + Sync + Send;
+
+/// Null Key Value Format
+///
+/// This function is intended to be passed to
+/// [`Builder::format_key_values`](crate::Builder::format_key_values).
+///
+/// This key value format simply ignores any key/value fields and doesn't include them in the
+/// output.
+pub fn hidden_kv_format(_formatter: &mut Formatter, _fields: &dyn Source) -> io::Result<()> {
+    Ok(())
+}
+
+/// Default Key Value Format
+///
+/// This function is intended to be passed to
+/// [`Builder::format_key_values`](crate::Builder::format_key_values).
+///
+/// This is the default key/value format. Which uses an "=" as the separator between the key and
+/// value and a " " between each pair.
+///
+/// For example: `ip=127.0.0.1 port=123456 path=/example`
+pub fn default_kv_format(formatter: &mut Formatter, fields: &dyn Source) -> io::Result<()> {
+    fields
+        .visit(&mut DefaultVisitor(formatter))
+        .map_err(|e| io::Error::new(io::ErrorKind::Other, e))
+}
+
+struct DefaultVisitor<'a>(&'a mut Formatter);
+
+impl<'a, 'kvs> Visitor<'kvs> for DefaultVisitor<'a> {
+    fn visit_pair(&mut self, key: Key, value: Value<'kvs>) -> Result<(), Error> {
+        // TODO: add styling
+        // tracing-subscriber uses italic for the key and dimmed for the =
+        write!(self.0, " {}={}", key, value)?;
+        Ok(())
+    }
+}