[tracing-subscriber]: add chrono crate implementations of FormatTime

tracing-subscriber/Cargo.toml

@@ -50,6 +50,7 @@ once_cell = { optional = true, version = "1.13.0" }
 tracing-log = { path = "../tracing-log", version = "0.2", optional = true, default-features = false, features = ["log-tracer", "std"] }
 nu-ansi-term = { version = "0.46.0", optional = true }
 time = { version = "0.3.2", features = ["formatting"], optional = true }
+chrono = { version = "0.4.26" }
 
 # only required by the json feature
 serde_json = { version = "1.0.82", optional = true }

tracing-subscriber/src/fmt/time/chrono_crate.rs

@@ -0,0 +1,57 @@
+use crate::fmt::format::Writer;
+use crate::fmt::time::FormatTime;
+
+/// Formats [local time]s and [UTC time]s with [formatter] implementations
+/// that use the [`chrono` crate].
+///
+/// [local time]: https://docs.rs/chrono/0.4.26/chrono/offset/struct.Local.html
+/// [UTC time]: https://docs.rs/chrono/0.4.26/chrono/offset/struct.Utc.html
+/// [`chrono` crate]: https://docs.rs/chrono/0.4.26/chrono/
+/// [formatter]: https://docs.rs/time/0.3/time/formatting/trait.Formattable.html
+
+/// Tag-type (indicating UTC timezone) enabling static dispatch
+/// to `chrono::Local` functions.
+#[derive(Debug)]
+pub struct LocalTime;
+
+impl FormatTime for LocalTime {
+    fn format_time(&self, w: &mut Writer<'_>) -> alloc::fmt::Result {
+        w.write_str(&chrono::Local::now().to_rfc3339())
+    }
+}
+
+/// Tag-type (indicating the "local" timezone) enabling static
+/// dispatch to `chrono::Utc` functions.
+#[derive(Debug)]
+pub struct Utc;
+
+impl FormatTime for Utc {
+    fn format_time(&self, w: &mut Writer<'_>) -> alloc::fmt::Result {
+        w.write_str(&chrono::Utc::now().to_rfc3339())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::fmt::format::Writer;
+    use crate::fmt::time::FormatTime;
+
+    use super::LocalTime;
+    use super::Utc;
+
+    #[test]
+    fn test_chrono_format_time_utc() {
+        let mut buf = String::new();
+        let mut dst: Writer<'_> = Writer::new(&mut buf);
+        assert!(FormatTime::format_time(&Utc, &mut dst).is_ok());
+        // e.g. `buf` contains "2023-08-18T19:05:08.662499+00:00"
+    }
+
+    #[test]
+    fn test_chrono_format_time_local() {
+        let mut buf = String::new();
+        let mut dst: Writer<'_> = Writer::new(&mut buf);
+        assert!(FormatTime::format_time(&LocalTime, &mut dst).is_ok());
+        // e.g. `buf` contains "2023-08-18T14:59:08.662499-04:00".
+    }
+}

tracing-subscriber/src/fmt/time/mod.rs

@@ -15,6 +15,9 @@ pub use time_crate::UtcTime;
 #[cfg_attr(docsrs, doc(cfg(all(unsound_local_offset, feature = "local-time"))))]
 pub use time_crate::LocalTime;
 
+/// [`Chrono`]-based implementation for time.
+pub mod chrono_crate;
+
 /// A type that can measure and format the current time.
 ///
 /// This trait is used by `Format` to include a timestamp with each `Event` when it is logged.