subscriber: add ChronoUtc and ChronoLocal timers, RFC 3339 support (#387)

* subscriber: add ChronoUtc and ChronoLocal timers
* subscriber: ChronoLocal::rfc3339,ChronoUtc::rfc3339
* subscriber: make with_format constructor for ChronoUtc, ChronoLocal

tracing-subscriber/src/fmt/format.rs

@@ -141,7 +141,17 @@ impl<F, T> Format<F, T> {
         }
     }
 
-    /// Use the given `timer` for log message timestamps.
+    /// Use the given [`timer`] for log message timestamps.
+    ///
+    /// See [`time`] for the provided timer implementations.
+    ///
+    /// Note that using the `chrono` feature flag enables the
+    /// additional time formatters [`ChronoUtc`] and [`ChronoLocal`].
+    ///
+    /// [`time`]: ./time/index.html
+    /// [`timer`]: ./time/trait.FormatTime.html
+    /// [`ChronoUtc`]: ./time/struct.ChronoUtc.html
+    /// [`ChronoLocal`]: ./time/struct.ChronoLocal.html
     pub fn with_timer<T2>(self, timer: T2) -> Format<F, T2> {
         Format {
             format: self.format,

tracing-subscriber/src/fmt/mod.rs

@@ -457,7 +457,17 @@ impl<N, L, T, F, W> Builder<N, format::Format<L, T>, F, W>
 where
     N: for<'writer> FormatFields<'writer> + 'static,
 {
-    /// Use the given `timer` for log message timestamps.
+    /// Use the given [`timer`] for log message timestamps.
+    ///
+    /// See [`time`] for the provided timer implementations.
+    ///
+    /// Note that using the `chrono` feature flag enables the
+    /// additional time formatters [`ChronoUtc`] and [`ChronoLocal`].
+    ///
+    /// [`time`]: ./time/index.html
+    /// [`timer`]: ./time/trait.FormatTime.html
+    /// [`ChronoUtc`]: ./time/struct.ChronoUtc.html
+    /// [`ChronoLocal`]: ./time/struct.ChronoLocal.html
     pub fn with_timer<T2>(self, timer: T2) -> Builder<N, format::Format<L, T2>, F, W> {
         Builder {
             fmt_fields: self.fmt_fields,

tracing-subscriber/src/fmt/time.rs

@@ -17,6 +17,10 @@ use std::time::Instant;
 /// current time as reported by `std::time::SystemTime`, and the latter does not print the current
 /// time at all. `FormatTime` is also automatically implemented for any function pointer with the
 /// appropriate signature.
+///
+/// The full list of provided implementations can be found in [`time`].
+///
+/// [`time`]: ./index.html
 pub trait FormatTime {
     /// Measure and write out the current time.
     ///
@@ -96,6 +100,111 @@ impl FormatTime for Uptime {
     }
 }
 
+/// The RFC 3339 format is used by default and using
+/// this struct allows chrono to bypass the parsing
+/// used when a custom format string is provided
+#[cfg(feature = "chrono")]
+#[derive(Debug, Clone, Eq, PartialEq)]
+enum ChronoFmtType {
+    Rfc3339,
+    Custom(String),
+}
+
+#[cfg(feature = "chrono")]
+impl Default for ChronoFmtType {
+    fn default() -> Self {
+        ChronoFmtType::Rfc3339
+    }
+}
+
+/// Retrieve and print the current UTC time.
+#[cfg(feature = "chrono")]
+#[derive(Debug, Clone, Eq, PartialEq, Default)]
+pub struct ChronoUtc {
+    format: ChronoFmtType,
+}
+
+#[cfg(feature = "chrono")]
+impl ChronoUtc {
+    /// Format the time using the [`RFC 3339`] format
+    /// (a subset of [`ISO 8601`]).
+    ///
+    /// [`RFC 3339`]: https://tools.ietf.org/html/rfc3339
+    /// [`ISO 8601`]: https://en.wikipedia.org/wiki/ISO_8601
+    pub fn rfc3339() -> Self {
+        ChronoUtc {
+            format: ChronoFmtType::Rfc3339,
+        }
+    }
+
+    /// Format the time using the given format string.
+    ///
+    /// See [`chrono::format::strftime`]
+    /// for details on the supported syntax.
+    ///
+    /// [`chrono::format::strftime`]: https://docs.rs/chrono/0.4.9/chrono/format/strftime/index.html
+    pub fn with_format(format_string: String) -> Self {
+        ChronoUtc {
+            format: ChronoFmtType::Custom(format_string),
+        }
+    }
+}
+
+#[cfg(feature = "chrono")]
+impl FormatTime for ChronoUtc {
+    fn format_time(&self, w: &mut dyn fmt::Write) -> fmt::Result {
+        let time = chrono::Utc::now();
+        match self.format {
+            ChronoFmtType::Rfc3339 => write!(w, "{} ", time.to_rfc3339()),
+            ChronoFmtType::Custom(ref format_str) => write!(w, "{} ", time.format(format_str)),
+        }
+    }
+}
+
+/// Retrieve and print the current local time.
+#[cfg(feature = "chrono")]
+#[derive(Debug, Clone, Eq, PartialEq, Default)]
+pub struct ChronoLocal {
+    format: ChronoFmtType,
+}
+
+#[cfg(feature = "chrono")]
+impl ChronoLocal {
+    /// Format the time using the [`RFC 3339`] format
+    /// (a subset of [`ISO 8601`]).
+    ///
+    /// [`RFC 3339`]: https://tools.ietf.org/html/rfc3339
+    /// [`ISO 8601`]: https://en.wikipedia.org/wiki/ISO_8601
+    pub fn rfc3339() -> Self {
+        ChronoLocal {
+            format: ChronoFmtType::Rfc3339,
+        }
+    }
+
+    /// Format the time using the given format string.
+    ///
+    /// See [`chrono::format::strftime`]
+    /// for details on the supported syntax.
+    ///
+    /// [`chrono::format::strftime`]: https://docs.rs/chrono/0.4.9/chrono/format/strftime/index.html
+    pub fn with_format(format_string: String) -> Self {
+        ChronoLocal {
+            format: ChronoFmtType::Custom(format_string),
+        }
+    }
+}
+
+#[cfg(feature = "chrono")]
+impl FormatTime for ChronoLocal {
+    fn format_time(&self, w: &mut dyn fmt::Write) -> fmt::Result {
+        let time = chrono::Local::now();
+        match self.format {
+            ChronoFmtType::Rfc3339 => write!(w, "{} ", time.to_rfc3339()),
+            ChronoFmtType::Custom(ref format_str) => write!(w, "{} ", time.format(format_str)),
+        }
+    }
+}
+
 #[inline(always)]
 #[cfg(feature = "ansi")]
 pub(crate) fn write<T>(timer: T, writer: &mut dyn fmt::Write, with_ansi: bool) -> fmt::Result