Add impls for the Defmt logging crate

- Add impls for the main structs where it is obvious how to do so
- Gate this behind a new cargo feature called "defmt"

Cargo.toml

@@ -28,6 +28,7 @@ now = ["std"]
 oldtime = []
 wasmbind = ["wasm-bindgen", "js-sys"]
 unstable-locales = ["pure-rust-locales"]
+defmt = []
 # Note that rkyv-16, rkyv-32, and rkyv-64 are mutually exclusive.
 rkyv = ["dep:rkyv", "rkyv/size_32"]
 rkyv-16 = ["dep:rkyv", "rkyv?/size_16"]
@@ -43,6 +44,7 @@ serde = { version = "1.0.99", default-features = false, optional = true }
 pure-rust-locales = { version = "0.8", optional = true }
 rkyv = { version = "0.7.43", optional = true, default-features = false }
 arbitrary = { version = "1.0.0", features = ["derive"], optional = true }
+defmt = "0.3.8"
 
 [target.'cfg(all(target_arch = "wasm32", not(any(target_os = "emscripten", target_os = "wasi"))))'.dependencies]
 wasm-bindgen = { version = "0.2", optional = true }

src/datetime/mod.rs

@@ -3,17 +3,6 @@
 
 //! ISO 8601 date and time with time zone.
 
-#[cfg(all(feature = "alloc", not(feature = "std"), not(test)))]
-use alloc::string::String;
-use core::borrow::Borrow;
-use core::cmp::Ordering;
-use core::fmt::Write;
-use core::ops::{Add, AddAssign, Sub, SubAssign};
-use core::time::Duration;
-use core::{fmt, hash, str};
-#[cfg(feature = "std")]
-use std::time::{SystemTime, UNIX_EPOCH};
-
 #[cfg(all(feature = "unstable-locales", feature = "alloc"))]
 use crate::format::Locale;
 use crate::format::{
@@ -30,6 +19,18 @@ use crate::offset::{FixedOffset, LocalResult, Offset, TimeZone, Utc};
 use crate::Date;
 use crate::{expect, try_opt};
 use crate::{Datelike, Months, TimeDelta, Timelike, Weekday};
+#[cfg(all(feature = "alloc", not(feature = "std"), not(test)))]
+use alloc::string::String;
+use core::borrow::Borrow;
+use core::cmp::Ordering;
+use core::fmt::Write;
+use core::ops::{Add, AddAssign, Sub, SubAssign};
+use core::time::Duration;
+use core::{fmt, hash, str};
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
+#[cfg(feature = "std")]
+use std::time::{SystemTime, UNIX_EPOCH};
 
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
@@ -1093,6 +1094,13 @@ impl DateTime<FixedOffset> {
     }
 }
 
+#[cfg(feature = "defmt")]
+impl<Tz: TimeZone> Format for DateTime<Tz> {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "{=i32:04}-{=u32:02}-{=u32:02}", self.year(), self.month(), self.day());
+    }
+}
+
 impl<Tz: TimeZone> DateTime<Tz>
 where
     Tz::Offset: fmt::Display,

src/lib.rs

@@ -521,6 +521,8 @@ pub use time_delta::TimeDelta;
 pub type Duration = TimeDelta;
 
 use core::fmt;
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 
 /// A convenience module appropriate for glob imports (`use chrono::prelude::*;`).
 pub mod prelude {
@@ -678,6 +680,13 @@ impl fmt::Display for OutOfRange {
     }
 }
 
+#[cfg(feature = "defmt")]
+impl Format for OutOfRange {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "out of range");
+    }
+}
+
 impl fmt::Debug for OutOfRange {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "out of range")

src/month.rs

@@ -1,5 +1,6 @@
 use core::fmt;
-
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
 
@@ -240,6 +241,13 @@ impl Months {
     }
 }
 
+#[cfg(feature = "defmt")]
+impl Format for Months {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "Months({=u32})", self.as_u32());
+    }
+}
+
 /// An error resulting from reading `<Month>` value with `FromStr`.
 #[derive(Clone, PartialEq, Eq)]
 pub struct ParseMonthError {

src/naive/date/mod.rs

@@ -19,7 +19,8 @@ use core::iter::FusedIterator;
 use core::num::NonZeroI32;
 use core::ops::{Add, AddAssign, Sub, SubAssign};
 use core::{fmt, str};
-
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
 
@@ -2238,6 +2239,13 @@ impl fmt::Debug for NaiveDate {
     }
 }
 
+#[cfg(feature = "defmt")]
+impl Format for NaiveDate {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "{=i32:04}-{=u32:02}-{=u32:02}", self.year(), self.month(), self.day());
+    }
+}
+
 /// The `Display` output of the naive date `d` is the same as
 /// [`d.format("%Y-%m-%d")`](crate::format::strftime).
 ///

src/naive/datetime/mod.rs

@@ -9,7 +9,8 @@ use core::fmt::Write;
 use core::ops::{Add, AddAssign, Sub, SubAssign};
 use core::time::Duration;
 use core::{fmt, str};
-
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
 
@@ -2149,3 +2150,10 @@ impl Default for NaiveDateTime {
         Self::UNIX_EPOCH
     }
 }
+
+#[cfg(feature = "defmt")]
+impl Format for NaiveDateTime {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "{}T{}", self.date(), self.time());
+    }
+}

src/naive/time/mod.rs

@@ -8,7 +8,8 @@ use core::borrow::Borrow;
 use core::ops::{Add, AddAssign, Sub, SubAssign};
 use core::time::Duration;
 use core::{fmt, str};
-
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
 
@@ -1641,3 +1642,10 @@ impl Default for NaiveTime {
         NaiveTime::from_hms_opt(0, 0, 0).unwrap()
     }
 }
+
+#[cfg(feature = "defmt")]
+impl Format for NaiveTime {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "{=u32}:{=u32}:{=u32}", self.hour(), self.minute(), self.second());
+    }
+}

src/offset/fixed.rs

@@ -5,7 +5,8 @@
 
 use core::fmt;
 use core::str::FromStr;
-
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
 
@@ -152,6 +153,13 @@ impl Offset for FixedOffset {
     }
 }
 
+#[cfg(feature = "defmt")]
+impl Format for FixedOffset {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "FixedOffset({=i32})", self.local_minus_utc());
+    }
+}
+
 impl fmt::Debug for FixedOffset {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let offset = self.local_minus_utc;

src/offset/utc.rs

@@ -4,6 +4,10 @@
 //! The UTC (Coordinated Universal Time) time zone.
 
 use core::fmt;
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
+#[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
+use rkyv::{Archive, Deserialize, Serialize};
 #[cfg(all(
     feature = "now",
     not(all(
@@ -14,9 +18,6 @@ use core::fmt;
 ))]
 use std::time::{SystemTime, UNIX_EPOCH};
 
-#[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
-use rkyv::{Archive, Deserialize, Serialize};
-
 use super::{FixedOffset, MappedLocalTime, Offset, TimeZone};
 use crate::naive::{NaiveDate, NaiveDateTime};
 #[cfg(feature = "now")]
@@ -150,3 +151,10 @@ impl fmt::Display for Utc {
         write!(f, "UTC")
     }
 }
+
+#[cfg(feature = "defmt")]
+impl Format for Utc {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(fmt, "UTC");
+    }
+}

src/time_delta.rs

@@ -10,14 +10,15 @@
 
 //! Temporal quantification
 
+use crate::{expect, try_opt};
 use core::fmt;
 use core::ops::{Add, AddAssign, Div, Mul, Neg, Sub, SubAssign};
 use core::time::Duration;
+#[cfg(feature = "defmt")]
+use defmt::{Format, Formatter};
 #[cfg(feature = "std")]
 use std::error::Error;
 
-use crate::{expect, try_opt};
-
 #[cfg(any(feature = "rkyv", feature = "rkyv-16", feature = "rkyv-32", feature = "rkyv-64"))]
 use rkyv::{Archive, Deserialize, Serialize};
 
@@ -593,6 +594,18 @@ impl fmt::Display for TimeDelta {
     }
 }
 
+#[cfg(feature = "defmt")]
+impl Format for TimeDelta {
+    fn format(&self, fmt: Formatter) {
+        defmt::write!(
+            fmt,
+            "TimeDelta {{ secs: {=i64}, nanos: {=i32} }}",
+            self.num_seconds(),
+            self.subsec_nanos()
+        );
+    }
+}
+
 /// Represents error when converting `TimeDelta` to/from a standard library
 /// implementation
 ///