Migrate to core::error::Error

Stabilized in rust 1.81, behind a feature flag so
as to not bump the current MSRV of 1.61.

Author: benbrittain

Cargo.toml

@@ -25,6 +25,7 @@ winapi = ["windows-link"]
 std = ["alloc"]
 clock = ["winapi", "iana-time-zone", "android-tzdata", "now"]
 now = ["std"]
+core_error = []
 oldtime = []
 wasmbind = ["wasm-bindgen", "js-sys"]
 unstable-locales = ["pure-rust-locales"]

src/format/mod.rs

@@ -33,6 +33,8 @@
 
 #[cfg(all(feature = "alloc", not(feature = "std"), not(test)))]
 use alloc::boxed::Box;
+#[cfg(all(feature = "core_error", not(feature = "std")))]
+use core::error::Error;
 use core::fmt;
 use core::str::FromStr;
 #[cfg(feature = "std")]
@@ -450,7 +452,7 @@ impl fmt::Display for ParseError {
     }
 }
 
-#[cfg(feature = "std")]
+#[cfg(any(feature = "core_error", feature = "std"))]
 impl Error for ParseError {
     #[allow(deprecated)]
     fn description(&self) -> &str {

src/lib.rs

@@ -512,8 +512,8 @@
 extern crate alloc;
 
 mod time_delta;
-#[cfg(feature = "std")]
 #[doc(no_inline)]
+#[cfg(any(feature = "std", feature = "core_error"))]
 pub use time_delta::OutOfRangeError;
 pub use time_delta::TimeDelta;
 
@@ -690,6 +690,9 @@ impl fmt::Debug for OutOfRange {
 #[cfg(feature = "std")]
 impl std::error::Error for OutOfRange {}
 
+#[cfg(all(not(feature = "std"), feature = "core_error"))]
+impl core::error::Error for OutOfRange {}
+
 /// Workaround because `?` is not (yet) available in const context.
 #[macro_export]
 #[doc(hidden)]

src/month.rs

@@ -272,6 +272,9 @@ pub struct ParseMonthError {
 #[cfg(feature = "std")]
 impl std::error::Error for ParseMonthError {}
 
+#[cfg(all(not(feature = "std"), feature = "core_error"))]
+impl core::error::Error for ParseMonthError {}
+
 impl fmt::Display for ParseMonthError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "ParseMonthError {{ .. }}")

src/round.rs

@@ -109,7 +109,11 @@ pub trait DurationRound: Sized {
     type Err: std::error::Error;
 
     /// Error that can occur in rounding or truncating
-    #[cfg(not(feature = "std"))]
+    #[cfg(all(not(feature = "std"), feature = "core_error"))]
+    type Err: core::error::Error;
+
+    /// Error that can occur in rounding or truncating
+    #[cfg(all(not(feature = "std"), not(feature = "core_error")))]
     type Err: fmt::Debug + fmt::Display;
 
     /// Return a copy rounded by TimeDelta.
@@ -362,6 +366,14 @@ impl std::error::Error for RoundingError {
     }
 }
 
+#[cfg(all(not(feature = "std"), feature = "core_error"))]
+impl core::error::Error for RoundingError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "error from rounding or truncating with DurationRound"
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::{DurationRound, RoundingError, SubsecRound, TimeDelta};

src/time_delta.rs

@@ -10,6 +10,8 @@
 
 //! Temporal quantification
 
+#[cfg(all(not(feature = "std"), feature = "core_error"))]
+use core::error::Error;
 use core::fmt;
 use core::ops::{Add, AddAssign, Div, Mul, Neg, Sub, SubAssign};
 use core::time::Duration;
@@ -630,7 +632,7 @@ impl fmt::Display for OutOfRangeError {
     }
 }
 
-#[cfg(feature = "std")]
+#[cfg(any(feature = "std", feature = "core_error"))]
 impl Error for OutOfRangeError {
     #[allow(deprecated)]
     fn description(&self) -> &str {

src/weekday.rs

@@ -238,6 +238,9 @@ pub struct ParseWeekdayError {
     pub(crate) _dummy: (),
 }
 
+#[cfg(all(not(feature = "std"), feature = "core_error"))]
+impl core::error::Error for ParseWeekdayError {}
+
 #[cfg(feature = "std")]
 impl std::error::Error for ParseWeekdayError {}