Use core::error::Error

This allows us to always use the Error rather than only when std feature
is enabled. This was stabilized in Rust 1.81, which is below our current
MSRV.

Author: Mark-Simulacrum

dc/s2n-quic-dc/src/stream/recv/error.rs

@@ -37,7 +37,7 @@ impl fmt::Display for Error {
     }
 }
 
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}
 
 impl IntoEvent<Error> for Error {
     fn into_event(self) -> Error {

dc/s2n-quic-dc/src/stream/send/error.rs

@@ -34,7 +34,7 @@ impl fmt::Display for Error {
     }
 }
 
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}
 
 impl IntoEvent<Error> for Error {
     fn into_event(self) -> Error {

quic/s2n-quic-core/src/application/error.rs

@@ -18,8 +18,7 @@ use core::{fmt, ops};
 #[derive(Copy, Clone, Eq, PartialEq)]
 pub struct Error(VarInt);
 
-#[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}
 
 impl fmt::Debug for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

quic/s2n-quic-core/src/buffer/deque.rs

@@ -225,8 +225,7 @@ impl fmt::Display for FillError {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for FillError {}
+impl core::error::Error for FillError {}
 
 #[cfg(feature = "std")]
 impl From<FillError> for std::io::Error {

quic/s2n-quic-core/src/connection/error.rs

@@ -101,8 +101,7 @@ pub enum Error {
     },
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

quic/s2n-quic-core/src/datagram/default.rs

@@ -49,8 +49,7 @@ pub enum BuilderError {
     ZeroCapacity,
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for BuilderError {}
+impl core::error::Error for BuilderError {}
 
 impl fmt::Display for BuilderError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

quic/s2n-quic-core/src/path/mtu.rs

@@ -241,8 +241,7 @@ impl Display for MtuError {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for MtuError {}
+impl core::error::Error for MtuError {}
 
 /// Information about the path that may be used when generating MTU configuration.
 #[non_exhaustive]

quic/s2n-quic-core/src/query.rs

@@ -132,5 +132,4 @@ impl core::fmt::Display for Error {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}

quic/s2n-quic-core/src/stream/error.rs

@@ -68,8 +68,7 @@ pub enum StreamError {
     },
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for StreamError {}
+impl core::error::Error for StreamError {}
 
 impl fmt::Display for StreamError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

quic/s2n-quic-core/src/transport/error.rs

@@ -29,8 +29,7 @@ pub struct Error {
     pub reason: &'static str,
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl core::error::Error for Error {}
 
 impl Error {
     /// Creates a new `Error`