Implement std::error::Error for TryFromIntError (#61)

Cargo.toml

@@ -19,7 +19,7 @@ categories = ["embedded", "no-std", "data-structures"]
 
 [features]
 default = []
-# This is a no-op feature only provided for compatibility in case it is
-# explicitly selected by a user. This crate works without explicit indication
-# both on std and no_std systems.
+# The std feature only toggles whether the Error trait is implemented for error
+# types. Apart from that, this crate works without explicit indication both on
+# std and no_std systems.
 std = []

src/conversion.rs

@@ -16,6 +16,15 @@ impl From<lib::core::convert::Infallible> for TryFromIntError {
     }
 }
 
+impl Display for TryFromIntError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> core::fmt::Result {
+        write!(f, "out of range integral type conversion attempted")
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for TryFromIntError {}
+
 // Only implement if $from can be converted into $name lossless
 macro_rules! implement_from {
     {[$($name:ident),*], [$($from:ident),*] } => {$(implement_from!($name, $from);)*};
@@ -1829,4 +1838,13 @@ mod tests {
         assert!(i6::try_from(i7(64)).is_err());
         assert!(i6::try_from(i7(-64)).is_err());
     }
+
+    #[test]
+    #[cfg(feature = "std")]
+    fn error_trait() {
+        assert_eq!(
+            (&TryFromIntError(()) as &dyn std::error::Error).to_string(),
+            "out of range integral type conversion attempted"
+        );
+    }
 }

src/lib.rs

@@ -12,7 +12,7 @@
 //! and thus does not use it:
 //! an `Option<u7>` still takes up two bytes.
 
-#![no_std]
+#![cfg_attr(not(feature = "std"), no_std)]
 
 mod lib {
     pub use core;