add unwrap_or_err to result

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/utils/result/unwrap_or_err.rs

@@ -0,0 +1,15 @@
+use std::convert::identity;
+
+use super::Result;
+
+/// Returns the Ok value or the Err value. Available when the Ok and Err types
+/// are the same. This is a way to default the result using the specific Err
+/// value rather than unwrap_or_default() using Ok's default.
+pub trait UnwrapOrErr<T> {
+	fn unwrap_or_err(self) -> T;
+}
+
+impl<T> UnwrapOrErr<T> for Result<T, T> {
+	#[inline]
+	fn unwrap_or_err(self) -> T { self.unwrap_or_else(identity::<T>) }
+}