Merge pull request #258 from Fishrock123/error-backtrace-stable-display

error: allow backtrace Option to be Display on stable

src/error.rs

@@ -19,6 +19,17 @@ pub struct Error {
  type_name: Option<&'static str>,
 }
 
+#[allow(unreachable_pub)]
+#[derive(Debug)]
+#[doc(hidden)]
+pub struct BacktracePlaceholder;
+
+impl Display for BacktracePlaceholder {
+ fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
+ unreachable!()
+ }
+}
+
 impl Error {
  /// Create a new error object from any error type.
  ///
@@ -108,7 +119,7 @@ impl Error {
 
  #[cfg(not(backtrace))]
  #[allow(missing_docs)]
- pub fn backtrace(&self) -> Option<()> {
+ pub const fn backtrace(&self) -> Option<BacktracePlaceholder> {
  None
  }