Add Error::from_boxed with documentation about bidirectional ?

src/error.rs

@@ -85,6 +85,67 @@ impl Error {
         Error::construct_from_adhoc(message, backtrace!())
     }
 
+    /// Construct an error object from a type-erased standard library error.
+    ///
+    /// This is mostly useful for interop with other error libraries.
+    ///
+    /// # Example
+    ///
+    /// Here is a skeleton of a library that provides its own error abstraction.
+    /// The pair of `From` impls provide bidirectional support for `?`
+    /// conversion between `Report` and `anyhow::Error`.
+    ///
+    /// ```
+    /// use std::error::Error as StdError;
+    ///
+    /// pub struct Report {/* ... */}
+    ///
+    /// impl<E> From<E> for Report
+    /// where
+    ///     E: Into<anyhow::Error>,
+    ///     Result<(), E>: anyhow::Context<(), E>,
+    /// {
+    ///     fn from(error: E) -> Self {
+    ///         let anyhow_error: anyhow::Error = error.into();
+    ///         let boxed_error: Box<dyn StdError + Send + Sync + 'static> = anyhow_error.into();
+    ///         Report::from_boxed(boxed_error)
+    ///     }
+    /// }
+    ///
+    /// impl From<Report> for anyhow::Error {
+    ///     fn from(report: Report) -> Self {
+    ///         let boxed_error: Box<dyn StdError + Send + Sync + 'static> = report.into_boxed();
+    ///         anyhow::Error::from_boxed(boxed_error)
+    ///     }
+    /// }
+    ///
+    /// impl Report {
+    ///     fn from_boxed(boxed_error: Box<dyn StdError + Send + Sync + 'static>) -> Self {
+    ///         todo!()
+    ///     }
+    ///     fn into_boxed(self) -> Box<dyn StdError + Send + Sync + 'static> {
+    ///         todo!()
+    ///     }
+    /// }
+    ///
+    /// // Example usage: can use `?` in both directions.
+    /// fn a() -> anyhow::Result<()> {
+    ///     b()?;
+    ///     Ok(())
+    /// }
+    /// fn b() -> Result<(), Report> {
+    ///     a()?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[cfg(any(feature = "std", not(anyhow_no_core_error)))]
+    #[cold]
+    #[must_use]
+    pub fn from_boxed(boxed_error: Box<dyn StdError + Send + Sync + 'static>) -> Self {
+        let backtrace = backtrace_if_absent!(&*boxed_error);
+        Error::construct_from_boxed(boxed_error, backtrace)
+    }
+
     #[cfg(any(feature = "std", not(anyhow_no_core_error)))]
     #[cold]
     pub(crate) fn construct_from_std<E>(error: E, backtrace: Option<Backtrace>) -> Self