Add a way to get the deepest Whatever error backtrace

compatibility-tests/backtrace-shim/tests/whatever_nested.rs

@@ -0,0 +1,20 @@
+use snafu::{whatever, ResultExt, Whatever};
+
+fn inner_outer() -> Result<(), Whatever> {
+    not_a_whatever().with_whatever_context(|_| format!("Outer failure"))
+}
+
+fn not_a_whatever() -> Result<(), Box<dyn std::error::Error>> {
+    inner_whatever().map_err(Into::into)
+}
+
+fn inner_whatever() -> Result<(), Whatever> {
+    whatever!("Inner failure");
+}
+
+#[test]
+fn backtrace_method_delegates_to_nested_whatever() {
+    let e = inner_outer().unwrap_err();
+    let bt = e.backtrace().expect("Must have a backtrace");
+    assert!(bt.to_string().contains("::inner_whatever::"));
+}

src/lib.rs

@@ -1110,6 +1110,18 @@ impl GenerateBacktrace for Backtrace {
 /// ```
 ///
 /// See [`whatever!`][] for detailed usage instructions.
+///
+/// ## Limitations
+///
+/// When wrapping errors, only the backtrace from the shallowest
+/// function is guaranteed to be available. If you need the deepest
+/// possible trace, consider creating a custom error type and [using
+/// `#[snafu(backtrace)]` on the `source`
+/// field](Snafu#controlling-backtraces). If a best-effort attempt is
+/// sufficient, see the [`backtrace`] method.
+///
+/// When the standard library stabilizes backtrace support, this
+/// behavior may change.
 #[derive(Debug, Snafu)]
 #[snafu(crate_root(crate))]
 #[snafu(whatever)]
@@ -1121,3 +1133,23 @@ pub struct Whatever {
     message: String,
     backtrace: Backtrace,
 }
+
+#[cfg(any(feature = "std", test))]
+impl Whatever {
+    /// Gets the backtrace from the deepest `Whatever` error. If none
+    /// of the underlying errors are `Whatever`, returns the backtrace
+    /// from when this instance was created.
+    pub fn backtrace(&self) -> Option<&Backtrace> {
+        let mut best_backtrace = &self.backtrace;
+
+        let mut source = self.source();
+        while let Some(s) = source {
+            if let Some(this) = s.downcast_ref::<Self>() {
+                best_backtrace = &this.backtrace;
+            }
+            source = s.source();
+        }
+
+        Some(best_backtrace)
+    }
+}