Add option to include tracing spans in anyhow errors.

Cargo.toml

@@ -21,6 +21,7 @@ std = []
 # besides bringing in an unused dependency, as `std::backtrace` is always
 # preferred.
 backtrace = { version = "0.3.51", optional = true }
+tracing = { version = "0.1.7", optional = true }
 
 [dev-dependencies]
 futures = { version = "0.3", default-features = false }

src/backtrace.rs

@@ -35,6 +35,13 @@ macro_rules! backtrace {
     };
 }
 
+#[cfg(feature = "tracing")]
+macro_rules! capture_span {
+    () => {
+        Some(::tracing::Span::current())
+    };
+}
+
 #[cfg(error_generic_member_access)]
 macro_rules! backtrace_if_absent {
     ($err:expr) => {
@@ -68,6 +75,23 @@ macro_rules! backtrace_if_absent {
     };
 }
 
+#[cfg(all(error_generic_member_access, feature = "tracing"))]
+macro_rules! span_if_absent {
+    ($err:expr) => {
+        match core::error::request_ref::<::tracing::Span>($err as &dyn core::error::Error) {
+            Some(_) => None,
+            None => capture_span!(),
+        }
+    };
+}
+
+#[cfg(all(not(error_generic_member_access), feature = "tracing"))]
+macro_rules! span_if_absent {
+    ($err:expr) => {
+        capture_span!()
+    };
+}
+
 #[cfg(all(not(std_backtrace), feature = "backtrace"))]
 mod capture {
     use alloc::borrow::{Cow, ToOwned as _};

src/context.rs

@@ -25,7 +25,15 @@ mod ext {
             C: Display + Send + Sync + 'static,
         {
             let backtrace = backtrace_if_absent!(&self);
-            Error::from_context(context, self, backtrace)
+            #[cfg(feature = "tracing")]
+            let span = span_if_absent!(&self);
+            Error::from_context(
+                context,
+                self,
+                backtrace,
+                #[cfg(feature = "tracing")]
+                span,
+            )
         }
     }
 
@@ -96,7 +104,12 @@ impl<T> Context<T, Infallible> for Option<T> {
         // backtrace.
         match self {
             Some(ok) => Ok(ok),
-            None => Err(Error::from_display(context, backtrace!())),
+            None => Err(Error::from_display(
+                context,
+                backtrace!(),
+                #[cfg(feature = "tracing")]
+                capture_span!(),
+            )),
         }
     }
 
@@ -107,7 +120,12 @@ impl<T> Context<T, Infallible> for Option<T> {
     {
         match self {
             Some(ok) => Ok(ok),
-            None => Err(Error::from_display(context(), backtrace!())),
+            None => Err(Error::from_display(
+                context(),
+                backtrace!(),
+                #[cfg(feature = "tracing")]
+                capture_span!(),
+            )),
         }
     }
 }