AFD failure now sources underlying I/O error

src/iocp/mod.rs

@@ -121,17 +121,19 @@ impl Poller {
     pub(super) fn new() -> io::Result<Self> {
         // Make sure AFD is able to be used.
         if let Err(e) = afd::NtdllImports::force_load() {
-            return Err(crate::unsupported_error(format!(
-                "Failed to initialize unstable Windows functions: {}\nThis usually only happens for old Windows or Wine.",
-                e
-            )));
+            return Err(io::Error::new(
+                io::ErrorKind::Unsupported,
+                AfdError::new("failed to initialize unstable Windows functions", e),
+            ));
         }
 
         // Create and destroy a single AFD to test if we support it.
-        Afd::<Packet>::new().map_err(|e| crate::unsupported_error(format!(
-            "Failed to initialize \\Device\\Afd: {}\nThis usually only happens for old Windows or Wine.",
-            e,
-        )))?;
+        Afd::<Packet>::new().map_err(|e| {
+            io::Error::new(
+                io::ErrorKind::Unsupported,
+                AfdError::new("failed to initialize \\Device\\Afd", e),
+            )
+        })?;
 
         let port = IoCompletionPort::new(0)?;
         tracing::trace!(handle = ?port, "new");
@@ -1326,6 +1328,54 @@ fn dur2timeout(dur: Duration) -> u32 {
         .unwrap_or(INFINITE)
 }
 
+/// An error type that wraps around failing to open AFD.
+struct AfdError {
+    /// String description of what happened.
+    description: &'static str,
+
+    /// The underlying system error.
+    system: io::Error,
+}
+
+impl AfdError {
+    #[inline]
+    fn new(description: &'static str, system: io::Error) -> Self {
+        Self {
+            description,
+            system,
+        }
+    }
+}
+
+impl fmt::Debug for AfdError {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("AfdError")
+            .field("description", &self.description)
+            .field("system", &self.system)
+            .field("note", &"probably caused by old Windows or Wine")
+            .finish()
+    }
+}
+
+impl fmt::Display for AfdError {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}: {}\nThis error is usually caused by running on old Windows or Wine",
+            self.description, &self.system
+        )
+    }
+}
+
+impl std::error::Error for AfdError {
+    #[inline]
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        Some(&self.system)
+    }
+}
+
 struct CallOnDrop<F: FnMut()>(F);
 
 impl<F: FnMut()> Drop for CallOnDrop<F> {