feat(core): Added error downcasting in the style of the master branch

sqlx-core/src/error/database.rs

@@ -9,4 +9,65 @@ use std::error::Error as StdError;
 pub trait DatabaseError: 'static + StdError + Send + Sync {
     /// Returns the primary, human-readable error message.
     fn message(&self) -> &str;
+
+    #[doc(hidden)]
+    fn as_error(&self) -> &(dyn StdError + Send + Sync + 'static);
+
+    #[doc(hidden)]
+    fn as_error_mut(&mut self) -> &mut (dyn StdError + Send + Sync + 'static);
+
+    #[doc(hidden)]
+    fn into_error(self: Box<Self>) -> Box<dyn StdError + Send + Sync + 'static>;
 }
+
+impl dyn DatabaseError {
+    /// Downcast a reference to this generic database error to a specific
+    /// database error type.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the database error type is not `E`. This is a deliberate contrast from
+    /// `Error::downcast_ref` which returns `Option<&E>`. In normal usage, you should know the
+    /// specific error type. In other cases, use `try_downcast_ref`.
+    pub fn downcast_ref<E: DatabaseError>(&self) -> &E {
+        self.try_downcast_ref().unwrap_or_else(|| {
+            panic!(
+                "downcast to wrong DatabaseError type; original error: {}",
+                self
+            )
+        })
+    }
+
+    /// Downcast this generic database error to a specific database error type.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the database error type is not `E`. This is a deliberate contrast from
+    /// `Error::downcast` which returns `Option<E>`. In normal usage, you should know the
+    /// specific error type. In other cases, use `try_downcast`.
+    pub fn downcast<E: DatabaseError>(self: Box<Self>) -> Box<E> {
+        self.try_downcast().unwrap_or_else(|e| {
+            panic!(
+                "downcast to wrong DatabaseError type; original error: {}",
+                e
+            )
+        })
+    }
+
+    /// Downcast a reference to this generic database error to a specific
+    /// database error type.
+    #[inline]
+    pub fn try_downcast_ref<E: DatabaseError>(&self) -> Option<&E> {
+        self.as_error().downcast_ref()
+    }
+
+    /// Downcast this generic database error to a specific database error type.
+    #[inline]
+    pub fn try_downcast<E: DatabaseError>(self: Box<Self>) -> std::result::Result<Box<E>, Box<Self>> {
+        if self.as_error().is::<E>() {
+            Ok(self.into_error().downcast().unwrap())
+        } else {
+            Err(self)
+        }
+    }
+}