feat: add new error type for nested transaction

src/bb8_postgres.rs

@@ -97,7 +97,7 @@ impl<'me> ConnectionClient for Transaction<'me> {
         self.0
             .transaction()
             .await
-            .map_err(|_| crate::Error::UpgradeToTransaction)
+            .map_err(|_| crate::Error::UpgradeToNestedTransaction)
             .map(Transaction)
     }
 }

src/error.rs

@@ -25,6 +25,9 @@ pub enum Error {
     /// Returns if we cannot upgrade connection to transaction.
     #[cfg(feature = "nightly")]
     UpgradeToTransaction,
+    /// Returns if we cannot upgrade transaction to a nested one.
+    #[cfg(feature = "nightly")]
+    UpgradeToNestedTransaction,
     /// Returns if we cannot commit transaction.
     #[cfg(feature = "nightly")]
     CommitTransaction,
@@ -42,6 +45,10 @@ impl fmt::Display for Error {
             #[cfg(feature = "nightly")]
             Error::UpgradeToTransaction => f.write_str("Cannot upgrade connection to transaction"),
             #[cfg(feature = "nightly")]
+            Error::UpgradeToNestedTransaction => {
+                f.write_str("Cannot upgrade transaction to a nested one")
+            }
+            #[cfg(feature = "nightly")]
             Error::CommitTransaction => f.write_str("Cannot commit changes of transaction"),
             #[cfg(feature = "nightly")]
             Error::RollbackTransaction => f.write_str("Cannot rolls transaction back"),

src/r2d2_postgres.rs

@@ -91,7 +91,7 @@ impl<'me> ConnectionClient for Transaction<'me> {
     fn start_transaction(&mut self) -> crate::Result<Self::Trx<'_>> {
         self.0
             .transaction()
-            .map_err(|_| crate::Error::UpgradeToTransaction)
+            .map_err(|_| crate::Error::UpgradeToNestedTransaction)
             .map(Transaction)
     }
 }

src/r2d2_sqlite.rs

@@ -76,9 +76,9 @@ impl<'me> ConnectionClient for Transaction<'me> {
     #[cfg(feature = "nightly")]
     fn start_transaction(&mut self) -> crate::Result<Self::Trx<'_>> {
         // At the moment we cannot implement nested transaction because
-        // the transaction in `rusqlite` requires DerefMut, which cannot be
-        // implemented yet 😣
-        unimplemented!()
+        // the transaction in `rusqlite` 😣 We need to implement
+        // our box for transaction if we want to use it here.
+        Err(crate::Error::UpgradeToNestedTransaction)
         // self.0
         //     .transaction()
         //     .map_err(|_| error::PersistenceError::UpgradeToTransaction)