Add constraint method to DatabaseError trait

Override `constraint` method for `PgDatabaseError`.

sqlx-core/src/error.rs

@@ -158,6 +158,15 @@ pub trait DatabaseError: 'static + Send + Sync + StdError {
 
     #[doc(hidden)]
     fn into_error(self: Box<Self>) -> Box<dyn StdError + Send + Sync + 'static>;
+
+    /// Returns the name of the constraint that triggered the error, if applicable.
+    /// If the error was caused by a conflict of a unique index, this will be the index name.
+    ///
+    /// ### Note
+    /// Currently only populated by the Postgres driver.
+    fn constraint(&self) -> Option<&str> {
+        None
+    }
 }
 
 impl dyn DatabaseError {

sqlx-core/src/postgres/error.rs

@@ -184,4 +184,8 @@ impl DatabaseError for PgDatabaseError {
     fn into_error(self: Box<Self>) -> Box<dyn Error + Send + Sync + 'static> {
         self
     }
+
+    fn constraint(&self) -> Option<&str> {
+        self.constraint()
+    }
 }