fix(core): require a SQL backend feature

Author: TheRustifyer

canyon_core/src/lib.rs

@@ -4,6 +4,11 @@
 //! and data mapping. It includes support for multiple database backends such as PostgreSQL,
 //! MySQL, and SQL Server, and defines traits and utilities for interacting with these databases.
 
+#[cfg(not(any(feature = "postgres", feature = "mysql", feature = "mssql")))]
+compile_error!(
+    "Canyon-SQL requires at least one SQL backend feature: `postgres`, `mysql` or `mssql`."
+);
+
 #[cfg(feature = "postgres")]
 pub extern crate tokio_postgres;
 
@@ -17,12 +22,20 @@ pub extern crate mysql_async;
 
 extern crate core;
 
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod canyon;
 
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod column;
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod connection;
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod mapper;
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod query;
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod row;
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod rows;
+#[cfg(any(feature = "postgres", feature = "mysql", feature = "mssql"))]
 pub mod transaction;