Macro dispatch_backend 

macro_rules! dispatch_backend {
    ($backend:expr, $pg_branch:expr, $sqlite_branch:expr) => { ... };
}

Dispatches to backend-specific code based on compile-time features.

Usage:

dispatch_backend!(
    backend_type_expr,
    postgres_expr,
    sqlite_expr
)

When both postgres and sqlite features are enabled, this expands to a match statement. When only one feature is enabled, this compiles only the relevant branch.