Function install_decorators 

pub fn install_decorators(
    env: OptimizationConfig,
    flags: &OptimizationConfig,
    required: Vec<Opt>,
) -> (OptimizationConfig, DecoratorGate)

Merge the file’s ## No flags into env, install the result as the thread-local active config (so EVERY backend’s active_config().is_on(..) gate honors them — AOT codegen, the VM compiler, the JIT), and open a fired-trace when any ## Require was declared. Returns the merged config to hand to the optimizer/codegen plus the gate to verify() after the compile has run. When a trace is already active (a nested compile), the outer gate owns the verification, so this one is a no-op verify().