Macro diag 

macro_rules! diag {
    ($f:expr) => { ... };
}

Run a diagnostic-counter update against the global counters, but ONLY when the diagnostics feature is enabled. When the feature is off this expands to nothing, so counter maintenance imposes zero cost on the render/event hot paths in a maximally-optimized build.

The closure receives &DiagnosticCounters:

diag!(|d| d.inc_render_calls());