Static AVCaptureExposureDurationCurrent 

pub unsafe static AVCaptureExposureDurationCurrent: CMTime

Available on crate features AVCaptureDevice and objc2-core-media only.

A special value that may be passed as the duration parameter of setExposureModeCustomWithDuration:ISO:completionHandler: to indicate that the caller does not wish to specify a value for the exposureDuration property, and that it should instead be set to its current value. Note that the device may be adjusting exposureDuration at the time of the call, in which case the value to which exposureDuration is set may differ from the value obtained by querying the exposureDuration property.

See also Apple’s documentation