QRhi * that 

QSGMaterialShader::GraphicsPipelineState

and also 

window()->rendererInterface()->getResource(window(),QSGRendererInterface::RhiResource)

expose are useless unless someone includes the private/qrhi_p.h header. Understood that Qt designers want to hide the QRhi internals..
But there are a couple of methods that QRhi has, that have to do with the current hardware capabilities, that would be of great help if we could have access to them without including private headers:

isFeatureSupported()
resourceLimit()
supportedSampleCounts()

 Please consider a "parent" QRhi class, exposing only the above methods (and the associated enums), that we can use to query the hardware capabilities of the QRhi renderer without resorting to including a private header.