High-harmonic generation (HHG) is considered on the basis of the interaction of a short intense laser pulse with a thin solid crystal layer. For a selective range of parameters its initial regular structure is preserved during the strong interaction, including the suppression of shock waves in the layer. For this situation, phase matching like in Bragg scattering may be achieved between the fundamental and high-harmonic waves in the presence of substantial free-electron background. This allows us to estimate the spectral intensity of HHG in terms of single-ion signals and to predict very high temporal and spatial coherence in the hard x-ray regime.