When aqua regia dissolves HgS, the function of the HCl in the mixture is to

Aqua regia functions as both an oxidising and a complexing agent while dissolving HgS. The ${\mathrm{NO}}_3^{-}$ ions oxidise the ${S^2}^{\mathit{-}}$ ions to free sulphur and the Cl ions to soluble complex-ions, ${\mathrm{HgCl}}_4^{2-}$, in the presence of hydronium ions.

$3{\mathrm{HgS}}_{\mathrm{S}}+2{\mathrm{NO}}_3^{-}+8{\mathrm{H}}_3{\mathrm{O}}^{+}+12{\mathrm{Cl}}^{-}\to 3{\mathrm{HgCl}}_4^{2-}+2\mathrm{NO}+3 \mathrm{S}+12{\mathrm{H}}_2\mathrm{O}$

Thus, When aqua regia dissolves HgS, the function of the HCl in the mixture is to complex the mercury.