  Semiclassical expansion of the Wigner function for spin-1/2 fermions having
an effective spacetime-dependent mass is used to analyze spin-polarization
effects. The existing framework is reformulated to obtain a differential
equation directly connecting the particle spin tensor with the effective mass.
It reflects the conservation of the total angular momentum in a system. In
general, we find that the gradients of mass act as a source of the spin
polarization. Although this effect is absent for simple boost-invariant
dynamics, an extension to non-boost-invariant systems displays a non-trivial
dependence of the spin density on the mass indicating that the spin
polarization effects may be intertwined with the phenomenon of chiral
restoration.
