  We consider properties of the structure functions of inclusive heavy meson
decays $B\to X_c$ and treat the $c$ quark mass as a free parameter. We show
that in two extreme cases of heavy and light $c$ quark the structure functions
of heavy--heavy and heavy--light transitions are given by a Fourier transform
of the matrix elements of Wilson lines containing a time--like and a
light--like segment, correspondingly. Using the renormalization properties of
Wilson lines we find the dependence of the structure functions on the
factorization scale, the structure function of heavy--heavy transition is
renormalized multiplicatively while that of heavy-light transition obeys the
GLAP--type evolution equation. We propose a generalization of the sum rules for
the moments of the structure functions (Bjorken, Voloshin, and the ``third''
sum rules) with a soft exponential factorization cut--off, which correctly
incorporates both perturbative and nonperturbative effects. We analyze
nonperturbative corrections by first considering infrared renormalon
contributions to the Wilson lines. Uncertainties induced by the leading
renormalon pole at $u=\frac12$ are exactly cancelled by the similar uncertainty
in the heavy quark pole mass. The leading nonperturbative corrections
associated with the next renormalon at $u=1$ are parameterized by the matrix
element $\mu_\pi^2$ which is proportional to the heavy quark kinetic energy.
