  Gas-giant planets, such as Jupiter, Saturn and massive exoplanets, were
formed via the gas accretion onto the solid cores each with a mass of roughly
ten Earth masses. However, rapid radial migration due to disk-planet
interaction prevents the formation of such massive cores via planetesimal
accretion. Comparably rapid core growth via pebble accretion requires very
massive protoplanetary disks because most pebbles fall into the central star.
Although planetesimal formation, planetary migration, and gas-giant core
formation have been studied with much effort, the full evolution path from dust
to planets are still uncertain. Here we report the result of full simulations
for collisional evolution from dust to planets in a whole disk. Dust growth
with realistic porosity allows the formation of icy planetesimals in the inner
disk (> 10 au), while pebbles formed in the outer disk drift to the inner disk
and there grow to planetesimals. The growth of those pebbles to planetesimals
suppresses their radial drift and supplies small planetesimals sustainably in
the vicinity of cores. This enables rapid formation of sufficiently massive
planetary cores within 0.2-0.4 million years, prior to the planetary migration.
Our models shows first gas giants form at 2-7 au in rather common
protoplanetary disks, in agreement with the exoplanet and solar systems.
