  Galactic cosmic-rays (GCRs) are thought to be accelerated in strong shocks
induced by massive star winds and supernova explosions sweeping across the
interstellar medium. But the phase of the interstellar medium from which the
CRs are extracted has remained elusive until now. Here, we study in detail the
GCR source composition deduced from recent measurements by the AMS-02, Voyager
1 and SuperTIGER experiments to obtain information on the composition,
ionisation state and dust content of the GCR source reservoirs. We show that
the volatile elements of the CR material are mainly accelerated from a plasma
of temperature higher than $\sim 2$ MK, which is typical of the hot medium
found in galactic superbubbles energised by the activity of massive star winds
and supernova explosions. Another GCR component, which is responsible for the
overabundance of $^{22}$Ne, most likely arises from acceleration of massive
star winds in their termination shocks. From the CR-related $\gamma$-ray
luminosity of the Milky Way, we estimate that the ion acceleration efficiency
in both supernova shocks and wind termination shocks is of the order of
$10^{-5}$. The GCR source composition also shows evidence for a preferential
acceleration of refractory elements contained in interstellar dust. We suggest
that the GCR refractories are also produced in superbubbles, from shock
acceleration and subsequent sputtering of dust grains continuously incorporated
into the hot plasma through thermal evaporation of embedded molecular clouds.
Our model explains well the measured abundances of all primary and mostly
primary CRs from H to Zr, including the overabundance of $^{22}$Ne.
