As an additional data point, thunderbird 1:60.9.1+build1-0ubuntu0.16.04.1 and 1:60.9.1+build1-0ubuntu0.19.04.1, which were FTBFS in xenial-proposed and disco-proposed because of this bug in cargo, are now successfully building (builds already completed for some of the architectures, others are still going).
As an additional data point, thunderbird 1:60.9. 1+build1- 0ubuntu0. 16.04.1 and 1:60.9. 1+build1- 0ubuntu0. 19.04.1, which were FTBFS in xenial-proposed and disco-proposed because of this bug in cargo, are now successfully building (builds already completed for some of the architectures, others are still going).