Example command line:
emerge -uvD --changed-use --with-bdeps=n --buildpkgonly --keep-going world
Typically an update like this will involve a mix of packages which can and cannot be built in this manner. Right now emerge refuses to run with this error:
!!! --buildpkgonly requires all dependencies to be merged.
!!! Cannot merge requested packages. Merge deps and try again.
Instead emerge should at least build packages when the build-time dependencies are satisfied. Obviously if the tree goes down more than one level or if there are rebuilds those packages will need to be deferred.