I experienced this on 1.4 release, but it's likely that this happens on Next too.

The build stops at a random point with very informative rust error messages like:

error: could not compile `termcolor`

or

error: could not compile `percent-encoding`

I believe this is due to too high level of parallelism for the available memory. In other words MAKEOPTS --jobs is too high. So, probably the memsaver eclass can fix this issue.