cargo and rustc (and other tools using jobserver) will fail with a "failed to acquire jobserver token: early EOF on jobserver pipe" error in the next scenario reported in rust-lang/rust#46981 (comment) as ICE in rustc:

• make executes a subprocess (e.g. shell) with the jobserver pipe closed (marked with FD_CLOEXEC), but the MAKEFLAGS env var will still contain --jobserver-auth=3,4 (or some other pair of integers) in the subprocess.
  This is where make is wrong, if it closes the descriptors for a subprocess, then it should probably clean up MAKEFLAGS for it as well, but it doesn't, so we have to live with make possibly providing garbage descriptors.
• the subprocess (e.g. shell) opens some files so descriptors 3 and 4 are taken again now, but refer to entirely unrelated files.
• the shell runs cargo, which sees that 3 and 4 are open, concludes that they refer to a jobserver, and fails when trying to read from them.

jobserver could be more resilient in the face of garbage descriptors if it checked not only that the descriptors are valid, but also that they actually refer to a pipe rather than random unrelated files.