rustc 'deadlock detected'

[Mike Pumford <mpumford%mudcovered.org.uk@localhost>]

I've reported this in the past as intermittent and for a long time it 
had completely disappeared so I wasn't really able to debug it.

However I've now got into a state where I'm seeing this pretty much 100% 
of the time both in a standalone build chroot and in my auto builds.

Build system is:
NetBSD trigati.mudcovered.org.uk 9.0_BETA NetBSD 9.0_BETA (GENERIC) #0: 
Fri Oct 18 20:30:55 BST 2019

Failure I'm seeing (Although not always in exactly the same place)
    Compiling proc-macro2 v0.4.30
    Compiling unicode-xid v0.1.0
    Compiling syn v0.15.35
    Compiling ryu v1.0.0
    Compiling serde v1.0.92
    Compiling libc v0.2.60
    Compiling fixedbitset v0.1.9
    Compiling ordermap v0.3.5
 dead lock detected
 error: Could not compile `proc-macro2`.
 warning: build failed, waiting for other jobs to finish...

It was pointed out last time I raised this that the message comes from 
rtld.c (Where it appears twice). I don't get any core dumps that I can 
debug to work out what is going wrong. Is there anything I can do to 
help track this down?

Mike