multiple logging sinks

[graydon]

Things like https://logging.apache.org/log4cxx/ and similar logging libraries tend to be able to send logging to a number of possible sinks (syslog, files, in-memory, etc.)

Our logging should know how to do this too.

See also #6810 , sub-bug of #3309

[pnkfelix]

visiting for triage, email from 2013 sep 09. Nothing to add.

[flaper87]

Triage bump, nothing to add

[alexcrichton]

cc @nick29581, could this move to https://github.com/rust-lang/log?

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/log#7