Backwards compatibility can't be broken, so marking as Go2.

https://golang.org/pkg/log/#Logger.SetOutput

mvdan: But if we use an interface that log.Logger matches, in theory it should not break anything, right ?

Possibly breaking compatibility.

proxy.ErrorLog = makeLogger()
passLogger(proxy.ErrorLog)

makeLogger may have return typed *log.Logger. argument of passLogger is typed it.

If the signature changes, that breaks the Go1 compatibility promise. @mattn showed the outline of a reasonable program that would break.

could we do something similar to how Cancel was deprecated in favor of Context in net/http.Request?

e.g., Logger.SetOutput maintains backwards compatibility and something like Logger.SetOutputFunc(f func(format string, args ...interface{})) with a new conditional to handle it here:

edit: currently have an issue with this, proposed solution for us is likely to be duplicating reverseproxy.go to make the changes we need

This should be handled as part of #5465. Closing in favor of that issue.