Make it clear when an unused binds warning is due to transitive unuse

Motivation

a :: Int
a = 4
  where bar = foo
        foo = 3

Given this file, with -Wunused-local-binds, GHC will output a warning saying that bar is unused, and a warning saying that foo is unused.

Every once in a while I find myself confused by this, thinking "I'm sure I used foo somewhere", until I remember that GHC warns about transitively unused bindings since version 6.0.

Proposal

Instead of saying "Defined but not used: foo", GHC should say something like "Defined but only used in unused bindings: foo".

Furthermore, ideally, the warning could be split into two warnings, -Wunused-local-binds to take care of genuinely unused bindings like bar, and -Wonly-in-unused-local-binds (or a better name) to take care of bindings like foo. This would allow users who wish to declutter the warning output to disable the latter warning.

To keep the warning behavior consistent with the current warning, -Wunused-local-binds could imply -Wonly-in-unused-local-binds.

Unresolved Questions

Would splitting the warning (even if the new warning is implied by the old warning) require a proper GHC proposal?