Wacky error message when RULE mentions out-of-scope variable

If I compile the following program with GHC 8.4 or later:

module Foo where

{-# RULES "test" forall x. f x = x #-}

The error message I get is somewhat eyebrow-raising:

$ /opt/ghc/8.4.3/bin/ghc Foo.hs
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )

Foo.hs:3:11: error:
    Rule "test":
        Not in scope: OutOfScope(f)
        in left-hand side: f x
    LHS must be of form (f e1 .. en) where f is not forall'd
  |
3 | {-# RULES "test" forall x. f x = x #-}
  |           ^^^^^^^^^^^^^^^^^^^^^^^^

I'm not sure what this OutOfScope business is, but I doubt we want to be referring to it in error messages. The error in GHC 8.2 and earlier was much less confusing:

$ /opt/ghc/8.2.2/bin/ghc Foo.hs
[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )

Foo.hs:3:11: error:
    Rule "test":
        Not in scope: f
        in left-hand side: f x
    LHS must be of form (f e1 .. en) where f is not forall'd
  |
3 | {-# RULES "test" forall x. f x = x #-}
  |           ^^^^^^^^^^^^^^^^^^^^^^^^

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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