One-shot module loops have hard to understand messages

Consider these two modules:

-- A.hs
module A where

-- B.hs
module B where
import A

ghc --make B.hs will compile this fine. Now edit A.hs to be circular:

-- A.hs
module A where
import B

and attempt to one-shot compile it with ghc -c A.hs. You will now get an uninformative error message:

A.hs:1:1: Circular imports: module `A' depends on itself

You can get a more informative message with --make:

Module imports form a cycle:
         module `A' (A.hs)
        imports `B' (./B.hs)
  which imports `A' (A.hs)

We should either suggest users try again with --make, or try harder to give a useful message in one-shot mode.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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