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)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import typechecker labels

+1 this is very annoying. This happens when working on GHC too. For example, add import StgSyn in TrieMap and you get this completely unhelpful error message.

Actually, I think there's something else going wrong about this in GHC HEAD:

➜  circular  ghc-stage1 --version
The Glorious Glasgow Haskell Compilation System, version 8.1.20160108
➜  circular  cat A.hs 
module A where

-- import B
➜  circular  cat B.hs 
module B where

import A
➜  circular  ghc-stage1 --make B.hs
[1 of 2] Compiling A                ( A.hs, A.o )
[2 of 2] Compiling B                ( B.hs, B.o )
➜  circular  cat A.hs 
module A where

import B
➜  circular  ghc-stage1 -c A.hs    
➜  circular  

added Pnormal label