mdo blocks in error messages are shown modified

{-# LANGUAGE RecursiveDo #-}
main = mdo
  Just x <- return (Right x)
  return ()

gives

MdoTypeError.hs:3:3:
    Couldn't match expected type ‘Either a0 b0’
                with actual type ‘Maybe b0’
    Relevant bindings include x :: b0 (bound at MdoTypeError.hs:3:8)
    In the pattern: Just x
    In a stmt of an 'mdo' block: Just x <- return (Right x)
    In a stmt of an 'mdo' block: rec { Just x <- return (Right x) }

Note that the error message mentions rec even though that is not what the user entered. In other instances, we are careful to show the code more similar to the original (e.g. do-notation instead of >>=). This should also be done here.

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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