[GHC] #7918: SrcSpan's associated with expanded quasi-quotes are inconsistent

[GHC]

#7918: SrcSpan's associated with expanded quasi-quotes are inconsistent
-------------------------------------+------------------------------------
        Reporter:  edsko             |            Owner:  edsko
            Type:  bug               |           Status:  new
        Priority:  high              |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by edsko):

 So here's what's happening: we have

 {{{
 runQuasiQuoteExpr :: HsQuasiQuote RdrName -> RnM (LHsExpr RdrName)
 }}}

 which executes a quasi-quote. Note that `runQuasiQuoteExpr` returns a
 ''located'' expression; it makes an attempt to set the location of the
 expanded quasi-quote to something sensible. However, `runQuasiQuoteExpr`
 gets called by `rnExpr`, which ''ignores'' the outermost location returned
 by `runQuasiQuoteExpr`:

 {{{
 rnExpr (HsQuasiQuoteE qq)
   = runQuasiQuoteExpr qq        `thenM` \ (L _ expr') ->
     rnExpr expr'
 }}}

 The result is that the outermost location is whatever location of the
 quasi-quote was, while all inner locations have been set by
 `runQuasiQuoteExpr`. That's why in the test case the case of a single
 `True` or `False` is different from a compound expression; in all cases
 the `runQuasiQuoteExpr` sets the location of all identifiers to the same
 thing, but in the case of a non-compound expression (a single `True` or
 `False`) this is then overwritten by `rnExpr`; for the compound expression
 it's the location of the compound expression (`HsApp` or `HsPar`) which
 gets overwritten instead.

 Now, we can fix this by adding a special case to `rnLExpr` instead:

 {{{
 rnLExpr :: LHsExpr RdrName -> RnM (LHsExpr Name, FreeVars)
 -- Don't ignore the new location set by runQuasiQuoteExpr (#7918)
 #ifndef GHCI
 rnLExpr (L _ e@(HsQuasiQuoteE _)) =
   pprPanic "Cant do quasiquotation without GHCi" (ppr e)
 #else
 rnLExpr (L loc (HsQuasiQuoteE qq)) = setSrcSpan loc $
   runQuasiQuoteExpr qq `thenM` rnLExpr
 #endif  /* GHCI */
 rnLExpr e = wrapLocFstM rnExpr e
 }}}

 I have done this and it works and it fixes the problem for the case of
 expressions. I've done it too for the case of types but there seems to be
 another location override in that case; still looking for that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7918#comment:8>
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