Refactor SrcLoc and SrcSpansrcloc

The "Unhelpful" cases are now in a separate type. This allows us to
improve various things, e.g.:
* Most of the panic's in SrcLoc are now gone
* The Lexer now works with RealSrcSpans rather than SrcSpans, i.e. it
  knows that it has real locations and thus can assume that the line
  number etc really exists
* Some of the more suspicious cases are no longer necessary, e.g.
  we no longer need this case in advanceSrcLoc:
      advanceSrcLoc loc _ = loc -- Better than nothing

More improvements can probably be made, e.g. tick locations can
probably use RealSrcSpans too.

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/rename/RnNames.lhs b/compiler/rename/RnNames.lhs
index 46058c4677..3867e17693 100644
--- a/compiler/rename/RnNames.lhs
+++ b/compiler/rename/RnNames.lhs
@@ -1256,7 +1256,9 @@ warnUnusedImportDecls gbl_env
        ; ifDOptM Opt_D_dump_minimal_imports $
          printMinimalImports usage }
   where
-    explicit_import (L loc _) = isGoodSrcSpan loc
+    explicit_import (L loc _) = case loc of
+                                UnhelpfulSpan _ -> False
+                                RealSrcSpan _ -> True
         -- Filter out the implicit Prelude import
         -- which we do not want to bleat about
 \end{code}