Refactor the GlobalRdrEnv, fixing #7672
This patch started innocently enough, by deleting a single call from rnImportDecl, namely let gbl_env = mkGlobalRdrEnv (filterOut from_this_mod gres) The 'filterOut' makes no sense, and was the cause of #7672. But that little loose end led to into a twisty maze of little passages, all alike, which has taken me an unreasonably long time to straighten out. Happily, I think the result is really much better. In particular: * INVARIANT 1 of the GlobalRdrEnv type was simply not true: we had multiple GlobalRdrElts in a list with the same gre_name field. This kludgily implmented one form of shadowing. * Meanwhile, extendGlobalRdrEnvRn implemented a second form of shadowing, by deleting stuff from the GlobalRdrEnv. * In turn, much of this shadowing stuff depended on the Names of the Ids bound in the GHCi InteractiveContext being Internal names, even though the TyCons and suchlike all had External Names. Very confusing. So I have made the following changes * I re-established INVARIANT 1 of GlobalRdrEnv. As a result some strange code in RdrName.pickGREs goes away. * RnNames.extendGlobalRdrEnvRn now makes one call to deal with shadowing, where necessary, and another to extend the environment. It deals separately with duplicate bindings. The very complicated RdrName.extendGlobalRdrEnv becomes much simpler; we need to export the shadowing function, now called RdrName.shadowNames; and we can nuke RdrName.findLocalDupsRdrEnv altogether. RdrName Note [GlobalRdrEnv shadowing] summarises the shadowing story * The Names of the Ids bound in the GHCi interactive context are now all External. See Note [Interactively-bound Ids in GHCi] in HscTypes. * Names for Ids created by the debugger are now made by IfaceEnv.newInteractiveBinder. This fixes a lurking bug which was that the debugger was using mkNewUniqueSupply 'I' to make uniques, which does NOT guarantee a fresh supply of uniques on successive calls. * Note [Template Haskell ambiguity] in RnEnv shows that one TH-related error is reported lazily (on occurrences) when it might be better reported when extending the environment. In some (but not all) cases this was done before; but now it's uniformly at occurrences. In some ways it'd be better to report when extending the environment, but it's a tiresome test and the error is rare, so I'm leaving it at the lookup site for now, with the above Note. * A small thing: RnNames.greAvail becomes RdrName.availFromGRE, where it joins the dual RdrName.gresFromAvail.
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