A raft of changes driven by Trac #8540
The root cause of #8450 is that the new Template Haskell story, with the renamer doing more of the work of Template Haskell, wasn't dealing correctly with the keepAlive problem. Consider g = ..blah... f = [| g |] Then f's RHS refers to g's name but not to g, so g was being discarded as dead code. Fixing this sucked me into a deep swamp of understanding how all the moving parts of hte new Template Haskell fit together, leading to a large collection of related changes and better documentation. Specifically: * Instead of putting the TH level of a binder in the LocalRdrEnv, there is now a separate field tcl_th_bndrs :: NameEnv (TopLevelFlag, ThLevel) in the TcLclEnv, which records for each binder a) whether it is syntactically a top-level binder or not b) its TH level This deals uniformly with top-level and non-top-level binders, which was previously dealt with via greviously-delicate meddling with Internal and External Names. Much better. * As a result I could remove the tct_level field of ATcId. * There are consequential changes in TcEnv too, which must also extend the level bindings. Again, more clarity. I renamed TcEnv.tcExtendTcTyThingEnv to tcExtendKindEnv2, since it's only used during kind inference, for (AThing kind) and APromotionErr; and that is relevant to whether we want to extend the tcl_th_bndrs field (no). * I de-crufted the code in RnEnv.extendGlobalRdrEnv, by getting rid of the qual_gre code which said "Seems like 5 times as much work as it deserves!". Instead, RdrName.pickGREs makes the Internal names shadow External ones. * I moved the checkThLocalName cross-stage test to finishHsVar; previously we weren't doing the test at all in the OpApp case! * Quite a few changes (shortening the code) in the cross-stage checking code in TcExpr and RnSplice, notably to move the keepAlive call to the renamer One leftover piece: * In TcEnv I removed tcExtendGhciEnv and refactored tcExtendGlobalTyVars; this is really related to the next commit, but it was too hard to disentangle.
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