Keep source locations in Core (optionally)

The people working on liquid haskell have to maintain their own copy of the desugarer because of one little change they have to do: Add lots and lots of CCs to keep source spans around. Their current patch is

diff --git a/src/Language/Haskell/Liquid/Desugar/DsExpr.hs b/src/Language/Haskell/Liquid/Desugar/DsExpr.hs
index 396095e9a..9ba60c56f 100644
--- a/src/Language/Haskell/Liquid/Desugar/DsExpr.hs
+++ b/src/Language/Haskell/Liquid/Desugar/DsExpr.hs
@@ -221,14 +221,13 @@ dsUnliftedBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
 dsLExpr :: LHsExpr Id -> DsM CoreExpr
 
 dsLExpr (L loc e)
-  = putSrcSpanDs loc $
-    do { core_expr <- dsExpr e
-   -- uncomment this check to test the hsExprType function in TcHsSyn
-   --    ; MASSERT2( exprType core_expr `eqType` hsExprType e
-   --              , ppr e <+> dcolon <+> ppr (hsExprType e) $$
-   --                ppr core_expr <+> dcolon <+> ppr (exprType core_expr) )
-       ; return core_expr }
+  = do ce <- putSrcSpanDs loc $ dsExpr e
+       m <- getModule
+       return $ Tick (srcSpanTick m loc) ce 
 
+srcSpanTick :: Module -> SrcSpan -> Tickish a
+srcSpanTick m loc
+  = ProfNote (AllCafsCC m loc) False True
 -- | Variant of 'dsLExpr' that ensures that the result is not levity
 -- polymorphic. This should be used when the resulting expression will
 -- be an argument to some other function.

It would make their live easier if they could just use GHC’s desugarer, and it might open the path to making Liquid Haskell a proper GHC plugin instead of a separate program, which could facilitate adoption.

Would it be ok to add a flag to GHC that enables this behaviour? Or is there another way of doing this?

Trac metadata

Trac field	Value
Version	8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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