diff options
Diffstat (limited to 'compiler/GHC/Hs/Extension.hs')
| -rw-r--r-- | compiler/GHC/Hs/Extension.hs | 24 |
1 files changed, 9 insertions, 15 deletions
diff --git a/compiler/GHC/Hs/Extension.hs b/compiler/GHC/Hs/Extension.hs index ce28e0355d..d58bd9efbc 100644 --- a/compiler/GHC/Hs/Extension.hs +++ b/compiler/GHC/Hs/Extension.hs @@ -32,8 +32,6 @@ import GHC.Types.SrcLoc (GenLocated(..), unLoc) import GHC.Utils.Panic import GHC.Parser.Annotation -import Data.Void - {- Note [IsPass] ~~~~~~~~~~~~~ @@ -115,12 +113,12 @@ instance MapXRec (GhcPass p) where -- wrapXRec = noLocA {- -Note [NoExtCon and strict fields] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Note [DataConCantHappen and strict fields] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Currently, any unused TTG extension constructor will generally look like the following: - type instance XXHsDecl (GhcPass _) = NoExtCon + type instance XXHsDecl (GhcPass _) = DataConCantHappen data HsDecl p = ... | XHsDecl !(XXHsDecl p) @@ -132,17 +130,17 @@ the following function which consumes an HsDecl: ex :: HsDecl GhcPs -> HsDecl GhcRn ... - ex (XHsDecl nec) = noExtCon nec + ex (XHsDecl nec) = dataConCantHappen nec Because `p` equals GhcPs (i.e., GhcPass 'Parsed), XHsDecl's field has the type -NoExtCon. But since (1) the field is strict and (2) NoExtCon is an empty data -type, there is no possible way to reach the right-hand side of the XHsDecl -case. As a result, the coverage checker concludes that the XHsDecl case is -inaccessible, so it can be removed. +DataConCantHappen. But since (1) the field is strict and (2) DataConCantHappen +is an empty data type, there is no possible way to reach the right-hand side +of the XHsDecl case. As a result, the coverage checker concludes that +the XHsDecl case is inaccessible, so it can be removed. (See Note [Strict argument type constraints] in GHC.HsToCore.Pmc.Solver for more on how this works.) -Bottom line: if you add a TTG extension constructor that uses NoExtCon, make +Bottom line: if you add a TTG extension constructor that uses DataConCantHappen, make sure that any uses of it as a field are strict. -} @@ -219,10 +217,6 @@ type OutputableBndrId pass = , IsPass pass ) --- | See Note [Constructor cannot occur] -dataConCantHappen :: Void -> a -dataConCantHappen = absurd - -- useful helper functions: pprIfPs :: forall p. IsPass p => (p ~ 'Parsed => SDoc) -> SDoc pprIfPs pp = case ghcPass @p of GhcPs -> pp |