Check if -XStaticPointers is enabled when renaming static expressions

Summary:
Trying to use `static` expressions without the `-XStaticPointers`
extension enabled can lead to runtime errors. Normally, such a situation isn't
possible, but Template Haskell provides a backdoor that allows it to happen,
as shown in #14204.

To prevent this, we ensure that `-XStaticPointers` is enabled when renaming
`static` expressions.

Test Plan: make test TEST=T14204

Reviewers: facundominguez, austin, bgamari, simonpj

Reviewed By: facundominguez, simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #14204

Differential Revision: https://phabricator.haskell.org/D3931

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/rename/RnExpr.hs b/compiler/rename/RnExpr.hs
index 5ccefb8467..99e3f7068a 100644
--- a/compiler/rename/RnExpr.hs
+++ b/compiler/rename/RnExpr.hs
@@ -375,6 +375,16 @@ wired-in. See the Notes about the NameSorts in Name.hs.
 -}
 
 rnExpr e@(HsStatic _ expr) = do
+    -- Normally, you wouldn't be able to construct a static expression without
+    -- first enabling -XStaticPointers in the first place, since that extension
+    -- is what makes the parser treat `static` as a keyword. But this is not a
+    -- sufficient safeguard, as one can construct static expressions by another
+    -- mechanism: Template Haskell (see #14204). To ensure that GHC is
+    -- absolutely prepared to cope with static forms, we check for
+    -- -XStaticPointers here as well.
+    unlessXOptM LangExt.StaticPointers $
+      addErr $ hang (text "Illegal static expression:" <+> ppr e)
+                  2 (text "Use StaticPointers to enable this extension")
     (expr',fvExpr) <- rnLExpr expr
     stage <- getStage
     case stage of