Force all Pos* to be non-null

This fixes a class of crashes and introduces ptr<T> to make the
code robust against this failure mode going forward.

Thanks regnat for the idea of a ref<T> without overhead!

Closes #4895
Closes #4893
Closes #5127
Closes #5113

1 files changed, 2 insertions, 2 deletions

diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 5ceb2ae67..4b574fe0a 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -131,9 +131,9 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     state.forceValue(topLevel);
     PathSet context;
     Attr & aDrvPath(*topLevel.attrs->find(state.sDrvPath));
-    auto topLevelDrv = state.store->parseStorePath(state.coerceToPath(aDrvPath.pos ? *(aDrvPath.pos) : noPos, *(aDrvPath.value), context));
+    auto topLevelDrv = state.store->parseStorePath(state.coerceToPath(*aDrvPath.pos, *(aDrvPath.value), context));
     Attr & aOutPath(*topLevel.attrs->find(state.sOutPath));
-    Path topLevelOut = state.coerceToPath(aOutPath.pos ? *(aOutPath.pos) : noPos, *(aOutPath.value), context);
+    Path topLevelOut = state.coerceToPath(*aOutPath.pos, *(aOutPath.value), context);
 
     /* Realise the resulting store expression. */
     debug("building user environment");