Do not treat Expr::Index with pure index as pure.

CHANGELOG.md

@@ -10,6 +10,7 @@ Bug fixes
 ---------
 
 * Fixed bug when position is zero in `insert` and `split_at` methods for arrays.
+* Indexing operations with pure index values are no longer considered pure due to the possibility of indexers.
 
 Breaking changes
 ----------------

src/ast.rs

@@ -1899,9 +1899,7 @@ impl Expr {
 
             Self::Map(x, _) => x.0.iter().map(|(_, v)| v).all(Self::is_pure),
 
-            Self::Index(x, _) | Self::And(x, _) | Self::Or(x, _) => {
-                x.lhs.is_pure() && x.rhs.is_pure()
-            }
+            Self::And(x, _) | Self::Or(x, _) => x.lhs.is_pure() && x.rhs.is_pure(),
 
             Self::Stmt(x) => x.0.iter().all(Stmt::is_pure),