Minor: Clarify use of infallable APIs (#13217)

Author: alamb

datafusion/expr/src/expr.rs

@@ -1563,13 +1563,13 @@ impl Expr {
     /// Returns true if there are any column references in this Expr
     pub fn any_column_refs(&self) -> bool {
         self.exists(|expr| Ok(matches!(expr, Expr::Column(_))))
-            .unwrap()
+            .expect("exists closure is infallible")
     }
 
-    /// Return true when the expression contains out reference(correlated) expressions.
+    /// Return true if the expression contains out reference(correlated) expressions.
     pub fn contains_outer(&self) -> bool {
         self.exists(|expr| Ok(matches!(expr, Expr::OuterReferenceColumn { .. })))
-            .unwrap()
+            .expect("exists closure is infallible")
     }
 
     /// Returns true if the expression node is volatile, i.e. whether it can return
@@ -1589,7 +1589,8 @@ impl Expr {
     ///
     /// See [`Volatility`] for more information.
     pub fn is_volatile(&self) -> bool {
-        self.exists(|expr| Ok(expr.is_volatile_node())).unwrap()
+        self.exists(|expr| Ok(expr.is_volatile_node()))
+            .expect("exists closure is infallible")
     }
 
     /// Recursively find all [`Expr::Placeholder`] expressions, and