fix(rust): Window function had incorrect output name on ExprIR (#18970)

crates/polars-plan/src/plans/conversion/expr_to_ir.rs

@@ -304,14 +304,16 @@ pub(super) fn to_aexpr_impl(
             order_by,
             options,
         } => {
+            // Process function first so name is correct.
+            let function = to_aexpr_impl(owned(function), arena, state)?;
             let order_by = if let Some((e, options)) = order_by {
                 Some((to_aexpr_impl(owned(e.clone()), arena, state)?, options))
             } else {
                 None
             };
 
             AExpr::Window {
-                function: to_aexpr_impl(owned(function), arena, state)?,
+                function,
                 partition_by: to_aexprs(partition_by, arena, state)?,
                 order_by,
                 options,

crates/polars-plan/src/plans/expr_ir.rs

@@ -28,17 +28,21 @@ pub enum OutputName {
 }
 
 impl OutputName {
-    pub fn unwrap(&self) -> &PlSmallStr {
+    pub fn get(&self) -> Option<&PlSmallStr> {
         match self {
-            OutputName::Alias(name) => name,
-            OutputName::ColumnLhs(name) => name,
-            OutputName::LiteralLhs(name) => name,
+            OutputName::Alias(name) => Some(name),
+            OutputName::ColumnLhs(name) => Some(name),
+            OutputName::LiteralLhs(name) => Some(name),
             #[cfg(feature = "dtype-struct")]
-            OutputName::Field(name) => name,
-            OutputName::None => panic!("no output name set"),
+            OutputName::Field(name) => Some(name),
+            OutputName::None => None,
         }
     }
 
+    pub fn unwrap(&self) -> &PlSmallStr {
+        self.get().expect("no output name set")
+    }
+
     pub(crate) fn is_none(&self) -> bool {
         matches!(self, OutputName::None)
     }