Add comments explaining that spreads always require runtime-tracked index

Copilot · CyrusNajmabadi · Copilot · commit 38fdecea1b79 · 2025-11-05T00:59:48.000Z
Added comments to clarify that when spread elements are present, we always
need a runtime-tracked index variable (indexTemp is not null).

Co-authored-by: CyrusNajmabadi &lt;4564579+CyrusNajmabadi@users.noreply.github.com&gt;
diff --git a/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs b/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs
@@ -906,6 +906,7 @@ private BoundExpression CreateAndPopulateArray(BoundCollectionExpression node, A
                 },
                 tryOptimizeSpreadElement: (ArrayBuilder<BoundExpression> sideEffects, BoundExpression arrayTemp, BoundCollectionExpressionSpreadElement spreadElement, BoundExpression rewrittenSpreadOperand) =>
                 {
+                    // When we have spreads, we always need a runtime-tracked index variable.
                     Debug.Assert(indexTemp is not null);
 
                     if (PrepareCopyToOptimization(spreadElement, rewrittenSpreadOperand) is not var (spanSliceMethod, spreadElementAsSpan, getLengthMethod, copyToMethod))
@@ -1266,6 +1267,7 @@ private BoundExpression CreateAndPopulateList(BoundCollectionExpression node, Ty
                     },
                     tryOptimizeSpreadElement: (ArrayBuilder<BoundExpression> sideEffects, BoundExpression spanTemp, BoundCollectionExpressionSpreadElement spreadElement, BoundExpression rewrittenSpreadOperand) =>
                     {
+                        // When we have spreads, we always need a runtime-tracked index variable.
                         Debug.Assert(indexTemp is not null);
 
                         if (PrepareCopyToOptimization(spreadElement, rewrittenSpreadOperand) is not var (spanSliceMethod, spreadElementAsSpan, getLengthMethod, copyToMethod))

Original file line number	Diff line number	Diff line change
`@@ -906,6 +906,7 @@ private BoundExpression CreateAndPopulateArray(BoundCollectionExpression node, A`
`906`	`906`	`},`
`907`	`907`	`tryOptimizeSpreadElement: (ArrayBuilder<BoundExpression> sideEffects, BoundExpression arrayTemp, BoundCollectionExpressionSpreadElement spreadElement, BoundExpression rewrittenSpreadOperand) =>`
`908`	`908`	`{`
	`909`	`+ // When we have spreads, we always need a runtime-tracked index variable.`
`909`	`910`	`Debug.Assert(indexTemp is not null);`
`910`	`911`
`911`	`912`	`if (PrepareCopyToOptimization(spreadElement, rewrittenSpreadOperand) is not var (spanSliceMethod, spreadElementAsSpan, getLengthMethod, copyToMethod))`
`@@ -1266,6 +1267,7 @@ private BoundExpression CreateAndPopulateList(BoundCollectionExpression node, Ty`
`1266`	`1267`	`},`
`1267`	`1268`	`tryOptimizeSpreadElement: (ArrayBuilder<BoundExpression> sideEffects, BoundExpression spanTemp, BoundCollectionExpressionSpreadElement spreadElement, BoundExpression rewrittenSpreadOperand) =>`
`1268`	`1269`	`{`
	`1270`	`+ // When we have spreads, we always need a runtime-tracked index variable.`
`1269`	`1271`	`Debug.Assert(indexTemp is not null);`
`1270`	`1272`
`1271`	`1273`	`if (PrepareCopyToOptimization(spreadElement, rewrittenSpreadOperand) is not var (spanSliceMethod, spreadElementAsSpan, getLengthMethod, copyToMethod))`