Addressed PR feedback.

src/Compilers/CSharp/Portable/BoundTree/UnboundLambda.cs

@@ -112,7 +112,7 @@ public TypeWithAnnotations GetInferredReturnType(ConversionsBase conversions, Nu
                 // will be called again from NullableWalker.ApplyConversion when the
                 // BoundLambda is converted to an anonymous function.
                 // https://github.com/dotnet/roslyn/issues/31752: Can we avoid generating extra
-                // diagnostics? And is this exponential when there are nested lambdas
+                // diagnostics? And is this exponential when there are nested lambdas?
                 var returnTypes = ArrayBuilder<(BoundReturnStatement, TypeWithAnnotations)>.GetInstance();
                 var diagnostics = DiagnosticBag.GetInstance();
                 var delegateType = Type.GetDelegateType();

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.DebugVerifier.cs

@@ -56,7 +56,7 @@ protected override BoundExpression VisitExpressionWithoutStackGuard(BoundExpress
 
             public override BoundNode Visit(BoundNode node)
             {
-                // Ensure that we always have a checkpoint for every BoundExpression in the map
+                // Ensure that we always have a snapshot for every BoundExpression in the map
                 if (_snapshotManager != null && node != null)
                 {
                     _snapshotManager.VerifyNode(node);

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.SnapshotManager.cs

@@ -14,7 +14,7 @@ internal sealed partial class NullableWalker
         internal sealed class SnapshotManager
         {
             /// <summary>
-            /// The int key corresponds to <see cref="Snapshot.GlobalStateIndex"/>.
+            /// The int key corresponds to <see cref="Snapshot.SharedStateIndex"/>.
             /// </summary>
             private readonly ImmutableArray<SharedWalkerState> _walkerGlobalStates;
             /// <summary>
@@ -53,7 +53,7 @@ private SnapshotManager(ImmutableArray<SharedWalkerState> walkerGlobalStates, Im
                     return null;
                 }
 
-                var globalState = _walkerGlobalStates[incrementalSnapshot.GlobalStateIndex];
+                var globalState = _walkerGlobalStates[incrementalSnapshot.SharedStateIndex];
                 var variableState = new VariableState(globalState.VariableSlot, globalState.VariableBySlot, globalState.VariableTypes, incrementalSnapshot.VariableState.Clone());
                 var method = globalState.Symbol as MethodSymbol;
                 return new NullableWalker(
@@ -79,7 +79,7 @@ internal void VerifyNode(BoundNode node)
                 }
 
                 Debug.Assert(_incrementalSnapshots.ContainsKey(node.Syntax.SpanStart), $"Did not find a snapshot for {node} `{node.Syntax}.`");
-                Debug.Assert(_walkerGlobalStates.Length > _incrementalSnapshots[node.Syntax.SpanStart].GlobalStateIndex, $"Did not find global state for {node} `{node.Syntax}`.");
+                Debug.Assert(_walkerGlobalStates.Length > _incrementalSnapshots[node.Syntax.SpanStart].SharedStateIndex, $"Did not find global state for {node} `{node.Syntax}`.");
             }
 #endif
 
@@ -89,6 +89,7 @@ internal void VerifyNode(BoundNode node)
             /// </summary>
             private sealed class DescendingIntComparer : IComparer<int>
             {
+                internal static readonly DescendingIntComparer Singleton = new DescendingIntComparer();
                 public int Compare(int x, int y) => y.CompareTo(x);
             }
 
@@ -99,7 +100,7 @@ internal sealed class Builder
                 /// <summary>
                 /// Snapshots are kept in a dictionary of position -> snapshot at that position. These are stored in descending order.
                 /// </summary>
-                private readonly ImmutableSortedDictionary<int, Snapshot>.Builder _incrementalSnapshots = ImmutableSortedDictionary.CreateBuilder<int, Snapshot>(new DescendingIntComparer());
+                private readonly ImmutableSortedDictionary<int, Snapshot>.Builder _incrementalSnapshots = ImmutableSortedDictionary.CreateBuilder<int, Snapshot>(DescendingIntComparer.Singleton);
                 /// <summary>
                 /// Every walker is walking a specific symbol, and can potentially walk each symbol multiple times
                 /// to get to a stable state. Each of these symbols gets a single global state slot, which this
@@ -161,7 +162,7 @@ internal void TakeIncrementalSnapshot(BoundNode node, LocalState currentState)
         }
 
         /// <summary>
-        /// Contains the shared state state used to restore the walker at a specific point
+        /// Contains the shared state used to restore the walker at a specific point
         /// </summary>
         internal struct SharedWalkerState
         {
@@ -190,12 +191,12 @@ internal struct SharedWalkerState
         private readonly struct Snapshot
         {
             internal readonly LocalState VariableState;
-            internal readonly int GlobalStateIndex;
+            internal readonly int SharedStateIndex;
 
             internal Snapshot(LocalState variableState, int globalStateIndex)
             {
                 VariableState = variableState;
-                GlobalStateIndex = globalStateIndex;
+                SharedStateIndex = globalStateIndex;
             }
         }
     }

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -1878,7 +1878,7 @@ private ArrayTypeSymbol VisitArrayInitializer(BoundArrayCreation node)
                     (BoundExpression expressionNoConversion, Conversion conversion) = RemoveConversion(expression, includeExplicitConversions: false);
                     expressionsNoConversions.Add(expressionNoConversion);
                     conversions.Add(conversion);
-                    SnapshotWalkerThroughConversionGroup(expression as BoundConversion, expressionNoConversion);
+                    SnapshotWalkerThroughConversionGroup(expression, expressionNoConversion);
                     var resultType = VisitRvalueWithState(expressionNoConversion);
                     resultTypes.Add(resultType);
                     placeholderBuilder.Add(CreatePlaceholderIfNecessary(expressionNoConversion, resultType.ToTypeWithAnnotations()));
@@ -2685,7 +2685,7 @@ public override BoundNode VisitConditionalOperator(BoundConditionalOperator node
 
                 BoundExpression operandNoConversion;
                 (operandNoConversion, conversion) = RemoveConversion(operand, includeExplicitConversions: false);
-                SnapshotWalkerThroughConversionGroup(operand as BoundConversion, operandNoConversion);
+                SnapshotWalkerThroughConversionGroup(operand, operandNoConversion);
                 Visit(operandNoConversion);
                 return (operandNoConversion, conversion, ResultType);
             }
@@ -3371,7 +3371,7 @@ void visitArgumentEvaluateAndUnsplit(int argumentIndex, bool assertsTrue, bool a
                         (argument, conversion) = RemoveConversion(argument, includeExplicitConversions: false);
                         if (argument != before)
                         {
-                            SnapshotWalkerThroughConversionGroup(before as BoundConversion, argument);
+                            SnapshotWalkerThroughConversionGroup(before, argument);
                             includedConversion = true;
                         }
                     }
@@ -3875,7 +3875,7 @@ public override BoundNode VisitConversion(BoundConversion node)
             Debug.Assert(targetType.HasType);
 
             (BoundExpression operand, Conversion conversion) = RemoveConversion(node, includeExplicitConversions: true);
-            SnapshotWalkerThroughConversionGroup(node as BoundConversion, operand);
+            SnapshotWalkerThroughConversionGroup(node, operand);
             TypeWithState operandType = VisitRvalueWithState(operand);
             SetResultType(node,
                 VisitConversion(
@@ -3909,7 +3909,7 @@ private TypeWithState VisitOptionalImplicitConversion(BoundExpression expr, Type
             }
 
             (BoundExpression operand, Conversion conversion) = RemoveConversion(expr, includeExplicitConversions: false);
-            SnapshotWalkerThroughConversionGroup(expr as BoundConversion, operand);
+            SnapshotWalkerThroughConversionGroup(expr, operand);
             var operandType = VisitRvalueWithState(operand);
             // If an explicit conversion was used in place of an implicit conversion, the explicit
             // conversion was created by initial binding after reporting "error CS0266:
@@ -4739,12 +4739,18 @@ static TypeWithState calculateResultType(TypeWithAnnotations targetTypeWithNulla
             return operandType;
         }
 
-        private void SnapshotWalkerThroughConversionGroup(BoundConversion conversionOpt, BoundExpression convertedNode)
+        private void SnapshotWalkerThroughConversionGroup(BoundExpression conversionExpression, BoundExpression convertedNode)
         {
+            if (_snapshotBuilderOpt is null)
+            {
+                return;
+            }
+
+            var conversionOpt = conversionExpression as BoundConversion;
             var conversionGroup = conversionOpt?.ConversionGroupOpt;
             while (conversionOpt != null &&
                    conversionOpt != convertedNode &&
-                   conversionOpt.Syntax.SpanStart != conversionOpt.Syntax.SpanStart)
+                   conversionOpt.Syntax.SpanStart != convertedNode.Syntax.SpanStart)
             {
                 Debug.Assert(conversionOpt.ConversionGroupOpt == conversionGroup);
                 TakeIncrementalSnapshot(conversionOpt);
@@ -5751,7 +5757,7 @@ protected override void VisitForEachExpression(BoundForEachStatement node)
             }
 
             var (expr, conversion) = RemoveConversion(node.Expression, includeExplicitConversions: false);
-            SnapshotWalkerThroughConversionGroup(node.Expression as BoundConversion, expr);
+            SnapshotWalkerThroughConversionGroup(node.Expression, expr);
 
             // There are 7 ways that a foreach can be created:
             //    1. The collection type is an array type. For this, initial binding will generate an implicit reference conversion to

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

@@ -503,10 +503,10 @@ public override BoundNode VisitSwitchExpression(BoundSwitchExpression node)
             foreach (var arm in node.SwitchArms)
             {
                 SetState(!arm.Pattern.HasErrors && labelStateMap.TryGetValue(arm.Label, out var labelState) ? labelState.state : UnreachableState());
-                // https://github.com/dotnet/roslyn/issues/35836 Is this where we want to take the checkpoint?
+                // https://github.com/dotnet/roslyn/issues/35836 Is this where we want to take the snapshot?
                 TakeIncrementalSnapshot(arm);
                 (BoundExpression expression, Conversion conversion) = RemoveConversion(arm.Value, includeExplicitConversions: false);
-                SnapshotWalkerThroughConversionGroup(arm.Value as BoundConversion, expression);
+                SnapshotWalkerThroughConversionGroup(arm.Value, expression);
                 expressions.Add(expression);
                 conversions.Add(conversion);
                 var armType = VisitRvalueWithState(expression);