Ensure we don't try and build an empty table when modifying entries w…

…ith no values (#54639)

src/Compilers/CSharp/Test/Semantic/SourceGeneration/StateTableTests.cs

@@ -155,6 +155,28 @@ public void Node_Table_Single_Returns_Second_Item_When_First_Is_Removed()
  Assert.Equal(2, table.Single());
  }
 
+ [Fact]
+ public void Node_Builder_Handles_Modification_When_Both_Tables_Have_Empty_Entries()
+ {
+ var builder = NodeStateTable<int>.Empty.ToBuilder();
+ builder.AddEntries(ImmutableArray.Create(1, 2), EntryState.Added);
+ builder.AddEntries(ImmutableArray<int>.Empty, EntryState.Added);
+ builder.AddEntries(ImmutableArray.Create(3, 4), EntryState.Added);
+ var previousTable = builder.ToImmutableAndFree();
+
+ var expected = ImmutableArray.Create((1, EntryState.Added), (2, EntryState.Added), (3, EntryState.Added), (4, EntryState.Added));
+ AssertTableEntries(previousTable, expected);
+
+ builder = previousTable.ToBuilder();
+ Assert.True(builder.TryModifyEntries(ImmutableArray.Create(3, 2), EqualityComparer<int>.Default)); // ((3, EntryState.Modified), (2, EntryState.Cached))
+ Assert.True(builder.TryModifyEntries(ImmutableArray<int>.Empty, EqualityComparer<int>.Default)); // nothing
+ Assert.True(builder.TryModifyEntries(ImmutableArray.Create(3, 5), EqualityComparer<int>.Default)); // ((3, EntryState.Cached), (5, EntryState.Modified))
+ var newTable = builder.ToImmutableAndFree();
+
+ expected = ImmutableArray.Create((3, EntryState.Modified), (2, EntryState.Cached), (3, EntryState.Cached), (5, EntryState.Modified));
+ AssertTableEntries(newTable, expected);
+ }
+
  [Fact]
  public void Driver_Table_Calls_Into_Node_With_Self()
  {

src/Compilers/Core/Portable/SourceGeneration/Nodes/NodeStateTable.cs

@@ -205,6 +205,14 @@ public bool TryModifyEntries(ImmutableArray<T> outputs, IEqualityComparer<T> com
  // - Added when new item position < previousTable.length
 
  var previousEntry = _previous._states[_states.Count];
+
+ // when both entries have no items, we can short circuit
+ if (previousEntry.Count == 0 && outputs.Length == 0)
+ {
+ _states.Add(previousEntry);
+ return true;
+ }
+
  var modified = new TableEntry.Builder();
  var sharedCount = Math.Min(previousEntry.Count, outputs.Length);