Replace ValueStringBuilder.set_Length with indexer (#27274)

Indexer that let's you see and edit the content of the pending string is more efficient and flexible.

Fixes #26643

Author: jkotas

src/Common/src/CoreLib/System/Collections/Generic/ValueListBuilder.cs

@@ -23,6 +23,15 @@ public ValueListBuilder(Span<T> initialSpan)
 
         public int Length => _pos;
 
+        public ref T this[int index]
+        {
+            get
+            {
+                Debug.Assert(index < _pos);
+                return ref _span[index];
+            }
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Append(T item)
         {

src/Common/src/CoreLib/System/Text/ValueStringBuilder.cs

@@ -21,27 +21,21 @@ public ValueStringBuilder(Span<char> initialBuffer)
             _pos = 0;
         }
 
-        public int Length
+        public int Length => _pos;
+
+        public ref char this[int index]
         {
-            get => _pos;
-            set
-            {
-                int delta = value - _pos;
-                if (delta > 0)
-                {
-                    Append('\0', delta);
-                }
-                else
-                {
-                    _pos = value;
-                }
+            get
+            {
+                Debug.Assert(index < _pos);
+                return ref _chars[index];
             }
         }
 
         public override string ToString()
         {
             var s = new string(_chars.Slice(0, _pos));
-            Clear();
+            Dispose();
             return s;
         }
 
@@ -50,13 +44,13 @@ public bool TryCopyTo(Span<char> destination, out int charsWritten)
             if (_chars.Slice(0, _pos).TryCopyTo(destination))
             {
                 charsWritten = _pos;
-                Clear();
+                Dispose();
                 return true;
             }
             else
             {
                 charsWritten = 0;
-                Clear();
+                Dispose();
                 return false;
             }
         }
@@ -185,7 +179,7 @@ private void Grow(int requiredAdditionalCapacity)
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private void Clear()
+        public void Dispose()
         {
             char[] toReturn = _arrayToReturnToPool;
             this = default; // for safety, to avoid using pooled array if this instance is erroneously appended to again

src/Common/tests/Tests/System/Text/ValueStringBuilderTests.cs

@@ -214,31 +214,16 @@ public void TryCopyTo_ClearsBuilder_ThenReusable()
         }
 
         [Fact]
-        public unsafe void Length_Growing_SetsNulls()
+        public unsafe void Indexer()
         {
             const string Text1 = "foobar";
             var vsb = new ValueStringBuilder();
 
-            // Shrink then grow within capacity
             vsb.Append(Text1);
-            Assert.Equal(Text1.Length, vsb.Length);
-            vsb.Length = 3;
-            Assert.Equal(3, vsb.Length);
-            vsb.Length = 6;
-            Assert.Equal(6, vsb.Length);
-            Assert.Equal("foo\0\0\0", vsb.ToString());
-
-            // Grow over capacity
-            const string Text2 = "bar";
-            Span<char> stackSpace = stackalloc char[Text2.Length];
-            var vsb2 = new ValueStringBuilder(stackSpace);
-            Assert.Equal(0, vsb2.Length);
-            vsb2.Append(Text2);
-            Assert.True(Text2.AsReadOnlySpan().SequenceEqual(stackSpace), "existing stack buffer should have been used");
-            Assert.Equal(Text2.Length, vsb2.Length);
-            vsb2.Length = 6;
-            Assert.Equal(6, vsb2.Length);
-            Assert.Equal("bar\0\0\0", vsb2.ToString());
+
+            Assert.Equal(vsb[3], 'b');
+            vsb[3] = 'c';
+            Assert.Equal(vsb[3], 'c');
         }
     }
 }

src/System.IO.FileSystem/src/System/IO/Enumeration/FileSystemName.cs

@@ -46,8 +46,7 @@ public static string TranslateDosExpression(string expression)
                         modified = true;
                         if (i > 1 && i == length - 1 && expression[i - 1] == '*')
                         {
-                            sb.Length--;
-                            sb.Append('<'); // DOS_STAR (ends in *.)
+                            sb[sb.Length - 1] = '<'; // DOS_STAR (ends in *.)
                         }
                         else if (i < length - 1 && (expression[i + 1] == '?' || expression[i + 1] == '*'))
                         {

src/System.Text.RegularExpressions/src/System/Text/RegularExpressions/ValueListBuilder.Pop.cs

@@ -11,8 +11,6 @@ namespace System.Collections.Generic
     /// </summary>
     internal ref partial struct ValueListBuilder<T>
     {
-        public ref T this[int index] => ref _span[index];
-        
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public T Pop()
         {