Use C# 10's enhanced struct initialization features (#63131)

* Use C# 10's enhanced struct initialization features

* Addressed usages

* Remove #pragmas

* Fixed build

Cf. #63131 (comment)

* Update src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.Linq.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>

Author: gfoidl

src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.Linq.cs

@@ -80,7 +80,7 @@ internal static T[] ToArray<T>(IEnumerable<T> source)
                 return result;
             }
 
-            var builder = new LargeArrayBuilder<T>(initialize: true);
+            LargeArrayBuilder<T> builder = new();
             builder.AddRange(source);
             return builder.ToArray();
         }

src/libraries/Common/src/System/Collections/Generic/LargeArrayBuilder.SizeOpt.cs

@@ -15,12 +15,10 @@ internal struct LargeArrayBuilder<T>
     {
         private ArrayBuilder<T> _builder; // mutable struct; do not make this readonly
 
-        public LargeArrayBuilder(bool initialize) : this()
-        {
-            // This is a workaround for C# not having parameterless struct constructors yet.
-            // Once it gets them, replace this with a parameterless constructor.
-            Debug.Assert(initialize);
-        }
+        /// <summary>
+        /// Constructs a new builder.
+        /// </summary>
+        public LargeArrayBuilder() => this = default;
 
         public int Count => _builder.Count;
 

src/libraries/Common/src/System/Collections/Generic/LargeArrayBuilder.SpeedOpt.cs

@@ -25,13 +25,9 @@ internal struct LargeArrayBuilder<T>
         /// <summary>
         /// Constructs a new builder.
         /// </summary>
-        /// <param name="initialize">Pass <c>true</c>.</param>
-        public LargeArrayBuilder(bool initialize)
+        public LargeArrayBuilder()
             : this(maxCapacity: int.MaxValue)
         {
-            // This is a workaround for C# not having parameterless struct constructors yet.
-            // Once it gets them, replace this with a parameterless constructor.
-            Debug.Assert(initialize);
         }
 
         /// <summary>
@@ -42,10 +38,10 @@ public LargeArrayBuilder(bool initialize)
         /// Do not add more than <paramref name="maxCapacity"/> items to this builder.
         /// </remarks>
         public LargeArrayBuilder(int maxCapacity)
-            : this()
         {
             Debug.Assert(maxCapacity >= 0);
 
+            this = default;
             _first = _current = Array.Empty<T>();
             _maxCapacity = maxCapacity;
         }

src/libraries/Common/src/System/Collections/Generic/SparseArrayBuilder.cs

@@ -71,15 +71,10 @@ internal struct SparseArrayBuilder<T>
         /// <summary>
         /// Constructs a new builder.
         /// </summary>
-        /// <param name="initialize">Pass <c>true</c>.</param>
-        public SparseArrayBuilder(bool initialize)
-            : this()
+        public SparseArrayBuilder()
         {
-            // Once C# gains parameterless struct constructors, please
-            // remove this workaround.
-            Debug.Assert(initialize);
-
-            _builder = new LargeArrayBuilder<T>(initialize: true);
+            this = default;
+            _builder = new LargeArrayBuilder<T>();
         }
 
         /// <summary>

src/libraries/Common/tests/Tests/System/Collections/Generic/LargeArrayBuilderTests.cs

@@ -14,7 +14,7 @@ namespace System.Collections.Generic.Tests
         [Fact]
         public void Constructor()
         {
-            var builder = new LargeArrayBuilder<T>(initialize: true);
+            var builder = new LargeArrayBuilder<T>();
 
             Assert.Equal(0, builder.Count);
             Assert.Same(Array.Empty<T>(), builder.ToArray());
@@ -24,8 +24,8 @@ public void Constructor()
         [MemberData(nameof(EnumerableData))]
         public void AddCountAndToArray(IEnumerable<T> seed)
         {
-            var builder1 = new LargeArrayBuilder<T>(initialize: true);
-            var builder2 = new LargeArrayBuilder<T>(initialize: true);
+            var builder1 = new LargeArrayBuilder<T>();
+            var builder2 = new LargeArrayBuilder<T>();
 
             int count = 0;
             foreach (T item in seed)
@@ -63,7 +63,7 @@ public void MaxCapacity(IEnumerable<T> seed, int maxCapacity)
         [MemberData(nameof(EnumerableData))]
         public void AddRange(IEnumerable<T> seed)
         {
-            var builder = new LargeArrayBuilder<T>(initialize: true);
+            var builder = new LargeArrayBuilder<T>();
 
             // Call AddRange multiple times and verify contents w/ each iteration.
             for (int i = 1; i <= 10; i++)
@@ -81,7 +81,7 @@ public void CopyTo(IEnumerable<T> seed, int index, int count)
         {
             var array = new T[seed.Count()];
 
-            var builder = new LargeArrayBuilder<T>(initialize: true);
+            var builder = new LargeArrayBuilder<T>();
             builder.AddRange(seed);
             builder.CopyTo(array, index, count);
 

src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs

@@ -23,7 +23,7 @@ private TSource[] LazyToArray()
             {
                 Debug.Assert(GetCount(onlyIfCheap: true) == -1);
 
-                var builder = new LargeArrayBuilder<TSource>(initialize: true);
+                LargeArrayBuilder<TSource> builder = new();
 
                 if (!_appending)
                 {
@@ -106,7 +106,7 @@ private TSource[] LazyToArray()
             {
                 Debug.Assert(GetCount(onlyIfCheap: true) == -1);
 
-                var builder = new SparseArrayBuilder<TSource>(initialize: true);
+                SparseArrayBuilder<TSource> builder = new();
 
                 if (_prepended != null)
                 {

src/libraries/System.Linq/src/System/Linq/Concat.SpeedOpt.cs

@@ -38,7 +38,7 @@ public override int GetCount(bool onlyIfCheap)
 
             public override TSource[] ToArray()
             {
-                var builder = new SparseArrayBuilder<TSource>(initialize: true);
+                SparseArrayBuilder<TSource> builder = new();
 
                 bool reservedFirst = builder.ReserveOrAdd(_first);
                 bool reservedSecond = builder.ReserveOrAdd(_second);
@@ -102,7 +102,7 @@ private TSource[] LazyToArray()
             {
                 Debug.Assert(!_hasOnlyCollections);
 
-                var builder = new SparseArrayBuilder<TSource>(initialize: true);
+                SparseArrayBuilder<TSource> builder = new();
                 ArrayBuilder<int> deferredCopies = default;
 
                 for (int i = 0; ; i++)

src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs

@@ -22,7 +22,7 @@ private sealed partial class SelectEnumerableIterator<TSource, TResult> : IIList
         {
             public TResult[] ToArray()
             {
-                var builder = new LargeArrayBuilder<TResult>(initialize: true);
+                LargeArrayBuilder<TResult> builder = new();
 
                 foreach (TSource item in _source)
                 {
@@ -585,7 +585,8 @@ private TResult[] LazyToArray()
             {
                 Debug.Assert(_source.GetCount(onlyIfCheap: true) == -1);
 
-                var builder = new LargeArrayBuilder<TResult>(initialize: true);
+                LargeArrayBuilder<TResult> builder = new();
+
                 foreach (TSource input in _source)
                 {
                     builder.Add(_selector(input));

src/libraries/System.Linq/src/System/Linq/SelectMany.SpeedOpt.cs

@@ -31,7 +31,7 @@ public int GetCount(bool onlyIfCheap)
 
             public TResult[] ToArray()
             {
-                var builder = new SparseArrayBuilder<TResult>(initialize: true);
+                SparseArrayBuilder<TResult> builder = new();
                 ArrayBuilder<IEnumerable<TResult>> deferredCopies = default;
 
                 foreach (TSource element in _source)

src/libraries/System.Linq/src/System/Linq/Where.SpeedOpt.cs

@@ -34,7 +34,7 @@ public int GetCount(bool onlyIfCheap)
 
             public TSource[] ToArray()
             {
-                var builder = new LargeArrayBuilder<TSource>(initialize: true);
+                LargeArrayBuilder<TSource> builder = new();
 
                 foreach (TSource item in _source)
                 {
@@ -332,7 +332,7 @@ public int GetCount(bool onlyIfCheap)
 
             public TResult[] ToArray()
             {
-                var builder = new LargeArrayBuilder<TResult>(initialize: true);
+                LargeArrayBuilder<TResult> builder = new();
 
                 foreach (TSource item in _source)
                 {