Added more readonly modifiers to enumerators

Author: Sergio0694

Microsoft.Toolkit.HighPerformance/Enumerables/ReadOnlySpanEnumerable{T}.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -42,8 +43,9 @@ public ReadOnlySpanEnumerable(ReadOnlySpan<T> span)
         /// Implements the duck-typed <see cref="IEnumerable{T}.GetEnumerator"/> method.
         /// </summary>
         /// <returns>An <see cref="ReadOnlySpanEnumerable{T}"/> instance targeting the current <see cref="ReadOnlySpan{T}"/> value.</returns>
+        [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ReadOnlySpanEnumerable<T> GetEnumerator() => this;
+        public readonly ReadOnlySpanEnumerable<T> GetEnumerator() => this;
 
         /// <summary>
         /// Implements the duck-typed <see cref="System.Collections.IEnumerator.MoveNext"/> method.
@@ -67,7 +69,7 @@ public bool MoveNext()
         /// <summary>
         /// Gets the duck-typed <see cref="IEnumerator{T}.Current"/> property.
         /// </summary>
-        public Item Current
+        public readonly Item Current
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get

Microsoft.Toolkit.HighPerformance/Enumerables/ReadOnlySpanTokenizer{T}.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 
 namespace Microsoft.Toolkit.HighPerformance.Enumerables
@@ -54,8 +55,9 @@ public ReadOnlySpanTokenizer(ReadOnlySpan<T> span, T separator)
         /// Implements the duck-typed <see cref="IEnumerable{T}.GetEnumerator"/> method.
         /// </summary>
         /// <returns>An <see cref="ReadOnlySpanTokenizer{T}"/> instance targeting the current <see cref="ReadOnlySpan{T}"/> value.</returns>
+        [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ReadOnlySpanTokenizer<T> GetEnumerator() => this;
+        public readonly ReadOnlySpanTokenizer<T> GetEnumerator() => this;
 
         /// <summary>
         /// Implements the duck-typed <see cref="System.Collections.IEnumerator.MoveNext"/> method.
@@ -94,7 +96,7 @@ public bool MoveNext()
         /// <summary>
         /// Gets the duck-typed <see cref="IEnumerator{T}.Current"/> property.
         /// </summary>
-        public ReadOnlySpan<T> Current
+        public readonly ReadOnlySpan<T> Current
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get => this.span.Slice(this.start, this.end - this.start);

Microsoft.Toolkit.HighPerformance/Enumerables/SpanEnumerable{T}.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -42,8 +43,9 @@ public SpanEnumerable(Span<T> span)
         /// Implements the duck-typed <see cref="IEnumerable{T}.GetEnumerator"/> method.
         /// </summary>
         /// <returns>An <see cref="SpanEnumerable{T}"/> instance targeting the current <see cref="Span{T}"/> value.</returns>
+        [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public SpanEnumerable<T> GetEnumerator() => this;
+        public readonly SpanEnumerable<T> GetEnumerator() => this;
 
         /// <summary>
         /// Implements the duck-typed <see cref="System.Collections.IEnumerator.MoveNext"/> method.
@@ -67,7 +69,7 @@ public bool MoveNext()
         /// <summary>
         /// Gets the duck-typed <see cref="IEnumerator{T}.Current"/> property.
         /// </summary>
-        public Item Current
+        public readonly Item Current
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get

Microsoft.Toolkit.HighPerformance/Enumerables/SpanTokenizer{T}.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.Contracts;
 using System.Runtime.CompilerServices;
 
 namespace Microsoft.Toolkit.HighPerformance.Enumerables
@@ -54,8 +55,9 @@ public SpanTokenizer(Span<T> span, T separator)
         /// Implements the duck-typed <see cref="IEnumerable{T}.GetEnumerator"/> method.
         /// </summary>
         /// <returns>An <see cref="SpanTokenizer{T}"/> instance targeting the current <see cref="Span{T}"/> value.</returns>
+        [Pure]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public SpanTokenizer<T> GetEnumerator() => this;
+        public readonly SpanTokenizer<T> GetEnumerator() => this;
 
         /// <summary>
         /// Implements the duck-typed <see cref="System.Collections.IEnumerator.MoveNext"/> method.
@@ -94,7 +96,7 @@ public bool MoveNext()
         /// <summary>
         /// Gets the duck-typed <see cref="IEnumerator{T}.Current"/> property.
         /// </summary>
-        public Span<T> Current
+        public readonly Span<T> Current
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get => this.span.Slice(this.start, this.end - this.start);