Mark Substitute.For<T> method as Pure (#844)

src/NSubstitute/Substitute.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.Contracts;
 using NSubstitute.Core;
 
 // Disable nullability for client API, so it does not affect clients.
@@ -6,18 +7,19 @@
 namespace NSubstitute;
 
 /// <summary>
-/// Create a substitute for one or more types. For example: <c>Substitute.For&lt;ISomeType&gt;()</c> 
+/// Create a substitute for one or more types. For example: <c>Substitute.For&lt;ISomeType&gt;()</c>
 /// </summary>
 public static class Substitute
 {
     /// <summary>
     /// Substitute for an interface or class.
-    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members 
+    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members
     /// can be recorded or have return values specified.</para>
     /// </summary>
     /// <typeparam name="T">The type of interface or class to substitute.</typeparam>
     /// <param name="constructorArguments">Arguments required to construct a class being substituted. Not required for interfaces or classes with default constructors.</param>
     /// <returns>A substitute for the interface or class.</returns>
+    [Pure]
     public static T For<T>(params object[] constructorArguments)
         where T : class
     {
@@ -26,13 +28,14 @@ public static T For<T>(params object[] constructorArguments)
 
     /// <summary>
     /// <para>Substitute for multiple interfaces or a class that implements an interface. At most one class can be specified.</para>
-    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members 
+    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members
     /// can be recorded or have return values specified.</para>
     /// </summary>
     /// <typeparam name="T1">The type of interface or class to substitute.</typeparam>
     /// <typeparam name="T2">An additional interface or class (maximum of one class) the substitute should implement.</typeparam>
     /// <param name="constructorArguments">Arguments required to construct a class being substituted. Not required for interfaces or classes with default constructors.</param>
     /// <returns>A substitute of type T1, that also implements T2.</returns>
+    [Pure]
     public static T1 For<T1, T2>(params object[] constructorArguments)
         where T1 : class
         where T2 : class
@@ -43,14 +46,15 @@ public static T1 For<T1, T2>(params object[] constructorArguments)
     /// <summary>
     /// <para>Substitute for multiple interfaces or a class that implements multiple interfaces. At most one class can be specified.</para>
     /// If additional interfaces are required use the <see cref="For(System.Type[], System.Object[])" /> overload.
-    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members 
+    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members
     /// can be recorded or have return values specified.</para>
     /// </summary>
     /// <typeparam name="T1">The type of interface or class to substitute.</typeparam>
     /// <typeparam name="T2">An additional interface or class (maximum of one class) the substitute should implement.</typeparam>
     /// <typeparam name="T3">An additional interface or class (maximum of one class) the substitute should implement.</typeparam>
     /// <param name="constructorArguments">Arguments required to construct a class being substituted. Not required for interfaces or classes with default constructors.</param>
     /// <returns>A substitute of type T1, that also implements T2 and T3.</returns>
+    [Pure]
     public static T1 For<T1, T2, T3>(params object[] constructorArguments)
         where T1 : class
         where T2 : class
@@ -61,12 +65,13 @@ public static T1 For<T1, T2, T3>(params object[] constructorArguments)
 
     /// <summary>
     /// <para>Substitute for multiple interfaces or a class that implements multiple interfaces. At most one class can be specified.</para>
-    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members 
+    /// <para>Be careful when specifying a class, as all non-virtual members will actually be executed. Only virtual members
     /// can be recorded or have return values specified.</para>
     /// </summary>
     /// <param name="typesToProxy">The types of interfaces or a type of class and multiple interfaces the substitute should implement.</param>
     /// <param name="constructorArguments">Arguments required to construct a class being substituted. Not required for interfaces or classes with default constructors.</param>
     /// <returns>A substitute implementing the specified types.</returns>
+    [Pure]
     public static object For(Type[] typesToProxy, object[] constructorArguments)
     {
         var substituteFactory = SubstitutionContext.Current.SubstituteFactory;
@@ -81,8 +86,9 @@ public static object For(Type[] typesToProxy, object[] constructorArguments)
     /// </summary>
     /// <typeparam name="T">The type to substitute for parts of. Must be a class; not a delegate or interface.</typeparam>
     /// <param name="constructorArguments"></param>
-    /// <returns>An instance of the class that will execute real methods when called, but allows parts to be selectively 
+    /// <returns>An instance of the class that will execute real methods when called, but allows parts to be selectively
     /// overridden via `Returns` and `When..DoNotCallBase`.</returns>
+    [Pure]
     public static T ForPartsOf<T>(params object[] constructorArguments)
         where T : class
     {
@@ -93,16 +99,17 @@ public static T ForPartsOf<T>(params object[] constructorArguments)
     /// <summary>
     /// Creates a proxy for a class that implements an interface, forwarding methods and properties to an instance of the class, effectively mimicking a real instance.
     /// Both the interface and the class must be provided as parameters.
-    /// The proxy will log calls made to the interface members and delegate them to an instance of the class. Specific members can be substituted 
+    /// The proxy will log calls made to the interface members and delegate them to an instance of the class. Specific members can be substituted
     /// by using <see cref="WhenCalled{T}.DoNotCallBase()">When(() => call).DoNotCallBase()</see> or by
     /// <see cref="SubstituteExtensions.Returns{T}(T,T,T[])">setting a value to return value</see> for that member.
     /// This extension supports sealed classes and non-virtual members, with some limitations. Since the substituted method is non-virtual, internal calls within the object will invoke the original implementation and will not be logged.
     /// </summary>
     /// <typeparam name="TInterface">The interface the substitute will implement.</typeparam>
     /// <typeparam name="TClass">The class type implementing the interface. Must be a class; not a delegate or interface. </typeparam>
     /// <param name="constructorArguments"></param>
-    /// <returns>An object implementing the selected interface. Calls will be forwarded to the actuall methods, but allows parts to be selectively 
+    /// <returns>An object implementing the selected interface. Calls will be forwarded to the actuall methods, but allows parts to be selectively
     /// overridden via `Returns` and `When..DoNotCallBase`.</returns>
+    [Pure]
     public static TInterface ForTypeForwardingTo<TInterface, TClass>(params object[] constructorArguments)
         where TInterface : class
     {