Add readonly to struct methods (#8656)

In recent dogfood VS builds we've gotten many IDE0251 suggestions, some warnings escalated to errors in StringTools. Follow many of those suggestions, but bump the rule down to suggestion always, like the related IDE0250.

Author: rainersigwald

.editorconfig

@@ -390,8 +390,11 @@ dotnet_diagnostic.IDE0241.severity = suggestion
 # Struct can be made 'readonly'
 dotnet_diagnostic.IDE0250.severity = suggestion
 
+# Struct methods can be made 'readonly'
+dotnet_diagnostic.IDE0251.severity = suggestion
+
 # Null check can be simplified
 dotnet_diagnostic.IDE0270.severity = suggestion
 
 # naming rule violation
-dotnet_diagnostic.IDE1006.severity = suggestion
+dotnet_diagnostic.IDE1006.severity = suggestion

src/Build/Collections/ConvertingEnumerable.cs

@@ -82,7 +82,7 @@ internal ConvertingEnumerator(IEnumerator<TFrom2> backingEnumerator, Func<TFrom2
             /// <summary>
             /// Get the current element, converted
             /// </summary>
-            public TTo2 Current
+            public readonly TTo2 Current
             {
                 get
                 {

src/Build/Construction/ProjectElementContainer.cs

@@ -742,7 +742,7 @@ internal ProjectElementSiblingEnumerable(ProjectElement initial, bool forwards =
             /// <summary>
             /// Get enumerator
             /// </summary>
-            public IEnumerator<ProjectElement> GetEnumerator()
+            public readonly IEnumerator<ProjectElement> GetEnumerator()
             {
                 return _enumerator;
             }
@@ -808,7 +808,7 @@ object System.Collections.IEnumerator.Current
                 /// <summary>
                 /// Dispose. Do nothing.
                 /// </summary>
-                public void Dispose()
+                public readonly void Dispose()
                 {
                 }
 

src/Build/Definition/Project.cs

@@ -2558,7 +2558,7 @@ public static CumulativeRemoveElementData Create()
                     };
                 }
 
-                public void AccumulateInformationFromRemoveItemSpec(EvaluationItemSpec removeSpec)
+                public readonly void AccumulateInformationFromRemoveItemSpec(EvaluationItemSpec removeSpec)
                 {
                     IEnumerable<string> removeSpecFragmentStrings = removeSpec.FlattenFragmentsAsStrings();
                     var removeGlob = removeSpec.ToMSBuildGlob();

src/Build/Definition/ToolsetReader.cs

@@ -817,7 +817,7 @@ private MSBuildExtensionsPathReferenceKind(string value)
         /// <summary>
         /// Returns the corresponding property name - eg. "$(MSBuildExtensionsPath32)"
         /// </summary>
-        public string MSBuildPropertyName => String.Format($"$({StringRepresentation})");
+        public readonly string MSBuildPropertyName => String.Format($"$({StringRepresentation})");
 
         /// <summary>
         /// Tries to find a reference to MSBuildExtensionsPath* property in the given string

src/Build/Evaluation/Expander.cs

@@ -210,7 +210,7 @@ public void Add(ReadOnlyMemory<char> span)
             /// concatenation of the string representation of the values, each additionally subjected
             /// to file path adjustment.
             /// </returns>
-            public object GetResult()
+            public readonly object GetResult()
             {
                 CheckDisposed();
                 if (_firstObject != null)
@@ -235,7 +235,7 @@ public void Dispose()
             /// <summary>
             /// Throws <see cref="ObjectDisposedException"/> if this concatenator is already disposed.
             /// </summary>
-            private void CheckDisposed() =>
+            private readonly void CheckDisposed() =>
                 ErrorUtilities.VerifyThrowObjectDisposed(!_disposed, nameof(SpanBasedConcatenator));
 
             /// <summary>
@@ -3142,7 +3142,7 @@ private struct FunctionBuilder<T>
             /// </summary>
             public UsedUninitializedProperties UsedUninitializedProperties { get; set; }
 
-            internal Function<T> Build()
+            internal readonly Function<T> Build()
             {
                 return new Function<T>(
                     ReceiverType,

src/Build/Evaluation/ItemsAndMetadataPair.cs

@@ -40,7 +40,7 @@ internal ItemsAndMetadataPair(HashSet<string> items, Dictionary<string, Metadata
         /// </summary>
         internal HashSet<string> Items
         {
-            get
+            readonly get
             {
                 return _items;
             }
@@ -58,7 +58,7 @@ internal HashSet<string> Items
         /// </summary>
         internal Dictionary<string, MetadataReference> Metadata
         {
-            get
+            readonly get
             {
                 return _metadata;
             }

src/Build/Evaluation/LazyItemEvaluator.cs

@@ -135,7 +135,7 @@ public ItemData(I item, ProjectItemElement originatingItemElement, int elementOr
                 _normalizedItemValue = normalizedItemValue;
             }
 
-            public ItemData Clone(IItemFactory<I, I> itemFactory, ProjectItemElement initialItemElementForFactory)
+            public readonly ItemData Clone(IItemFactory<I, I> itemFactory, ProjectItemElement initialItemElementForFactory)
             {
                 // setting the factory's item element to the original item element that produced the item
                 // otherwise you get weird things like items that appear to have been produced by update elements

src/Build/Graph/ProjectGraph.cs

@@ -766,7 +766,7 @@ public ProjectGraphBuildRequest(ProjectGraphNode node, ImmutableList<string> tar
 
             public ImmutableList<string> RequestedTargets { get; }
 
-            public bool Equals(ProjectGraphBuildRequest other)
+            public readonly bool Equals(ProjectGraphBuildRequest other)
             {
                 if (Node != other.Node
                     || RequestedTargets.Count != other.RequestedTargets.Count)
@@ -786,12 +786,12 @@ public bool Equals(ProjectGraphBuildRequest other)
                 return true;
             }
 
-            public override bool Equals(object obj)
+            public override readonly bool Equals(object obj)
             {
                 return !(obj is null) && obj is ProjectGraphBuildRequest graphNodeWithTargets && Equals(graphNodeWithTargets);
             }
 
-            public override int GetHashCode()
+            public override readonly int GetHashCode()
             {
                 unchecked
                 {

src/Build/Graph/ProjectGraphEntryPoint.cs

@@ -61,7 +61,7 @@ internal static IEnumerable<ProjectGraphEntryPoint> CreateEnumerable(IEnumerable
             }
         }
 
-        internal IEnumerable<ProjectGraphEntryPoint> AsEnumerable()
+        internal readonly IEnumerable<ProjectGraphEntryPoint> AsEnumerable()
         {
             yield return this;
         }