Add signature help support for ref struct interfaces (#73624)

The selected display part building code for method constraints in signature help had not yet been modified to support ITypeParameterSymbol.AllowsRefLikeType

This is in support of the "allows ref struct" on interfaces feature outlined here: #72124

This ref structs for interfaces feature was merged via this PR: #73567

Author: ToddGrun

src/EditorFeatures/CSharpTest/SignatureHelp/GenericNameSignatureHelpProviderTests.cs

@@ -451,6 +451,30 @@ void Goo()
             await TestAsync(markup, expectedOrderedItems);
         }
 
+        [Fact]
+        public async Task DeclaringGenericTypeWithConstraintsAllowRefStruct()
+        {
+            var markup = """
+                class G<S> where S : allows ref struct
+                { };
+
+                class C
+                {
+                    void Goo()
+                    {
+                        [|G<$$|]>
+                    }
+                }
+                """;
+
+            var expectedOrderedItems = new List<SignatureHelpTestItem>
+            {
+                new SignatureHelpTestItem("G<S> where S : allows ref struct", string.Empty, string.Empty, currentParameterIndex: 0)
+            };
+
+            await TestAsync(markup, expectedOrderedItems);
+        }
+
         #endregion
 
         #region "Generic member invocation"

src/Features/CSharp/Portable/SignatureHelp/GenericNameSignatureHelpProvider.cs

@@ -284,6 +284,22 @@ private static IList<SymbolDisplayPart> GetSelectedDisplayParts(
                 parts.Add(Keyword(SyntaxKind.NewKeyword));
                 parts.Add(Punctuation(SyntaxKind.OpenParenToken));
                 parts.Add(Punctuation(SyntaxKind.CloseParenToken));
+                needComma = true;
+            }
+
+            if (typeParam.AllowsRefLikeType)
+            {
+                if (needComma)
+                {
+                    parts.Add(Punctuation(SyntaxKind.CommaToken));
+                    parts.Add(Space());
+                }
+
+                parts.Add(Keyword(SyntaxKind.AllowsKeyword));
+                parts.Add(Space());
+                parts.Add(Keyword(SyntaxKind.RefKeyword));
+                parts.Add(Space());
+                parts.Add(Keyword(SyntaxKind.StructKeyword));
             }
         }
 
@@ -293,6 +309,7 @@ private static IList<SymbolDisplayPart> GetSelectedDisplayParts(
     private static bool TypeParameterHasConstraints(ITypeParameterSymbol typeParam)
     {
         return !typeParam.ConstraintTypes.IsDefaultOrEmpty || typeParam.HasConstructorConstraint ||
-            typeParam.HasReferenceTypeConstraint || typeParam.HasValueTypeConstraint;
+            typeParam.HasReferenceTypeConstraint || typeParam.HasValueTypeConstraint ||
+            typeParam.AllowsRefLikeType;
     }
 }

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Utilities/SignatureComparer.cs

@@ -223,7 +223,8 @@ private bool HaveSameConstraints(ITypeParameterSymbol typeParameter1, ITypeParam
     {
         if (typeParameter1.HasConstructorConstraint != typeParameter2.HasConstructorConstraint ||
             typeParameter1.HasReferenceTypeConstraint != typeParameter2.HasReferenceTypeConstraint ||
-            typeParameter1.HasValueTypeConstraint != typeParameter2.HasValueTypeConstraint)
+            typeParameter1.HasValueTypeConstraint != typeParameter2.HasValueTypeConstraint ||
+            typeParameter1.AllowsRefLikeType != typeParameter2.AllowsRefLikeType)
         {
             return false;
         }