Prefix TagHelpers with tag helper prefix in completion list.

- We don't want tooling applying tag helper prefix to what we deem `TagHelper`s. Ultimately we should consume all the necessary data and do that work.
- Added two tests to validate that we're applying tag helper prefix to `TagHelper` element completions for catch-all's and non-catch-alls.

#1224

src/Microsoft.VisualStudio.LanguageServices.Razor/DefaultTagHelperCompletionService.cs

@@ -45,6 +45,7 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
                 StringComparer.OrdinalIgnoreCase);
 
             var catchAllDescriptors = new HashSet<TagHelperDescriptor>();
+            var prefix = completionContext.DocumentContext.Prefix ?? string.Empty;
             var possibleChildDescriptors = _tagHelperFactsService.GetTagHelpersGivenParent(completionContext.DocumentContext, completionContext.ContainingTagName);
             foreach (var possibleDescriptor in possibleChildDescriptors)
             {
@@ -76,7 +77,7 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
 
                     if (addRuleCompletions)
                     {
-                        UpdateCompletions(rule.TagName, possibleDescriptor);
+                        UpdateCompletions(prefix + rule.TagName, possibleDescriptor);
                     }
                 }
             }
@@ -85,16 +86,18 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
             // This way, any TagHelper added completions will also have catch-alls listed under their entries.
             foreach (var catchAllDescriptor in catchAllDescriptors)
             {
-                foreach (var completionTagNames in elementCompletions.Keys)
+                foreach (var completionTagName in elementCompletions.Keys)
                 {
-                    UpdateCompletions(completionTagNames, catchAllDescriptor);
+                    if (completionTagName.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+                    {
+                        UpdateCompletions(completionTagName, catchAllDescriptor);
+                    }
                 }
             }
 
             var result = ElementCompletionResult.Create(elementCompletions);
             return result;
 
-
             void UpdateCompletions(string tagName, TagHelperDescriptor possibleDescriptor)
             {
                 if (!elementCompletions.TryGetValue(tagName, out var existingRuleDescriptors))

test/Microsoft.VisualStudio.LanguageServices.Razor.Test/DefaultTagHelperCompletionServiceTest.cs

@@ -10,6 +10,75 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
 {
     public class DefaultTagHelperCompletionServiceTest
     {
+        [Fact]
+        public void GetElementCompletions_CatchAllsOnlyApplyToCompletionsStartingWithPrefix()
+        {
+            // Arrange
+            var documentDescriptors = new[]
+            {
+                TagHelperDescriptorBuilder.Create("CatchAllTagHelper", "TestAssembly")
+                    .TagMatchingRule(rule => rule.RequireTagName("*"))
+                    .Build(),
+                TagHelperDescriptorBuilder.Create("LiTagHelper", "TestAssembly")
+                    .TagMatchingRule(rule => rule.RequireTagName("li"))
+                    .Build(),
+            };
+            var expectedCompletions = ElementCompletionResult.Create(new Dictionary<string, HashSet<TagHelperDescriptor>>()
+            {
+                ["th:li"] = new HashSet<TagHelperDescriptor> { documentDescriptors[1], documentDescriptors[0] },
+                ["li"] = new HashSet<TagHelperDescriptor>(),
+            });
+
+            var existingCompletions = new[] { "li" };
+            var completionContext = BuildCompletionContext(
+                documentDescriptors,
+                existingCompletions,
+                containingTagName: "ul",
+                tagHelperPrefix: "th:");
+            var service = CreateTagHelperCompletionFactsService();
+
+            // Act
+            var completions = service.GetElementCompletions(completionContext);
+
+            // Assert
+            AssertCompletionsAreEquivalent(expectedCompletions, completions);
+        }
+
+        [Fact]
+        public void GetElementCompletions_TagHelperPrefixIsPrependedToTagHelperCompletions()
+        {
+            // Arrange
+            var documentDescriptors = new[]
+            {
+                TagHelperDescriptorBuilder.Create("SuperLiTagHelper", "TestAssembly")
+                    .TagMatchingRule(rule => rule.RequireTagName("superli"))
+                    .Build(),
+                TagHelperDescriptorBuilder.Create("LiTagHelper", "TestAssembly")
+                    .TagMatchingRule(rule => rule.RequireTagName("li"))
+                    .Build(),
+            };
+            var expectedCompletions = ElementCompletionResult.Create(new Dictionary<string, HashSet<TagHelperDescriptor>>()
+            {
+                ["th:superli"] = new HashSet<TagHelperDescriptor> { documentDescriptors[0] },
+                ["th:li"] = new HashSet<TagHelperDescriptor> { documentDescriptors[1] },
+                ["li"] = new HashSet<TagHelperDescriptor>(),
+            });
+
+            var existingCompletions = new[] { "li" };
+            var completionContext = BuildCompletionContext(
+                documentDescriptors,
+                existingCompletions,
+                containingTagName: "ul",
+                tagHelperPrefix: "th:");
+            var service = CreateTagHelperCompletionFactsService();
+
+            // Act
+            var completions = service.GetElementCompletions(completionContext);
+
+            // Assert
+            AssertCompletionsAreEquivalent(expectedCompletions, completions);
+        }
+
         [Fact]
         public void GetElementCompletions_CatchAllsApplyToAllCompletions()
         {
@@ -382,9 +451,10 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
             IEnumerable<TagHelperDescriptor> descriptors,
             IEnumerable<string> existingCompletions,
             string containingTagName,
-            string containingParentTagName = "body")
+            string containingParentTagName = "body",
+            string tagHelperPrefix = "")
         {
-            var documentContext = TagHelperDocumentContext.Create(string.Empty, descriptors);
+            var documentContext = TagHelperDocumentContext.Create(tagHelperPrefix, descriptors);
             var completionContext = new ElementCompletionContext(
                 documentContext,
                 existingCompletions,