Add warning for unnecessary at

Author: jjonescz

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Components/ComponentLoweringPass.cs

@@ -163,6 +163,7 @@ private static ComponentIntermediateNode RewriteAsComponent(TagHelperIntermediat
         }
 
         ValidateRequiredAttributes(node, tagHelper, component);
+        WarnForUnnecessaryAt(component);
 
         return component;
     }
@@ -213,6 +214,20 @@ static bool IsPresentAsAttribute(string attributeName, ComponentIntermediateNode
         }
     }
 
+    private static void WarnForUnnecessaryAt(ComponentIntermediateNode component)
+    {
+        foreach (var attribute in component.Attributes)
+        {
+            // IntParam="@x" has unnecessary `@`, can just use IntParam="x" -> warn
+            // StrParam="@x" is different than StrParam="x" -> don't warn
+            if (!attribute.BoundAttribute.IsStringProperty &&
+                attribute.Children is [CSharpExpressionIntermediateNode])
+            {
+                attribute.Diagnostics.Add(RazorDiagnosticFactory.CreateComponentParameter_UnnecessaryAt(attribute.Source));
+            }
+        }
+    }
+
     private static MarkupElementIntermediateNode RewriteAsElement(TagHelperIntermediateNode node)
     {
         var result = new MarkupElementIntermediateNode()

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/RazorDiagnosticFactory.cs

@@ -480,6 +480,14 @@ public static RazorDiagnostic CreateTagHelper_InconsistentTagStructure(SourceSpa
     public static RazorDiagnostic CreateComponent_EditorRequiredParameterNotSpecified(SourceSpan? location, string tagName, string parameterName)
         => RazorDiagnostic.Create(Component_EditorRequiredParameterNotSpecified, location, tagName, parameterName);
 
+    internal static readonly RazorDiagnosticDescriptor ComponentParameter_UnnecessaryAt =
+        new($"{DiagnosticPrefix}2013",
+            Resources.ComponentParameter_UnnecessaryAt,
+            RazorDiagnosticSeverity.Warning);
+
+    public static RazorDiagnostic CreateComponentParameter_UnnecessaryAt(SourceSpan? location)
+        => RazorDiagnostic.Create(ComponentParameter_UnnecessaryAt, location);
+
     #endregion
 
     #region TagHelper Errors

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Resources.resx

@@ -574,6 +574,9 @@
   <data name="Component_EditorRequiredParameterNotSpecified" xml:space="preserve">
     <value>Component '{0}' expects a value for the parameter '{1}', but a value may not have been provided.</value>
   </data>
+  <data name="ComponentParameter_UnnecessaryAt" xml:space="preserve">
+    <value>The '@' prefix is not necessary for component parameters whose type is not string.</value>
+  </data>
   <data name="An_item_with_the_same_key_has_already_been_added_Key_0" xml:space="preserve">
     <value>An item with the same key has already been added. Key: {0}</value>
   </data>

src/Compiler/test/Microsoft.NET.Sdk.Razor.SourceGenerators.Tests/RazorSourceGeneratorComponentTests.cs

@@ -309,7 +309,13 @@ public async Task ComponentParameter_UnnecessaryAt()
         var result = RunGenerator(compilation!, ref driver, out compilation);
 
         // Assert
-        Assert.Empty(result.Diagnostics);
+        result.Diagnostics.VerifyRazor(project,
+            // Shared/Component1.razor(6,23): warning RZ2013: The '@' prefix is not necessary for component parameters whose type is not string.
+            // <Component2 IntParam="@(43)" StrParam="@hi" />
+            Diagnostic("RZ2013", "@(43)").WithLocation(6, 23),
+            // Shared/Component1.razor(7,23): warning RZ2013: The '@' prefix is not necessary for component parameters whose type is not string.
+            // <Component2 IntParam="@x" StrParam="@("lit")" />
+            Diagnostic("RZ2013", "@x").WithLocation(7, 23));
         Assert.Equal(3, result.GeneratedSources.Length);
         await VerifyRazorPageMatchesBaselineAsync(compilation, "Views_Home_Index");
     }