Add compiler support for @formname attribute

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/CodeGeneration/DefaultDocumentWriter.cs

@@ -314,6 +314,11 @@ public override void VisitSplat(SplatIntermediateNode node)
             Context.NodeWriter.WriteSplat(Context, node);
         }
 
+        public override void VisitFormName(FormNameIntermediateNode node)
+        {
+            Context.NodeWriter.WriteFormName(Context, node);
+        }
+
         public override void VisitDefault(IntermediateNode node)
         {
             Context.RenderChildren(node);

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/CodeGeneration/IntermediateNodeWriter.cs

@@ -76,6 +76,11 @@ public virtual void WriteSplat(CodeRenderingContext context, SplatIntermediateNo
         throw new NotSupportedException("This writer does not support components.");
     }
 
+    public virtual void WriteFormName(CodeRenderingContext context, FormNameIntermediateNode node)
+    {
+        throw new NotSupportedException("This writer does not support components.");
+    }
+
     public abstract void BeginWriterScope(CodeRenderingContext context, string writer);
 
     public abstract void EndWriterScope(CodeRenderingContext context);

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/ComponentResources.resx

@@ -177,6 +177,9 @@
   <data name="EventHandlerTagHelper_StopPropagation_Documentation" xml:space="preserve">
     <value>Specifies whether to prevent further propagation of the '{0}' event in the capturing and bubbling phases.</value>
   </data>
+  <data name="FormNameTagHelper_Documentation" xml:space="preserve">
+    <value>Name used to match the form handler on the server.</value>
+  </data>
   <data name="ImplementsDirective_Description" xml:space="preserve">
     <value>Declares an interface implementation for the current class.</value>
   </data>

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentDesignTimeNodeWriter.cs

@@ -1151,6 +1151,32 @@ private void WriteSplatInnards(CodeRenderingContext context, SplatIntermediateNo
         }
     }
 
+    public sealed override void WriteFormName(CodeRenderingContext context, FormNameIntermediateNode node)
+    {
+        var tokens = node.FindDescendantNodes<IntermediateToken>();
+        if (tokens.Count == 0)
+        {
+            return;
+        }
+
+        // Either all tokens should be C# or none of them.
+        if (tokens[0].IsCSharp)
+        {
+            context.CodeWriter.Write(ComponentsApi.RuntimeHelpers.TypeCheck);
+            context.CodeWriter.Write("<string>(");
+            foreach (var token in tokens)
+            {
+                Debug.Assert(token.IsCSharp);
+                WriteCSharpToken(context, token);
+            }
+            context.CodeWriter.Write(");");
+        }
+        else
+        {
+            Debug.Assert(!tokens.Any(t => t.IsCSharp));
+        }
+    }
+
     public override void WriteReferenceCapture(CodeRenderingContext context, ReferenceCaptureIntermediateNode node)
     {
         if (context == null)

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentDiagnosticFactory.cs

@@ -549,4 +549,26 @@ public static RazorDiagnostic CreateBindAttributeParameter_UnsupportedSyntaxBind
             attribute);
         return diagnostic;
     }
+
+    public static readonly RazorDiagnosticDescriptor FormName_MissingOnSubmit =
+        new RazorDiagnosticDescriptor(
+        $"{DiagnosticPrefix}10021",
+        () => "Attribute '@formname' can only be used when '@onsubmit' event handler is also present.",
+        RazorDiagnosticSeverity.Warning);
+
+    public static RazorDiagnostic CreateFormName_MissingOnSubmit(SourceSpan? source)
+    {
+        return RazorDiagnostic.Create(FormName_MissingOnSubmit, source ?? SourceSpan.Undefined);
+    }
+
+    public static readonly RazorDiagnosticDescriptor FormName_NotAForm =
+        new RazorDiagnosticDescriptor(
+        $"{DiagnosticPrefix}10022",
+        () => "Attribute '@formname' can only be applied to 'form' elements.",
+        RazorDiagnosticSeverity.Warning);
+
+    public static RazorDiagnostic CreateFormName_NotAForm(SourceSpan? source)
+    {
+        return RazorDiagnostic.Create(FormName_NotAForm, source ?? SourceSpan.Undefined);
+    }
 }

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentFormNameLoweringPass.cs

@@ -0,0 +1,94 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Linq;
+using Microsoft.AspNetCore.Razor.Language.Intermediate;
+
+namespace Microsoft.AspNetCore.Razor.Language.Components;
+
+internal sealed class ComponentFormNameLoweringPass : ComponentIntermediateNodePassBase, IRazorOptimizationPass
+{
+    // Run after component lowering pass
+    public override int Order => 50;
+
+    protected override void ExecuteCore(RazorCodeDocument codeDocument, DocumentIntermediateNode documentNode)
+    {
+        if (!IsComponentDocument(documentNode))
+        {
+            return;
+        }
+
+        var references = documentNode.FindDescendantReferences<TagHelperDirectiveAttributeIntermediateNode>();
+        foreach (var reference in references)
+        {
+            var node = (TagHelperDirectiveAttributeIntermediateNode)reference.Node;
+            if (node.TagHelper.IsFormNameTagHelper())
+            {
+                var parent = reference.Parent;
+
+                if (parent is not MarkupElementIntermediateNode { TagName: "form" })
+                {
+                    node.Diagnostics.Add(ComponentDiagnosticFactory.CreateFormName_NotAForm(node.Source));
+                    reference.Replace(RewriteForErrorRecovery(node, parent));
+                    continue;
+                }
+
+                if (!parent.Children.Any(c => c is HtmlAttributeIntermediateNode { AttributeName: "@onsubmit" }))
+                {
+                    node.Diagnostics.Add(ComponentDiagnosticFactory.CreateFormName_MissingOnSubmit(node.Source));
+                    reference.Replace(RewriteForErrorRecovery(node, parent));
+                    continue;
+                }
+
+                reference.Replace(Rewrite(node));
+            }
+        }
+
+        static IntermediateNode Rewrite(TagHelperDirectiveAttributeIntermediateNode node)
+        {
+            return RewriteCore(node, new FormNameIntermediateNode
+            {
+                Source = node.Source
+            });
+        }
+
+        static IntermediateNode RewriteForErrorRecovery(TagHelperDirectiveAttributeIntermediateNode node, IntermediateNode parent)
+        {
+            if (parent is ComponentIntermediateNode)
+            {
+                return RewriteCore(node, new ComponentAttributeIntermediateNode
+                {
+                    Source = node.Source,
+                    AttributeName = node.OriginalAttributeName,
+                });
+            }
+
+            var replacement = new HtmlAttributeIntermediateNode
+            {
+                Source = node.Source,
+                AttributeName = node.OriginalAttributeName,
+                Prefix = node.OriginalAttributeName + "=\"",
+                Suffix = "\"",
+            };
+            replacement.Children.AddRange(node.Children.Select(static child =>
+            {
+                IntermediateNode result = child is CSharpExpressionIntermediateNode
+                    ? new CSharpExpressionAttributeValueIntermediateNode()
+                    : new HtmlAttributeValueIntermediateNode();
+                result.Source = child.Source;
+                result.Children.AddRange(child.Children);
+                result.Diagnostics.AddRange(child.Diagnostics);
+                return result;
+            }));
+            replacement.Diagnostics.AddRange(node.Diagnostics);
+            return replacement;
+        }
+
+        static IntermediateNode RewriteCore(TagHelperDirectiveAttributeIntermediateNode node, IntermediateNode replacement)
+        {
+            replacement.Children.AddRange(node.Children);
+            replacement.Diagnostics.AddRange(node.Diagnostics);
+            return replacement;
+        }
+    }
+}

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentMetadata.cs

@@ -135,6 +135,12 @@ public static class EventHandler
         public const string TagHelperKind = "Components.EventHandler";
     }
 
+    public static class FormName
+    {
+        public const string TagHelperKind = "Components.FormName";
+        public const string RuntimeName = "Components.None";
+    }
+
     public static class Key
     {
         public const string TagHelperKind = "Components.Key";

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentNodeWriter.cs

@@ -5,6 +5,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using Microsoft.AspNetCore.Razor.Language.CodeGeneration;
 using Microsoft.AspNetCore.Razor.Language.Extensions;

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentRuntimeNodeWriter.cs

@@ -198,6 +198,8 @@ public override void WriteMarkupElement(CodeRenderingContext context, MarkupElem
             .WriteStringLiteral(node.TagName)
             .WriteEndMethodInvocation();
 
+        bool hasFormName = false;
+
         // Render attributes and splats (in order) before creating the scope.
         foreach (var child in node.Children)
         {
@@ -213,6 +215,12 @@ public override void WriteMarkupElement(CodeRenderingContext context, MarkupElem
             {
                 context.RenderNode(splat);
             }
+            else if (child is FormNameIntermediateNode formName)
+            {
+                Debug.Assert(!hasFormName);
+                context.RenderNode(formName);
+                hasFormName = true;
+            }
         }
 
         foreach (var setKey in node.SetKeys)
@@ -231,6 +239,20 @@ public override void WriteMarkupElement(CodeRenderingContext context, MarkupElem
             context.RenderNode(child);
         }
 
+        // AddNamedEvent must be called last.
+        if (hasFormName)
+        {
+            // _builder.AddNamedEvent("onsubmit", __formName);
+            context.CodeWriter.Write(_scopeStack.BuilderVarName);
+            context.CodeWriter.Write(".");
+            context.CodeWriter.Write(ComponentsApi.RenderTreeBuilder.AddNamedEvent);
+            context.CodeWriter.Write("(\"onsubmit\", ");
+            context.CodeWriter.Write(_scopeStack.FormNameVarName);
+            context.CodeWriter.Write(");");
+            context.CodeWriter.WriteLine();
+            _scopeStack.IncrementFormName();
+        }
+
         context.CodeWriter
             .WriteStartMethodInvocation($"{_scopeStack.BuilderVarName}.{ComponentsApi.RenderTreeBuilder.CloseElement}")
             .WriteEndMethodInvocation();
@@ -900,6 +922,19 @@ private void WriteSplatInnards(CodeRenderingContext context, SplatIntermediateNo
         }
     }
 
+    public sealed override void WriteFormName(CodeRenderingContext context, FormNameIntermediateNode node)
+    {
+        // string __formName = expression;
+        context.CodeWriter.Write("string ");
+        context.CodeWriter.Write(_scopeStack.FormNameVarName);
+        context.CodeWriter.Write(" = ");
+        context.CodeWriter.Write(ComponentsApi.RuntimeHelpers.TypeCheck);
+        context.CodeWriter.Write("<string>(");
+        WriteAttributeValue(context, node.FindDescendantNodes<IntermediateToken>());
+        context.CodeWriter.Write(")");
+        context.CodeWriter.WriteLine(";");
+    }
+
     public override void WriteReferenceCapture(CodeRenderingContext context, ReferenceCaptureIntermediateNode node)
     {
         // Looks like:
@@ -949,7 +984,7 @@ protected override void WriteReferenceCaptureInnards(CodeRenderingContext contex
         }
     }
 
-    private void WriteAttribute(CodeRenderingContext context, string key, IList<IntermediateToken> value)
+    private void WriteAttribute(CodeRenderingContext context, string key, IReadOnlyList<IntermediateToken> value)
     {
         BeginWriteAttribute(context, key);
 
@@ -969,7 +1004,7 @@ private void WriteAttribute(CodeRenderingContext context, string key, IList<Inte
         context.CodeWriter.WriteEndMethodInvocation();
     }
 
-    private void WriteAttribute(CodeRenderingContext context, IntermediateNode nameExpression, IList<IntermediateToken> value)
+    private void WriteAttribute(CodeRenderingContext context, IntermediateNode nameExpression, IReadOnlyList<IntermediateToken> value)
     {
         BeginWriteAttribute(context, nameExpression);
         if (value.Count > 0)
@@ -1023,7 +1058,7 @@ private static string GetHtmlContent(HtmlContentIntermediateNode node)
     // Only the mixed case is complicated, we want to turn it into code that will concatenate
     // the values into a string at runtime.
 
-    private static void WriteAttributeValue(CodeRenderingContext context, IList<IntermediateToken> tokens)
+    private static void WriteAttributeValue(CodeRenderingContext context, IReadOnlyList<IntermediateToken> tokens)
     {
         if (tokens == null)
         {

src/Compiler/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentsApi.cs

@@ -71,6 +71,8 @@ public static class RenderTreeBuilder
 
         public const string BuilderParameter = "__builder";
 
+        public const string FormNameVariableName = "__formName";
+
         public const string OpenElement = nameof(OpenElement);
 
         public const string CloseElement = nameof(CloseElement);
@@ -86,8 +88,11 @@ public static class RenderTreeBuilder
         public const string AddAttribute = nameof(AddAttribute);
 
         public const string AddMultipleAttributes = nameof(AddMultipleAttributes);
+
         public const string AddComponentParameter = nameof(AddComponentParameter);
 
+        public const string AddNamedEvent = nameof(AddNamedEvent);
+
         public const string AddElementReferenceCapture = nameof(AddElementReferenceCapture);
 
         public const string AddComponentReferenceCapture = nameof(AddComponentReferenceCapture);