Fix for https://github.com/aspnet/Mvc/issues/4903. (#4907)

Fix for https://github.com/aspnet/Mvc/issues/4903.
- `HtmlHelperTextAreaTest` added
- fix up `HtmlHelperTextBoxTest`

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs

@@ -662,6 +662,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             }
 
             tagBuilder.MergeAttribute("name", fullName, true);
+
+            AddPlaceholderAttribute(viewContext.ViewData, tagBuilder, modelExplorer, expression);
             AddValidationAttributes(viewContext, tagBuilder, modelExplorer, expression);
 
             // If there are any errors for a named field, we add this CSS attribute.
@@ -1170,12 +1172,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             var suppliedTypeString = tagBuilder.Attributes["type"];
             if (_placeholderInputTypes.Contains(suppliedTypeString))
             {
-                modelExplorer = modelExplorer ?? ExpressionMetadataProvider.FromStringExpression(expression, viewContext.ViewData, _metadataProvider);
+                AddPlaceholderAttribute(viewContext.ViewData, tagBuilder, modelExplorer, expression);
-                var placeholder = modelExplorer.Metadata.Placeholder;
-                if (!string.IsNullOrEmpty(placeholder))
-                {
-                    tagBuilder.MergeAttribute("placeholder", placeholder);
-                }
             }
 
             var valueParameter = FormatValue(value, format);
@@ -1288,6 +1285,27 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             return tagBuilder;
         }
 
+        /// <summary>
+        /// Adds a placeholder attribute to the <paramref name="tagBuilder" />.
+        /// </summary>
+        /// <param name="viewData">A <see cref="ViewDataDictionary"/> instance for the current scope.</param>
+        /// <param name="tagBuilder">A <see cref="TagBuilder"/> instance.</param>
+        /// <param name="modelExplorer">The <see cref="ModelExplorer"/> for the <paramref name="expression"/>.</param>
+        /// <param name="expression">Expression name, relative to the current model.</param>
+        protected virtual void AddPlaceholderAttribute(
+            ViewDataDictionary viewData,
+            TagBuilder tagBuilder,
+            ModelExplorer modelExplorer,
+            string expression)
+        {
+            modelExplorer = modelExplorer ?? ExpressionMetadataProvider.FromStringExpression(expression, viewData, _metadataProvider);
+            var placeholder = modelExplorer.Metadata.Placeholder;
+            if (!string.IsNullOrEmpty(placeholder))
+            {
+                tagBuilder.MergeAttribute("placeholder", placeholder);
+            }
+        }
+
         /// <summary>
         /// Adds validation attributes to the <paramref name="tagBuilder" /> if client validation
         /// is enabled.

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/HtmlHelperTextAreaTest.cs

@@ -0,0 +1,54 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.ComponentModel.DataAnnotations;
+using Microsoft.AspNetCore.Mvc.TestCommon;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.Rendering
+{
+    public class HtmlHelperTextBoxAreaTest
+    {
+        [Fact]
+        public void TextAreaFor_GeneratesPlaceholderAttribute_WhenDisplayAttributePromptIsSetAndTypeIsValid()
+        {
+            // Arrange            
+            var model = new TextAreaModelWithAPlaceholder();
+            var helper = DefaultTemplatesUtilities.GetHtmlHelper(model);
+
+            // Act
+            var textArea = helper.TextAreaFor(m => m.Property1);
+
+            // Assert 
+            var result = HtmlContentUtilities.HtmlContentToString(textArea);
+            Assert.Contains(@"placeholder=""HtmlEncode[[placeholder]]""", result, StringComparison.Ordinal);
+        }
+
+        [Fact]
+        public void TextAreaFor_DoesNotGeneratePlaceholderAttribute_WhenNoPlaceholderPresentInModel()
+        {
+            // Arrange            
+            var model = new TextAreaModelWithoutAPlaceholder();
+            var helper = DefaultTemplatesUtilities.GetHtmlHelper(model);
+
+            // Act
+            var textArea = helper.TextAreaFor(m => m.Property1);
+
+            // Assert 
+            var result = HtmlContentUtilities.HtmlContentToString(textArea);
+            Assert.DoesNotContain(@"placeholder=""HtmlEncode[[placeholder]]""", result, StringComparison.Ordinal);
+        }
+
+        private class TextAreaModelWithAPlaceholder
+        {
+            [Display(Prompt = "placeholder")]
+            public string Property1 { get; set; }
+        }
+
+        private class TextAreaModelWithoutAPlaceholder
+        {
+            public string Property1 { get; set; }
+        }
+    }
+}

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/HtmlHelperTextBoxTest.cs

@@ -1,8 +1,9 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using Microsoft.AspNetCore.Mvc.TestCommon;
+using System;
 using System.ComponentModel.DataAnnotations;
+using Microsoft.AspNetCore.Mvc.TestCommon;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Mvc.Rendering
@@ -23,10 +24,11 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
             var helper = DefaultTemplatesUtilities.GetHtmlHelper(model);
 
             // Act
-            var result = HtmlContentUtilities.HtmlContentToString(helper.TextBoxFor(m => m.Property1, new { type }));
+            var textBox = helper.TextBoxFor(m => m.Property1, new { type });
 
-            // Assert
+            // Assert 
-            Assert.True(result.Contains(@"placeholder=""HtmlEncode[[placeholder]]"""));
+            var result = HtmlContentUtilities.HtmlContentToString(textBox);
+            Assert.Contains(@"placeholder=""HtmlEncode[[placeholder]]""", result, StringComparison.Ordinal);
         }
 
         [Theory]
@@ -49,10 +51,11 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
             var helper = DefaultTemplatesUtilities.GetHtmlHelper(model);
 
             // Act
-            var result = HtmlContentUtilities.HtmlContentToString(helper.TextBoxFor(m => m.Property1, new { type }));
+            var textBox = helper.TextBoxFor(m => m.Property1, new { type });
 
-            // Assert
+            // Assert 
-            Assert.False(result.Contains(@"placeholder=""HtmlEncode[[placeholder]]"""));
+            var result = HtmlContentUtilities.HtmlContentToString(textBox);
+            Assert.DoesNotContain(@"placeholder=""HtmlEncode[[placeholder]]""", result, StringComparison.Ordinal);
         }
 
         private class TextBoxModel