Use Utf8.IsValid in System.Text.Json (#88165)

* Use Utf8.IsValid in System.Text.Json

* Fix TestingGetStringInvalidUTF8 test

It's expecting there to always be a DecoderFallbackException as the inner exception.

src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.Unescaping.cs

@@ -5,7 +5,7 @@
 using System.Buffers.Text;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
-using System.Runtime.CompilerServices;
+using System.Text.Unicode;
 
 namespace System.Text.Json
 {
@@ -277,6 +277,12 @@ public static int TranscodeHelper(ReadOnlySpan<byte> utf8Unescaped, Span<char> d
 
         public static void ValidateUtf8(ReadOnlySpan<byte> utf8Buffer)
         {
+#if NET8_0_OR_GREATER
+            if (!Utf8.IsValid(utf8Buffer))
+            {
+                throw ThrowHelper.GetInvalidOperationException_ReadInvalidUTF8();
+            }
+#else
             try
             {
 #if NETCOREAPP
@@ -304,6 +310,7 @@ public static void ValidateUtf8(ReadOnlySpan<byte> utf8Buffer)
                 // Therefore, wrapping the DecoderFallbackException around an InvalidOperationException.
                 throw ThrowHelper.GetInvalidOperationException_ReadInvalidUTF8(ex);
             }
+#endif
         }
 
         internal static int GetUtf8ByteCount(ReadOnlySpan<char> text)

src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.cs

@@ -515,7 +515,7 @@ public static void ThrowInvalidOperationException_ReadIncompleteUTF16()
             throw GetInvalidOperationException(SR.CannotReadIncompleteUTF16);
         }
 
-        public static InvalidOperationException GetInvalidOperationException_ReadInvalidUTF8(DecoderFallbackException innerException)
+        public static InvalidOperationException GetInvalidOperationException_ReadInvalidUTF8(DecoderFallbackException? innerException = null)
         {
             return GetInvalidOperationException(SR.CannotTranscodeInvalidUtf8, innerException);
         }
@@ -525,7 +525,7 @@ public static ArgumentException GetArgumentException_ReadInvalidUTF16(EncoderFal
             return new ArgumentException(SR.CannotTranscodeInvalidUtf16, innerException);
         }
 
-        public static InvalidOperationException GetInvalidOperationException(string message, Exception innerException)
+        public static InvalidOperationException GetInvalidOperationException(string message, Exception? innerException)
         {
             InvalidOperationException ex = new InvalidOperationException(message, innerException);
             ex.Source = ExceptionSourceValueToRethrowAsJsonException;

src/libraries/System.Text.Json/src/System/Text/Json/Writer/JsonWriterHelper.cs

@@ -225,10 +225,15 @@ internal static void ValidateNumber(ReadOnlySpan<byte> utf8FormattedNumber)
             }
         }
 
+#if !NET8_0_OR_GREATER
         private static readonly UTF8Encoding s_utf8Encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
+#endif
 
         public static unsafe bool IsValidUtf8String(ReadOnlySpan<byte> bytes)
         {
+#if NET8_0_OR_GREATER
+            return Utf8.IsValid(bytes);
+#else
             try
             {
 #if NETCOREAPP
@@ -248,6 +253,7 @@ public static unsafe bool IsValidUtf8String(ReadOnlySpan<byte> bytes)
             {
                 return false;
             }
+#endif
         }
 
         internal static unsafe OperationStatus ToUtf8(ReadOnlySpan<char> source, Span<byte> destination, out int written)

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Utf8JsonReaderTests.TryGet.cs

@@ -1125,13 +1125,22 @@ public static void TestingGetStringInvalidUTF8(byte[] dataUtf8)
                         int length = json.HasValueSequence ? (int)json.ValueSequence.Length : json.ValueSpan.Length;
 
                         InvalidOperationException ex = JsonTestHelper.AssertThrows<InvalidOperationException>(ref json, (ref Utf8JsonReader json) => json.GetString());
-                        Assert.IsType<DecoderFallbackException>(ex.InnerException);
+                        if (ex.InnerException is not null)
+                        {
+                            Assert.IsType<DecoderFallbackException>(ex.InnerException);
+                        }
 
                         ex = JsonTestHelper.AssertThrows<InvalidOperationException>(ref json, (ref Utf8JsonReader json) => json.CopyString(new byte[length]));
-                        Assert.IsType<DecoderFallbackException>(ex.InnerException);
+                        if (ex.InnerException is not null)
+                        {
+                            Assert.IsType<DecoderFallbackException>(ex.InnerException);
+                        }
 
                         ex = JsonTestHelper.AssertThrows<InvalidOperationException>(ref json, (ref Utf8JsonReader json) => json.CopyString(new char[length]));
-                        Assert.IsType<DecoderFallbackException>(ex.InnerException);
+                        if (ex.InnerException is not null)
+                        {
+                            Assert.IsType<DecoderFallbackException>(ex.InnerException);
+                        }
                     }
                 }
             }