Add dotnet test --list-tests json via SDK-side rendering

src/Cli/Microsoft.DotNet.Cli.Definitions/CommandDefinitionStrings.resx

@@ -452,7 +452,7 @@ This is equivalent to deleting project.assets.json.</value>
     <value>Include packages with symbols in addition to regular packages in output directory.</value>
   </data>
   <data name="CmdListTestsDescription" xml:space="preserve">
-    <value>List the discovered tests instead of running the tests.</value>
+    <value>List the discovered tests instead of running the tests. Optionally accepts a format: 'text' (default) for human-readable output or 'json' for machine-readable output.</value>
   </data>
   <data name="CmdLockedModeOptionDescription" xml:space="preserve">
     <value>Don't allow updating project lock file.</value>

src/Cli/Microsoft.DotNet.Cli.Definitions/Commands/Test/TestCommandDefinition.MicrosoftTestingPlatform.cs

@@ -116,11 +116,16 @@ public sealed class MicrosoftTestingPlatform : TestCommandDefinition, ICustomHel
 
         public const string ListTestsOptionName = "--list-tests";
 
-        public readonly Option<string> ListTestsOption = new(ListTestsOptionName)
+        public const string ListTestsFormatText = "text";
+
+        public const string ListTestsFormatJson = "json";
+
+        public readonly Option<string> ListTestsOption = new Option<string>(ListTestsOptionName)
         {
             Description = CommandDefinitionStrings.CmdListTestsDescription,
-            Arity = ArgumentArity.Zero
-        };
+            HelpName = $"{ListTestsFormatText}|{ListTestsFormatJson}",
+            Arity = ArgumentArity.ZeroOrOne
+        }.AcceptOnlyFromAmong(ListTestsFormatText, ListTestsFormatJson);
 
         public readonly Option<bool> NoLaunchProfileOption = new("--no-launch-profile")
         {

src/Cli/dotnet/Commands/Test/MTP/MicrosoftTestingPlatformTestCommand.cs

@@ -78,6 +78,7 @@ public int Run(ParseResult parseResult, bool isHelp)
         var testOptions = new TestOptions(
             IsHelp: isHelp,
             IsDiscovery: parseResult.HasOption(definition.ListTestsOption),
+            ListTestsFormat: GetListTestsFormat(parseResult, definition),
             EnvironmentVariables: parseResult.GetValue(definition.EnvOption) ?? ImmutableDictionary<string, string>.Empty);
 
         var output = InitializeOutput(degreeOfParallelism, parseResult, testOptions);
@@ -110,6 +111,16 @@ public int Run(ParseResult parseResult, bool isHelp)
         }
     }
 
+    private static TestListFormat GetListTestsFormat(ParseResult parseResult, TestCommandDefinition.MicrosoftTestingPlatform definition)
+    {
+        // '--list-tests' has ZeroOrOne arity. A bare '--list-tests' (no value) defaults to text.
+        // The accepted values are constrained to 'text'/'json' by the option definition.
+        string? value = parseResult.GetValue(definition.ListTestsOption);
+        return string.Equals(value, TestCommandDefinition.MicrosoftTestingPlatform.ListTestsFormatJson, StringComparison.Ordinal)
+            ? TestListFormat.Json
+            : TestListFormat.Text;
+    }
+
     private static TerminalTestReporter InitializeOutput(int degreeOfParallelism, ParseResult parseResult, TestOptions testOptions)
     {
         var definition = (TestCommandDefinition.MicrosoftTestingPlatform)parseResult.CommandResult.Command;
@@ -119,6 +130,16 @@ private static TerminalTestReporter InitializeOutput(int degreeOfParallelism, Pa
         var noProgress = parseResult.HasOption(definition.NoProgressOption);
         var noAnsi = parseResult.HasOption(definition.NoAnsiOption);
 
+        // When emitting machine-readable JSON discovery output, stdout must contain only the JSON
+        // document. Force off ANSI, progress rendering and the per-assembly "Discovering tests from..."
+        // banners so nothing else is interleaved with the JSON.
+        bool isJsonDiscovery = testOptions.IsDiscovery && testOptions.ListTestsFormat == TestListFormat.Json;
+        if (isJsonDiscovery)
+        {
+            noProgress = true;
+            noAnsi = true;
+        }
+
         // TODO: Replace this with proper CI detection that we already have in telemetry. https://github.com/microsoft/testfx/issues/5533#issuecomment-2838893327
         bool inCI = string.Equals(Environment.GetEnvironmentVariable("TF_BUILD"), "true", StringComparison.OrdinalIgnoreCase) || string.Equals(Environment.GetEnvironmentVariable("GITHUB_ACTIONS"), "true", StringComparison.OrdinalIgnoreCase);
 
@@ -142,9 +163,10 @@ private static TerminalTestReporter InitializeOutput(int degreeOfParallelism, Pa
             ShowProgress = !noProgress,
             ShowActiveTests = !noProgress && ansiMode == AnsiMode.AnsiIfPossible,
             AnsiMode = ansiMode,
-            ShowAssembly = true,
-            ShowAssemblyStartAndComplete = true,
+            ShowAssembly = !isJsonDiscovery,
+            ShowAssemblyStartAndComplete = !isJsonDiscovery,
             MinimumExpectedTests = parseResult.GetValue(definition.MinimumExpectedTestsOption),
+            ListTestsFormat = testOptions.ListTestsFormat,
         });
 
         // Ctrl+C handling is wired in Run() through CtrlCCancellationManager so that

src/Cli/dotnet/Commands/Test/MTP/Options.cs

@@ -5,7 +5,20 @@
 
 namespace Microsoft.DotNet.Cli.Commands.Test;
 
-internal record TestOptions(bool IsHelp, bool IsDiscovery, IReadOnlyDictionary<string, string> EnvironmentVariables);
+internal enum TestListFormat
+{
+    /// <summary>
+    /// Human-readable discovery output (the default when '--list-tests' is passed without a value).
+    /// </summary>
+    Text,
+
+    /// <summary>
+    /// Machine-readable JSON discovery output ('--list-tests json').
+    /// </summary>
+    Json,
+}
+
+internal record TestOptions(bool IsHelp, bool IsDiscovery, TestListFormat ListTestsFormat, IReadOnlyDictionary<string, string> EnvironmentVariables);
 
 internal record PathOptions(string? ProjectOrSolutionPath, string? SolutionPath, string? TestModules, string? ResultsDirectoryPath, string? ConfigFilePath, string? DiagnosticOutputDirectoryPath);