From 1e0c9f0e887329f212075e28f01f8870f7043eab Mon Sep 17 00:00:00 2001
From: Will Fuqua <wafuqua@gmail.com>
Date: Fri, 26 Jun 2026 22:08:23 +0700
Subject: [PATCH] Auto-insert semicolon for simple variable declarations

---
 .../CSharpRepl.Services.csproj                |  2 +-
 CSharpRepl.Services/Roslyn/RoslynServices.cs  |  7 +++
 .../Roslyn/Scripting/AutoInsertSemicolon.cs   | 54 +++++++++++++++++++
 CSharpRepl/CSharpRepl.csproj                  |  2 +-
 CSharpRepl/CSharpReplPromptCallbacks.cs       | 13 ++++-
 .../AutoInsertSemicolonTests.cs               | 49 +++++++++++++++++
 .../CSharpRepl.Tests/CSharpRepl.Tests.csproj  |  2 +-
 .../PromptConfigurationTests.cs               | 53 +++++++++++++++++-
 Tests/CSharpRepl.Tests/RoslynServicesTests.cs |  1 +
 9 files changed, 177 insertions(+), 6 deletions(-)
 create mode 100644 CSharpRepl.Services/Roslyn/Scripting/AutoInsertSemicolon.cs
 create mode 100644 Tests/CSharpRepl.Tests/AutoInsertSemicolonTests.cs

diff --git a/CSharpRepl.Services/CSharpRepl.Services.csproj b/CSharpRepl.Services/CSharpRepl.Services.csproj
index d98f2bea..23b13cd7 100644
--- a/CSharpRepl.Services/CSharpRepl.Services.csproj
+++ b/CSharpRepl.Services/CSharpRepl.Services.csproj
@@ -41,7 +41,7 @@
     <PackageReference Include="Microsoft.SymbolStore" Version="1.0.731401" />
     <PackageReference Include="Microsoft.Extensions.AI.Abstractions" Version="10.7.0" />
     <PackageReference Include="Microsoft.Extensions.AI.OpenAI" Version="10.7.0" />
-    <PackageReference Include="PrettyPrompt" Version="6.0.2" />
+    <PackageReference Include="PrettyPrompt" Version="6.0.3" />
     <PackageReference Include="Spectre.Console.Cli" Version="0.55.0" />
     <PackageReference Include="Spectre.Console.Ansi" Version="0.57.0" />
     <PackageReference Include="System.IO.Abstractions" Version="22.1.1" />
diff --git a/CSharpRepl.Services/Roslyn/RoslynServices.cs b/CSharpRepl.Services/Roslyn/RoslynServices.cs
index 2c2325f7..6b97d2b4 100644
--- a/CSharpRepl.Services/Roslyn/RoslynServices.cs
+++ b/CSharpRepl.Services/Roslyn/RoslynServices.cs
@@ -340,6 +340,13 @@ public async Task<bool> IsTextCompleteStatementAsync(string text)
         return root is null || SyntaxFactory.IsCompleteSubmission(root.SyntaxTree); // if something's wrong and we can't get the syntax tree, we don't want to prevent evaluation.
     }
 
+    /// <summary>
+    /// When <paramref name="text"/> is a single-line statement the user didn't terminate (e.g. <c>int i = 0</c>),
+    /// returns it with the missing semicolon appended; otherwise <see langword="null"/>.
+    /// It is deliberately editor-only; piped input is not auto-terminated.
+    /// </summary>
+    public string? TryAutoInsertSemicolon(string text) => AutoInsertSemicolon.TryAppend(text, parseOptions);
+
     public async Task<PrettyPromptTextSpan> GetSpanToReplaceByCompletionAsync(string text, int caret, CancellationToken cancellationToken)
     {
         await Initialization.ConfigureAwait(false);
diff --git a/CSharpRepl.Services/Roslyn/Scripting/AutoInsertSemicolon.cs b/CSharpRepl.Services/Roslyn/Scripting/AutoInsertSemicolon.cs
new file mode 100644
index 00000000..9479f8c1
--- /dev/null
+++ b/CSharpRepl.Services/Roslyn/Scripting/AutoInsertSemicolon.cs
@@ -0,0 +1,54 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+using System;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace CSharpRepl.Services.Roslyn.Scripting;
+
+/// <summary>
+/// If the user types "var x = 0" and submits, autoinsert the trailing semicolon.
+///
+/// This is an <em>interactive editor</em> concern: the prompt uses it to decide whether Enter submits and to
+/// show the inserted <c>;</c> on the committed line. Evaluation itself is left untouched (piped input is not
+/// auto-terminated).
+/// </summary>
+internal static class AutoInsertSemicolon
+{
+    /// <summary>
+    /// When <paramref name="text"/> is a single-line statement the user didn't terminate, returns it (trimmed)
+    /// with the missing semicolon appended, otherwise return <see langword="null"/>
+    /// </summary>
+    public static string? TryAppend(string text, CSharpParseOptions parseOptions)
+    {
+        var trimmed = text.Trim();
+        var tree = CSharpSyntaxTree.ParseText(trimmed, parseOptions);
+        if (SyntaxFactory.IsCompleteSubmission(tree))
+        {
+            return null; // already complete — nothing to append (e.g. the expression `1 + 1`)
+        }
+
+        // Only single-line input is auto-semicoloned; a statement spread across lines (e.g. a fluent chain whose
+        // cursor is at the end of line 2) is left alone so it can keep being edited.
+        if (trimmed.AsSpan().IndexOfAny('\n', '\r') >= 0)
+        {
+            return null;
+        }
+
+        if (tree.GetRoot() is not CompilationUnitSyntax { Members: [.., FieldDeclarationSyntax { SemicolonToken.IsMissing: true }] })
+        {
+            return null;
+        }
+
+        var terminated = trimmed + ";";
+        var fixedTree = CSharpSyntaxTree.ParseText(terminated, parseOptions);
+        return SyntaxFactory.IsCompleteSubmission(fixedTree) &&
+               !fixedTree.GetDiagnostics().Any(d => d.Severity == DiagnosticSeverity.Error)
+            ? terminated
+            : null;
+    }
+}
diff --git a/CSharpRepl/CSharpRepl.csproj b/CSharpRepl/CSharpRepl.csproj
index 90c46fd0..39578d4e 100644
--- a/CSharpRepl/CSharpRepl.csproj
+++ b/CSharpRepl/CSharpRepl.csproj
@@ -25,7 +25,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="PrettyPrompt" Version="6.0.2" />
+    <PackageReference Include="PrettyPrompt" Version="6.0.3" />
     <PackageReference Include="System.CommandLine" Version="3.0.0-preview.5.26302.115" />
     <PackageReference Include="System.Reflection.MetadataLoadContext" Version="10.0.9" />
     <!-- Keep MSBuild/StringTools assemblies out of the output dir; they're loaded from the SDK via
diff --git a/CSharpRepl/CSharpReplPromptCallbacks.cs b/CSharpRepl/CSharpReplPromptCallbacks.cs
index 835041d0..92891abc 100644
--- a/CSharpRepl/CSharpReplPromptCallbacks.cs
+++ b/CSharpRepl/CSharpReplPromptCallbacks.cs
@@ -214,11 +214,13 @@ FormatSpan FullSpanWithColor(AnsiColor color)
 
     protected override async Task<KeyPress> TransformKeyPressAsync(string text, int caret, KeyPress keyPress, CancellationToken cancellationToken)
     {
-        // user submitted the prompt but it's incomplete. Insert a newline automatically with the correct level of indentation.
+        // User submitted the prompt but it's incomplete. Insert a newline automatically with the correct level of indentation.
+        // A single-line statement the user didn't terminate (e.g. `int i = 0`) counts as submittable, we'll auto-insert the semicolon for them.
         if (keyPress.ConsoleKeyInfo.Key == ConsoleKey.Enter &&
             keyPress.ConsoleKeyInfo.Modifiers == default &&
             configuration.KeyBindings.SubmitPrompt.Matches(keyPress.ConsoleKeyInfo) &&
-            !await roslyn.IsTextCompleteStatementAsync(text).ConfigureAwait(false))
+            !await roslyn.IsTextCompleteStatementAsync(text).ConfigureAwait(false) &&
+            roslyn.TryAutoInsertSemicolon(text) is null)
         {
             return NewLineWithIndentation(GetSmartIndentationLevel(text, caret));
         }
@@ -301,6 +303,13 @@ protected override Task<bool> ConfirmCompletionCommit(string text, int caret, Ke
     {
         var keyChar = keyPress.ConsoleKeyInfo.KeyChar;
 
+        // user submitted the prompt but it's incomplete. Auto-insert a semicolon.
+        if (configuration.KeyBindings.SubmitPrompt.Matches(keyPress.ConsoleKeyInfo) &&
+            roslyn.TryAutoInsertSemicolon(text) is string textWithSemicolon)
+        {
+            return await roslyn.FormatInput(textWithSemicolon, textWithSemicolon.Length, formatParentNodeOnly: false, cancellationToken).ConfigureAwait(false);
+        }
+
         if (caret > 0)
         {
             switch (keyChar)
diff --git a/Tests/CSharpRepl.Tests/AutoInsertSemicolonTests.cs b/Tests/CSharpRepl.Tests/AutoInsertSemicolonTests.cs
new file mode 100644
index 00000000..f5601bc6
--- /dev/null
+++ b/Tests/CSharpRepl.Tests/AutoInsertSemicolonTests.cs
@@ -0,0 +1,49 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+#nullable enable
+
+using CSharpRepl.Services.Roslyn.Scripting;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Xunit;
+
+namespace CSharpRepl.Tests;
+
+/// <summary>
+/// Unit tests for the issue #356 single-line auto-semicolon rule. These are pure (no Roslyn workspace/fixture):
+/// the rule only needs the same Script <see cref="CSharpParseOptions"/> that <c>RoslynServices</c> uses.
+/// </summary>
+public class AutoInsertSemicolonTests
+{
+    private static readonly CSharpParseOptions ParseOptions =
+        CSharpParseOptions.Default.WithKind(SourceCodeKind.Script).WithLanguageVersion(LanguageVersion.Latest);
+
+    [Theory]
+    // single-line declaration the user didn't terminate -> complete, and TryAppend adds the ';'
+    [InlineData("int i = 0", true)]
+    [InlineData("var x = 5", true)]
+    [InlineData("int j", true)]
+    [InlineData("int a = 1, b = 2", true)]
+    [InlineData("var x = Enumerable.Range(1, 5)", true)]
+    // already-complete submissions -> complete, but nothing to append
+    [InlineData("var x = 5;", false)]
+    [InlineData("1 + 1", false)]
+    [InlineData("if (x == 4) return;", false)]
+    // genuinely incomplete / not a single-line field declaration -> neither complete nor appended
+    [InlineData("var x = ", false)]
+    [InlineData("if (x == 4)", false)]
+    [InlineData("int Square(int x)", false)]
+    [InlineData("var x = Enumerable\n.Range(1, 5)", false)]
+    [InlineData("if you're happy and you know it, syntax error!", false)]
+    public void TryAppend_AppendsSemicolonOnlyForUnterminatedSingleLineDeclarations(string code, bool shouldAppend)
+    {
+        var appended = AutoInsertSemicolon.TryAppend(code, ParseOptions);
+
+        if (shouldAppend)
+            Assert.Equal(code.Trim() + ";", appended);
+        else
+            Assert.Null(appended);
+    }
+}
diff --git a/Tests/CSharpRepl.Tests/CSharpRepl.Tests.csproj b/Tests/CSharpRepl.Tests/CSharpRepl.Tests.csproj
index dd1c237c..38950ef0 100644
--- a/Tests/CSharpRepl.Tests/CSharpRepl.Tests.csproj
+++ b/Tests/CSharpRepl.Tests/CSharpRepl.Tests.csproj
@@ -16,7 +16,7 @@
 
   <ItemGroup>
     <PackageReference Include="NSubstitute" Version="5.3.0" />
-    <PackageReference Include="PrettyPrompt" Version="6.0.2" />
+    <PackageReference Include="PrettyPrompt" Version="6.0.3" />
     <PackageReference Include="Spectre.Console.Testing" Version="0.57.0" />
     <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="22.1.1" />
     <PackageReference Include="xunit.v3.mtp-v2" Version="4.0.0-pre.128" />
diff --git a/Tests/CSharpRepl.Tests/PromptConfigurationTests.cs b/Tests/CSharpRepl.Tests/PromptConfigurationTests.cs
index bdfab955..65228103 100644
--- a/Tests/CSharpRepl.Tests/PromptConfigurationTests.cs
+++ b/Tests/CSharpRepl.Tests/PromptConfigurationTests.cs
@@ -204,7 +204,55 @@ public async Task HighlightCallback_RegularCode_DelegatesToRoslynClassification(
         Assert.NotEmpty(spans);
     }
 
-    /// <summary>Exposes the protected highlight callback so the REPL-keyword highlighting can be tested.</summary>
+    [Fact]
+    public async Task TransformKeyPress_EnterOnUnterminatedDeclaration_Submits()
+    {
+        IPromptCallbacks configuration = new CSharpReplPromptCallbacks(console, services, new Configuration());
+        var enterKey = new KeyPress(new ConsoleKeyInfo('\r', ConsoleKey.Enter, shift: false, alt: false, control: false));
+
+        var transformed = await configuration.TransformKeyPressAsync("int i = 0", caret: 9, enterKey, CancellationToken.None);
+
+        // submittable -> Enter passes through unchanged (it is not turned into an indented newline)
+        Assert.Null(transformed.PastedText);
+    }
+
+    [Fact]
+    public async Task TransformKeyPress_EnterOnIncompleteStatement_InsertsNewline()
+    {
+        IPromptCallbacks configuration = new CSharpReplPromptCallbacks(console, services, new Configuration());
+        var enterKey = new KeyPress(new ConsoleKeyInfo('\r', ConsoleKey.Enter, shift: false, alt: false, control: false));
+
+        var transformed = await configuration.TransformKeyPressAsync("if (x == 4)", caret: 11, enterKey, CancellationToken.None);
+
+        Assert.Equal("\n", transformed.PastedText);
+    }
+
+    [Fact]
+    public async Task FormatInput_SubmitOnUnterminatedDeclaration_InsertsSemicolon()
+    {
+        var callbacks = new TestableCallbacks(console, services, new Configuration());
+        var enter = new KeyPress(new ConsoleKeyInfo('\r', ConsoleKey.Enter, shift: false, alt: false, control: false));
+
+        var (text, caret) = await callbacks.Format("int i = 0", caret: 9, enter);
+
+        Assert.Equal("int i = 0;", text);
+        Assert.Equal(text.Length, caret);
+    }
+
+    [Theory]
+    [InlineData("1 + 1")]        // already complete - no semicolon to add
+    [InlineData("if (x == 4)")]  // not a single-line declaration the user merely didn't terminate
+    public async Task FormatInput_SubmitOnNonAppendable_LeavesTextUnchanged(string code)
+    {
+        var callbacks = new TestableCallbacks(console, services, new Configuration());
+        var enter = new KeyPress(new ConsoleKeyInfo('\r', ConsoleKey.Enter, shift: false, alt: false, control: false));
+
+        var (text, _) = await callbacks.Format(code, caret: code.Length, enter);
+
+        Assert.Equal(code, text);
+    }
+
+    /// <summary>Exposes the protected highlight and format callbacks for testing.</summary>
     private sealed class TestableCallbacks : CSharpReplPromptCallbacks
     {
         public TestableCallbacks(IConsoleService console, RoslynServices roslyn, Configuration configuration)
@@ -212,5 +260,8 @@ public TestableCallbacks(IConsoleService console, RoslynServices roslyn, Configu
 
         public async Task<IReadOnlyCollection<FormatSpan>> Highlight(string text)
             => await HighlightCallbackAsync(text, default);
+
+        public Task<(string Text, int Caret)> Format(string text, int caret, KeyPress key)
+            => FormatInput(text, caret, key, default);
     }
 }
\ No newline at end of file
diff --git a/Tests/CSharpRepl.Tests/RoslynServicesTests.cs b/Tests/CSharpRepl.Tests/RoslynServicesTests.cs
index 9e83b012..ad09385e 100644
--- a/Tests/CSharpRepl.Tests/RoslynServicesTests.cs
+++ b/Tests/CSharpRepl.Tests/RoslynServicesTests.cs
@@ -42,6 +42,7 @@ public RoslynServicesTests(RoslynServicesFixture fixture)
     [InlineData("if (x == 4)", false)]
     [InlineData("if (x == 4) return;", true)]
     [InlineData("if you're happy and you know it, syntax error!", false)]
+    [InlineData("int i = 0", false)] // A single-line declaration without a trailing semicolon is NOT a complete submission at this level. It's an editor-only convenience to automatically append the semicolon.
     public async Task IsCompleteStatement(string code, bool shouldBeCompleteStatement)
     {
         bool isCompleteStatement = await services.IsTextCompleteStatementAsync(code);