From 93f4546242f410f0e68686f2c22b3722a9e3d71d Mon Sep 17 00:00:00 2001
From: Tanner Gooding <tagoo@outlook.com>
Date: Sun, 12 Jul 2026 22:09:28 -0700
Subject: [PATCH] Concatenate adjacent string literals in deprecated attribute
 messages

A C deprecated message can be written as adjacent string literals, which
the compiler concatenates into a single value. The generator extracted the
text between the first and last quote verbatim, so the emitted Obsolete
attribute contained the raw boundary quotes and inter-literal whitespace,
producing invalid C#. Scan the attribute source for each literal and join
their contents into a single valid Obsolete message.

Fixes #694

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
---
 .../PInvokeGenerator.cs                       | 51 ++++++++++++++++---
 .../DeprecatedAdjacentStringTest.cs           | 40 +++++++++++++++
 2 files changed, 85 insertions(+), 6 deletions(-)
 create mode 100644 tests/ClangSharp.PInvokeGenerator.UnitTests/DeprecatedAdjacentStringTest.cs

diff --git a/sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.cs b/sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.cs
index aba0735e..56191fd8 100644
--- a/sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.cs
+++ b/sources/ClangSharp.PInvokeGenerator/PInvokeGenerator.cs
@@ -2060,14 +2060,11 @@ private void WithAttributes(NamedDecl namedDecl, bool onlySupportedOSPlatform =
                         }
 
                         var attrText = GetSourceRangeContents(namedDecl.TranslationUnit.Handle, attr.Extent);
+                        var message = GetDeprecatedMessage(attrText);
 
-                        var textStart = attrText.IndexOf('"', StringComparison.Ordinal);
-                        var textLength = attrText.LastIndexOf('"') - textStart;
-
-                        if (textLength > 1)
+                        if (!string.IsNullOrEmpty(message))
                         {
-                            var text = attrText.AsSpan(textStart + 1, textLength - 1);
-                            outputBuilder.WriteCustomAttribute($"Obsolete(\"{text}\")");
+                            outputBuilder.WriteCustomAttribute($"Obsolete(\"{message}\")");
                         }
                         else
                         {
@@ -2109,6 +2106,48 @@ private void WithAttributes(NamedDecl namedDecl, bool onlySupportedOSPlatform =
         }
     }
 
+    private static string? GetDeprecatedMessage(string attrText)
+    {
+        // The attribute source looks like `deprecated("message")`, but C allows the message to be
+        // written as adjacent string literals (e.g. `deprecated("part1" "part2")`), which the
+        // compiler concatenates into a single value. Gather the contents of every literal and join
+        // them so the emitted `Obsolete("...")` is a single, valid C# string.
+
+        var builder = new StringBuilder();
+        var hasLiteral = false;
+        var index = 0;
+
+        while (index < attrText.Length)
+        {
+            if (attrText[index] != '"')
+            {
+                index++;
+                continue;
+            }
+
+            hasLiteral = true;
+            index++;
+
+            while ((index < attrText.Length) && (attrText[index] != '"'))
+            {
+                // Preserve escape sequences verbatim so an escaped quote isn't treated as the end
+                // of the literal and the contents are emitted unchanged (as before).
+                if ((attrText[index] == '\\') && ((index + 1) < attrText.Length))
+                {
+                    _ = builder.Append(attrText[index]);
+                    index++;
+                }
+
+                _ = builder.Append(attrText[index]);
+                index++;
+            }
+
+            index++;
+        }
+
+        return hasLiteral ? builder.ToString() : null;
+    }
+
     private string GetLibraryPath(string remappedName)
     {
         return !_config.WithLibraryPaths.TryGetValue(remappedName, out var libraryPath) && !_config.WithLibraryPaths.TryGetValue("*", out libraryPath)
diff --git a/tests/ClangSharp.PInvokeGenerator.UnitTests/DeprecatedAdjacentStringTest.cs b/tests/ClangSharp.PInvokeGenerator.UnitTests/DeprecatedAdjacentStringTest.cs
new file mode 100644
index 00000000..3b659878
--- /dev/null
+++ b/tests/ClangSharp.PInvokeGenerator.UnitTests/DeprecatedAdjacentStringTest.cs
@@ -0,0 +1,40 @@
+// Copyright (c) .NET Foundation and Contributors. All Rights Reserved. Licensed under the MIT License (MIT). See License.md in the repository root for more information.
+
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace ClangSharp.UnitTests;
+
+/// <summary>
+/// Regression test for https://github.com/dotnet/ClangSharp/issues/694.
+/// A deprecated message written as adjacent C string literals must be concatenated into a single
+/// C# string literal, otherwise the emitted <c>[Obsolete(...)]</c> attribute is invalid C#.
+/// </summary>
+[Platform("win")]
+public sealed class DeprecatedAdjacentStringTest : PInvokeGeneratorTest
+{
+    [Test]
+    public Task AdjacentStringLiteralsAreConcatenated()
+    {
+        var inputContents = @"extern ""C"" [[deprecated(""Use Bar() or ""
+                            ""Baz() instead"")]]
+void MyFunction();
+";
+
+        var expectedOutputContents = @"using System;
+using System.Runtime.InteropServices;
+
+namespace ClangSharp.Test
+{
+    public static partial class Methods
+    {
+        [DllImport(""ClangSharpPInvokeGenerator"", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
+        [Obsolete(""Use Bar() or Baz() instead"")]
+        public static extern void MyFunction();
+    }
+}
+";
+
+        return ValidateGeneratedCSharpLatestWindowsBindingsAsync(inputContents, expectedOutputContents);
+    }
+}