Ensure source paths are comparable with editorconfig directory paths

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -210,6 +210,63 @@ class C
             Assert.Null(cmd.AnalyzerOptions);
         }
 
+        [Theory, CombinatorialData, WorkItem("https://github.com/dotnet/roslyn/issues/72657")]
+        public void AnalyzerConfig_DoubleSlash(bool doubleSlash1, bool doubleSlash2)
+        {
+            var dir = Temp.CreateDirectory();
+            var analyzer = new CompilationAnalyzerWithSeverity(DiagnosticSeverity.Warning, configurable: true);
+            var src = dir.CreateFile("Class1.cs").WriteAllText("""
+                public class C
+                {
+                    public void M() { }
+                }
+                """);
+
+            // The analyzer should produce a warning.
+            var output = VerifyOutput(dir, src, includeCurrentAssemblyAsAnalyzerReference: false, analyzers: [analyzer], expectedWarningCount: 1);
+            AssertEx.Equal("Class1.cs(1,1): warning ID1000:", output.Trim());
+
+            // But not when this editorconfig is applied.
+            var editorconfig = dir.CreateFile(".editorconfig").WriteAllText("""
+                root = true
+
+                [*.cs]
+                dotnet_analyzer_diagnostic.severity = none
+
+                generated_code = true
+                """);
+            var cmd = CreateCSharpCompiler(
+                [
+                    "/nologo",
+                    "/preferreduilang:en",
+                    "/t:library",
+                    "/analyzerconfig:" + modifyPath(editorconfig.Path, doubleSlash1),
+                    modifyPath(src.Path, doubleSlash2),
+                ],
+                [analyzer]);
+            var outWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var exitCode = cmd.Run(outWriter);
+            Assert.Equal(0, exitCode);
+            AssertEx.Equal("", outWriter.ToString());
+
+            static string modifyPath(string path, bool doubleSlash)
+            {
+                if (!doubleSlash)
+                {
+                    return path;
+                }
+
+                // Find the second-to-last slash.
+                char[] separators = ['/', '\\'];
+                var lastSlashIndex = path.LastIndexOfAny(separators);
+                lastSlashIndex = path.LastIndexOfAny(separators, lastSlashIndex - 1);
+
+                // Duplicate that slash.
+                var lastSlash = path[lastSlashIndex];
+                return path[0..lastSlashIndex] + lastSlash + path[lastSlashIndex..];
+            }
+        }
+
         [Fact]
         public void AnalyzerConfigWithOptions()
         {

src/Compilers/Core/CodeAnalysisTest/Analyzers/AnalyzerConfigTests.cs

@@ -932,6 +932,33 @@ public void EditorConfigToDiagnostics()
             }, options.Select(o => o.TreeOptions).ToArray());
         }
 
+        [Theory, WorkItem("https://github.com/dotnet/roslyn/issues/72657")]
+        [InlineData("/", "/")]
+        [InlineData("/a/b/c/", "/a/b/c/")]
+        [InlineData("/a/b//c/", "/a/b/c/")]
+        [InlineData("/a/b/c/", "/a/b//c/")]
+        [InlineData("/a/b//c/", "/a/b//c/")]
+        [InlineData("/a/b/c//", "/a/b/c/")]
+        [InlineData("/a/b/c/", "/a/b/c//")]
+        [InlineData("/a/b/c//", "/a/b/c//")]
+        [InlineData("/a/b//c/", "/a/b///c/")]
+        public void EditorConfigToDiagnostics_DoubleSlash(string prefix1, string prefix2)
+        {
+            var configs = ArrayBuilder<AnalyzerConfig>.GetInstance();
+            configs.Add(Parse("""
+                [*.cs]
+                dotnet_diagnostic.cs000.severity = none
+                """,
+                prefix1 + ".editorconfig"));
+
+            var options = GetAnalyzerConfigOptions([prefix2 + "test.cs"], configs);
+            configs.Free();
+
+            Assert.Equal([
+                CreateImmutableDictionary(("cs000", ReportDiagnostic.Suppress))
+            ], options.Select(o => o.TreeOptions).ToArray());
+        }
+
         [Fact]
         public void LaterSectionOverrides()
         {

src/Compilers/Core/CodeAnalysisTest/FileSystem/PathUtilitiesTests.cs

@@ -420,5 +420,20 @@ public void GetRelativePath_EnsureNo_IndexOutOfRangeException_Unix()
             var result = PathUtilities.GetRelativePath(@"/A/B/", @"/A/B");
             Assert.Equal(expected, result);
         }
+
+        [Theory]
+        [InlineData(@"//a/b/c", @"//a/b/c")]
+        [InlineData(@"/a\b/c/", @"/a/b/c/")]
+        [InlineData(@"\a\b/c/", @"/a/b/c/")]
+        [InlineData(@"C:\\a", @"C:/a")]
+        [InlineData(@"C:\a\b\c\", @"C:/a/b/c/")]
+        [InlineData(@"/\a", @"//a")]
+        [InlineData(@"a\\\b", @"a/b")]
+        [InlineData(@"\\\a\b\c", @"///a/b/c")]
+        [InlineData(@"\\\\a\b\c", @"///a/b/c")]
+        public void CollapseWithForwardSlash(string input, string output)
+        {
+            AssertEx.Equal(output, PathUtilities.CollapseWithForwardSlash(input.AsSpan()));
+        }
     }
 }

src/Compilers/Core/Portable/CommandLine/AnalyzerConfigSet.cs

@@ -183,7 +183,7 @@ public AnalyzerConfigOptionsResult GetOptionsForSourcePath(string sourcePath)
 
             var sectionKey = _sectionKeyPool.Allocate();
 
-            var normalizedPath = PathUtilities.NormalizeWithForwardSlash(sourcePath);
+            var normalizedPath = PathUtilities.CollapseWithForwardSlash(sourcePath.AsSpan());
             normalizedPath = PathUtilities.ExpandAbsolutePathWithRelativeParts(normalizedPath);
 
             // If we have a global config, add any sections that match the full path. We can have at most one section since

src/Compilers/Core/Portable/FileSystem/PathUtilities.cs

@@ -9,6 +9,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
+using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.PooledObjects;
 
@@ -780,6 +781,42 @@ public static bool IsValidFilePath([NotNullWhen(true)] string? fullPath)
         public static string NormalizeWithForwardSlash(string p)
             => DirectorySeparatorChar == '/' ? p : p.Replace(DirectorySeparatorChar, '/');
 
+        /// <summary>
+        /// Replaces all sequences of '\' or '/' with a single '/' but preserves UNC prefix '//'.
+        /// </summary>
+        public static string CollapseWithForwardSlash(ReadOnlySpan<char> path)
+        {
+            var sb = new StringBuilder(path.Length);
+
+            int start = 0;
+            if (path.Length > 1 && IsAnyDirectorySeparator(path[0]) && IsAnyDirectorySeparator(path[1]))
+            {
+                // Preserve UNC paths.
+                sb.Append("//");
+                start = 2;
+            }
+
+            bool wasDirectorySeparator = false;
+            for (int i = start; i < path.Length; i++)
+            {
+                if (IsAnyDirectorySeparator(path[i]))
+                {
+                    if (!wasDirectorySeparator)
+                    {
+                        sb.Append('/');
+                    }
+                    wasDirectorySeparator = true;
+                }
+                else
+                {
+                    sb.Append(path[i]);
+                    wasDirectorySeparator = false;
+                }
+            }
+
+            return sb.ToString();
+        }
+
         /// <summary>
         /// Takes an absolute path and attempts to expand any '..' or '.' into their equivalent representation.
         /// </summary>