Cache the unescaped form of globalconfig section names

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

@@ -2536,6 +2536,29 @@ public void EquivalentSourcePathNames(string sourcePath, bool shouldMatch)
             }
         }
 
+
+        [Fact]
+        public void CorrectlyMergeGlobalConfigWithEscapedPaths()
+        {
+            var configs = ArrayBuilder<AnalyzerConfig>.GetInstance();
+            configs.Add(Parse(@"
+is_global = true
+[/Test.cs]
+a = a
+[/\Test.cs]
+b = b
+", "/.editorconfig"));
+
+            var configSet = AnalyzerConfigSet.Create(configs, out var diagnostics);
+            configs.Free();
+
+            var options = configSet.GetOptionsForSourcePath("/Test.cs");
+
+            Assert.Equal(2, options.AnalyzerOptions.Count);
+            Assert.Equal("a", options.AnalyzerOptions["a"]);
+            Assert.Equal("b", options.AnalyzerOptions["b"]);
+        }
+
         #endregion
     }
 }

src/Compilers/Core/Portable/CommandLine/AnalyzerConfig.SectionNameMatching.cs

@@ -114,7 +114,7 @@ public bool IsMatch(string s)
                 numberRangePairs.ToImmutableAndFree());
         }
 
-        internal static bool TryUnescapeSectionName(string sectionName, out string? escapedSectionName)
+        internal static string UnescapeSectionName(string sectionName)
         {
             var sb = new StringBuilder();
             SectionNameLexer lexer = new SectionNameLexer(sectionName);
@@ -125,9 +125,14 @@ internal static bool TryUnescapeSectionName(string sectionName, out string? esca
                 {
                     sb.Append(lexer.EatCurrentCharacter());
                 }
+                else
+                {
+                    // We only call this on strings that were already passed through IsAbsoluteEditorConfigPath, so
+                    // we shouldn't have any other token kinds here.
+                    throw ExceptionUtilities.UnexpectedValue(tokenKind);
+                }
             }
-            escapedSectionName = sb.ToString();
-            return true;
+            return sb.ToString();
         }
 
         /// <summary>

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

@@ -298,7 +298,8 @@ public Section(string name, ImmutableDictionary<string, string> properties)
             }
 
             /// <summary>
-            /// The name as present directly in the section specification of the editorconfig file.
+            /// For regular files, the name as present directly in the section specification of the editorconfig file. For sections in
+            /// global configs, this is the unescaped full file path.
             /// </summary>
             public string Name { get; }
 

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

@@ -28,6 +28,7 @@ public sealed class AnalyzerConfigSet
     {
         /// <summary>
         /// The list of <see cref="AnalyzerConfig" />s in this set. This list has been sorted per <see cref="AnalyzerConfig.DirectoryLengthComparer"/>.
+        /// This does not include any of the global configs that were merged into <see cref="_globalConfig"/>.
         /// </summary>
         private readonly ImmutableArray<AnalyzerConfig> _analyzerConfigs;
 
@@ -197,13 +198,14 @@ public AnalyzerConfigOptionsResult GetOptionsForSourcePath(string sourcePath)
             var normalizedPath = PathUtilities.NormalizeWithForwardSlash(sourcePath);
             normalizedPath = PathUtilities.ExpandAbsolutePathWithRelativeParts(normalizedPath);
 
-            // If we have a global config, add any sections that match the full path 
+            // If we have a global config, add any sections that match the full path. We can have at most one section since
+            // we would have merged them earlier.
             foreach (var section in _globalConfig.NamedSections)
             {
-                var escapedSectionName = TryUnescapeSectionName(section.Name, out var sectionName);
-                if (escapedSectionName && normalizedPath.Equals(sectionName, Section.NameComparer))
+                if (normalizedPath.Equals(section.Name, Section.NameComparer))
                 {
                     sectionKey.Add(section);
+                    break;
                 }
             }
             int globalConfigOptionsCount = sectionKey.Count;
@@ -502,7 +504,10 @@ internal void MergeIntoGlobalConfig(AnalyzerConfig config, DiagnosticBag diagnos
                 {
                     if (IsAbsoluteEditorConfigPath(section.Name))
                     {
-                        MergeSection(config.PathToFile, section, config.GlobalLevel, isGlobalSection: false);
+                        // Let's recreate the section with the name unescaped, since we can then properly merge and match it later
+                        var unescapedSection = new Section(UnescapeSectionName(section.Name), section.Properties);
+
+                        MergeSection(config.PathToFile, unescapedSection, config.GlobalLevel, isGlobalSection: false);
                     }
                     else
                     {