Consider pure functions as part of CA1806. Fixes dotnet#1164.

src/Analyzer.Utilities/WellKnownTypes.cs

@@ -340,5 +340,10 @@ public static INamedTypeSymbol ObsoleteAttribute(Compilation compilation)
         {
             return compilation.GetTypeByMetadataName("System.ObsoleteAttribute");
         }
+
+        public static INamedTypeSymbol PureAttribute(Compilation compilation)
+        {
+            return compilation.GetTypeByMetadataName("System.Diagnostics.Contracts.PureAttribute");
+        }
     }
 }

src/Microsoft.CodeQuality.Analyzers/Core/Maintainability/DoNotIgnoreMethodResults.cs

@@ -6,6 +6,8 @@
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Semantics;
+using System.Diagnostics.Contracts;
+using System.Linq;
 
 namespace Microsoft.Maintainability.Analyzers
 {
@@ -46,6 +48,7 @@ public sealed class DoNotIgnoreMethodResultsAnalyzer : DiagnosticAnalyzer
         private static readonly LocalizableString s_localizableMessageObjectCreation = new LocalizableResourceString(nameof(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessageObjectCreation), MicrosoftMaintainabilityAnalyzersResources.ResourceManager, typeof(MicrosoftMaintainabilityAnalyzersResources));
         private static readonly LocalizableString s_localizableMessageStringCreation = new LocalizableResourceString(nameof(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessageStringCreation), MicrosoftMaintainabilityAnalyzersResources.ResourceManager, typeof(MicrosoftMaintainabilityAnalyzersResources));
         private static readonly LocalizableString s_localizableMessageHResultOrErrorCode = new LocalizableResourceString(nameof(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessageHResultOrErrorCode), MicrosoftMaintainabilityAnalyzersResources.ResourceManager, typeof(MicrosoftMaintainabilityAnalyzersResources));
+        private static readonly LocalizableString s_localizableMessagePureMethod = new LocalizableResourceString(nameof(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessagePureMethod), MicrosoftMaintainabilityAnalyzersResources.ResourceManager, typeof(MicrosoftMaintainabilityAnalyzersResources));
         private static readonly LocalizableString s_localizableMessageTryParse = new LocalizableResourceString(nameof(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessageTryParse), MicrosoftMaintainabilityAnalyzersResources.ResourceManager, typeof(MicrosoftMaintainabilityAnalyzersResources));
         private static readonly LocalizableString s_localizableDescription = new LocalizableResourceString(nameof(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsDescription), MicrosoftMaintainabilityAnalyzersResources.ResourceManager, typeof(MicrosoftMaintainabilityAnalyzersResources));
 
@@ -79,6 +82,17 @@ public sealed class DoNotIgnoreMethodResultsAnalyzer : DiagnosticAnalyzer
                                                                              helpLinkUri: "https://msdn.microsoft.com/en-us/library/ms182273.aspx",
                                                                              customTags: WellKnownDiagnosticTags.Telemetry);
 
+        internal static DiagnosticDescriptor PureMethodRule = new DiagnosticDescriptor(RuleId,
+                                                                             s_localizableTitle,
+                                                                             s_localizableMessagePureMethod,
+                                                                             DiagnosticCategory.Performance,
+                                                                             DiagnosticHelpers.DefaultDiagnosticSeverity,
+                                                                             isEnabledByDefault: true,
+                                                                             description: s_localizableDescription,
+                                                                             helpLinkUri: "https://msdn.microsoft.com/en-us/library/ms182273.aspx",
+                                                                             customTags: WellKnownDiagnosticTags.Telemetry);
+
+
         internal static DiagnosticDescriptor TryParseRule = new DiagnosticDescriptor(RuleId,
                                                                              s_localizableTitle,
                                                                              s_localizableMessageTryParse,
@@ -140,6 +154,10 @@ public override void Initialize(AnalysisContext analysisContext)
                             {
                                 rule = HResultOrErrorCodeRule;
                             }
+                            else if (IsPureMethod(targetMethod, opContext.Compilation))
+                            {
+                                rule = PureMethodRule;
+                            }
 
                             targetMethodName = targetMethod.Name;
                             break;
@@ -175,5 +193,10 @@ private static bool IsHResultOrErrorCodeReturningMethod(IMethodSymbol method)
                 (method.ReturnType.SpecialType == SpecialType.System_Int32 ||
                 method.ReturnType.SpecialType == SpecialType.System_UInt32);
         }
+
+        private static bool IsPureMethod(IMethodSymbol method, Compilation compilation)
+        {
+            return method.GetAttributes().Any(attr => attr.AttributeClass.Equals(WellKnownTypes.PureAttribute(compilation)));
+        }
     }
 }

src/Microsoft.CodeQuality.Analyzers/Core/Maintainability/MicrosoftMaintainabilityAnalyzersResources.resx

@@ -189,4 +189,7 @@
   <data name="CategoryPerformance" xml:space="preserve">
     <value>Performance</value>
   </data>
+  <data name="DoNotIgnoreMethodResultsMessagePureMethod" xml:space="preserve">
+    <value>{0} calls {1} but does not use the value the method returns. Because {1} is marked as a Pure method, it cannot have side effects. Use the result in a conditional statement, assign the result to a variable, or pass it as an argument to another method.</value>
+  </data>
 </root>

src/Microsoft.CodeQuality.Analyzers/UnitTests/Maintainability/DoNotIgnoreMethodResultsTests.cs

@@ -251,6 +251,43 @@ End Interface
     GetBasicHResultOrErrorCodeResultAt(6, 9, "M", "NativeMethod"));
         }
 
+        [Fact]
+        [WorkItem(1164, "https://github.com/dotnet/roslyn-analyzers/issues/1164")]
+        public void UnusedPureMethodTriggersError()
+        {
+            VerifyCSharp(@"
+using System.Diagnostics.Contracts;
+
+class C
+{
+    [Pure]
+    public int Returns1() => 1;
+
+    public void DoesNotUseResult()
+    {
+        Returns1();
+    }
+}",
+    GetCSharpPureMethodResultAt(11, 9, "DoesNotUseResult", "Returns1"));
+
+            VerifyBasic(@"
+Imports System.Diagnostics.Contracts
+
+Module Module1
+    <Pure>
+    Function Returns1() As Integer
+        Return 1
+    End Function
+
+    Sub DoesNotUseResult()
+        Returns1()
+    End Sub
+
+End Module
+",
+    GetBasicPureMethodResultAt(11, 9, "DoesNotUseResult", "Returns1"));
+        }
+
         #endregion
 
         #region Helpers
@@ -308,6 +345,18 @@ private static DiagnosticResult GetBasicHResultOrErrorCodeResultAt(int line, int
             return GetBasicResultAt(line, column, DoNotIgnoreMethodResultsAnalyzer.RuleId, message);
         }
 
+        private static DiagnosticResult GetCSharpPureMethodResultAt(int line, int column, string containingMethodName, string invokedMethodName)
+        {
+            string message = string.Format(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessagePureMethod, containingMethodName, invokedMethodName);
+            return GetCSharpResultAt(line, column, DoNotIgnoreMethodResultsAnalyzer.RuleId, message);
+        }
+
+        private static DiagnosticResult GetBasicPureMethodResultAt(int line, int column, string containingMethodName, string invokedMethodName)
+        {
+            string message = string.Format(MicrosoftMaintainabilityAnalyzersResources.DoNotIgnoreMethodResultsMessagePureMethod, containingMethodName, invokedMethodName);
+            return GetBasicResultAt(line, column, DoNotIgnoreMethodResultsAnalyzer.RuleId, message);
+        }
+
         #endregion
     }
 }