Set ErrorLog property if not set by user

Author: MattKotsenas

README.md

@@ -86,30 +86,11 @@ development processes.
 
 ## Getting Started
 
-### Enabling SARIF logs
+### Using SARIF logs
 
-SquiggleCop uses [SARIF](https://sarifweb.azurewebsites.net/) v2.1 files to work its magic. If you aren't already
-producing SARIF files as part of your build, set the
-[`ErrorLog`](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/errors-warnings#errorlog)
-property, either in a
-[Directory.Build.props](https://learn.microsoft.com/en-us/visualstudio/msbuild/customize-by-directory?view=vs-2022)
-so it automatically applies to all projects:
-
-```xml
-<ErrorLog>$(MSBuildProjectFile).diagnostics.sarif,version=2.1</ErrorLog>
-```
-
-> [!TIP]
-> We recommend you add `*.sarif` to your `.gitignore` file
-
-or on the command-line for ad-hoc validation:
-
-```powershell
-dotnet build -p:ErrorLog=diagnostics.sarif%2cversion=2.1
-```
-
-> [!IMPORTANT]
-> The comma or semi-colon character in the log path must be XML-escaped
+SquiggleCop uses [SARIF](https://sarifweb.azurewebsites.net/) v2.1 files to work its magic. SquiggleCop automatically
+enables SARIF logs if needed and places them in the `obj/` folder. If you want / need to customize the SARIF output
+path, see [Set the SARIF output path](#set-the-sarif-output-path).
 
 ### CLI Tool
 
@@ -285,6 +266,28 @@ Upload your SARIF reports as pipeline artifacts to help narrow down issues.
 
 ## Advanced configuration
 
+### Set the SARIF output path
+
+If the [`ErrorLog`](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/errors-warnings#errorlog)
+property is unset, SquiggleCop sets it just before the `CoreCompile` target. To set a custom SARIF output path, set the
+property to something like this:
+
+```xml
+<ErrorLog>$(MSBuildProjectFile).sarif,version=2.1</ErrorLog>
+```
+
+> [!TIP]
+> We recommend you add `*.sarif` to your `.gitignore` file
+
+or set the property on the command-line as part of ad-hoc validation:
+
+```powershell
+dotnet build -p:ErrorLog=diagnostics.sarif%2cversion=2.1
+```
+
+> [!IMPORTANT]
+> The comma or semi-colon character in the log path must be XML-escaped
+
 ### Alternate baseline paths
 
 By default, SquiggleCop expects the baseline file to be named `SquiggleCop.Baseline.yaml` and placed next to the project

src/Tasks/build/SquiggleCop.Tasks.targets

@@ -26,4 +26,20 @@
       AutoBaseline="$(SquiggleCop_AutoBaseline)"
       BaselineFiles="@(SquiggleCop_BaselineFiles)"/>
   </Target>
+
+  <Target Name="_SquiggleCop_SetErrorLog" BeforeTargets="CoreCompile">
+    <!--
+      ErrorLog is needed for SquiggleCop.
+
+      The value is set in a Target and not directly as a property because `$(IntermediateOutputPath)` and `$(OutputPath)`
+      are calculated properties and thus shouldn't be relied on during the initial property evaluation phase.
+      See https://github.com/dotnet/sdk/issues/41852.
+
+      We use `$(IntermediateOutputPath)` to ensure the file ends up in the `obj/` folder and not with sources to clearly
+      delineate inputs and outputs.
+    -->
+    <PropertyGroup Condition=" '$(ErrorLog)' == '' ">
+      <ErrorLog>$(IntermediateOutputPath)$(MSBuildProjectName).sarif,version=2.1</ErrorLog>
+    </PropertyGroup>
+  </Target>
 </Project>

tests/Tasks.Tests/ErrorLogParameterTests.cs

@@ -8,6 +8,10 @@ public class ErrorLogParameterTests : TestBase
     public async Task NoErrorLogReportsWarning()
     {
         ProjectCreator.Templates.SimpleBuild()
+            .Target(name: "_UnsetErrorLogPropertyBeforeSquiggleCopRuns", beforeTargets: "AfterCompile")
+                .TaskMessage("Unsetting ErrorLog...")
+                .TargetPropertyGroup()
+                    .TargetProperty(name: "ErrorLog", unevaluatedValue: string.Empty)
             .Save(Path.Combine(TestRootPath, "project.csproj"))
             .TryBuild(restore: true, out bool result, out BuildOutput output);
 

tests/Tasks.Tests/ErrorLogPropertyDefaultValueTests.cs

@@ -0,0 +1,39 @@
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities.ProjectCreation;
+
+namespace SquiggleCop.Tasks.Tests;
+
+public class ErrorLogPropertyDefaultValueTests : TestBase
+{
+    [Fact]
+    public void NotExplicitlySet()
+    {
+        ProjectCreator project = ProjectCreator.Templates.SimpleBuild()
+            .Target(name: "OutputErrorLog", beforeTargets: "AfterCompile")
+                .TaskMessage("ErrorLog=$(ErrorLog)", importance: MessageImportance.High);
+
+        project
+            .Save(Path.Combine(TestRootPath, "project.csproj"))
+            .TryBuild(restore: true, out bool result, out BuildOutput output);
+
+        string[] logs = output.Messages.Where(message => message.Contains("ErrorLog=")).ToArray();
+        logs.Should().ContainSingle().Which.Should().Be($"ErrorLog=obj{Path.DirectorySeparatorChar}Debug{Path.DirectorySeparatorChar}net8.0{Path.DirectorySeparatorChar}project.sarif,version=2.1");
+    }
+
+    [Fact]
+    public void ExplicitlySet()
+    {
+        ProjectCreator project = ProjectCreator.Templates.SimpleBuild()
+            .PropertyGroup()
+                .ErrorLog("explicit.log", "2.1")
+            .Target(name: "OutputErrorLog", beforeTargets: "AfterCompile")
+                .TaskMessage("ErrorLog=$(ErrorLog)", importance: MessageImportance.High);
+
+        project
+            .Save(Path.Combine(TestRootPath, "project.csproj"))
+            .TryBuild(restore: true, out bool result, out BuildOutput output);
+
+        string[] logs = output.Messages.Where(message => message.Contains("ErrorLog=")).ToArray();
+        logs.Should().ContainSingle().Which.Should().Be($"ErrorLog=explicit.log,version=2.1");
+    }
+}