Add analyzer to warn for assignment to dependency fields.

RELEASE-NOTES.md

@@ -44,6 +44,7 @@ END TEMPLATE-->
 * Control layout properties such as `Margin` can now be set via style sheets.
 * Distance between lines of a `RichTextLabel` can now be modified with `LineHeightScale`.
 * UI theme prototypes are now updated when reloaded.
+* New `RA0025` analyzer diagnostic warns for manual assignment to `[Dependency]` fields.
 
 ### Bugfixes
 

Robust.Analyzers.Tests/DependencyAssignAnalyzerTest.cs

@@ -0,0 +1,65 @@
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CSharp.Testing;
+using Microsoft.CodeAnalysis.Testing;
+using Microsoft.CodeAnalysis.Testing.Verifiers;
+using NUnit.Framework;
+using VerifyCS =
+    Microsoft.CodeAnalysis.CSharp.Testing.NUnit.AnalyzerVerifier<Robust.Analyzers.DependencyAssignAnalyzer>;
+
+namespace Robust.Analyzers.Tests;
+
+[Parallelizable(ParallelScope.All | ParallelScope.Fixtures)]
+[TestFixture]
+public sealed class DependencyAssignAnalyzerTest
+{
+    private const string BaseCode = """
+        using System;
+
+        namespace Robust.Shared.IoC;
+
+        [AttributeUsage(AttributeTargets.Field)]
+        public sealed class DependencyAttribute : Attribute
+        {
+        }
+        """;
+
+    private static Task Verifier(string code, params DiagnosticResult[] expected)
+    {
+        var test = new CSharpAnalyzerTest<DependencyAssignAnalyzer, NUnitVerifier>()
+        {
+            TestState =
+            {
+                AdditionalReferences = { typeof(DependencyAssignAnalyzer).Assembly },
+                Sources = { code, BaseCode }
+            },
+        };
+
+        // ExpectedDiagnostics cannot be set, so we need to AddRange here...
+        test.TestState.ExpectedDiagnostics.AddRange(expected);
+
+        return test.RunAsync();
+    }
+
+    [Test]
+    public async Task Test()
+    {
+        const string code = """
+            using Robust.Shared.IoC;
+
+            public sealed class Foo
+            {
+                [Dependency]
+                private object? Field;
+
+                public Foo()
+                {
+                    Field = "A";
+                }
+            }
+            """;
+
+        await Verifier(code,
+            // /0/Test0.cs(10,9): warning RA0025: Tried to assign to [Dependency] field 'Field'. Remove [Dependency] or inject it via field injection instead.
+            VerifyCS.Diagnostic().WithSpan(10, 9, 10, 20).WithArguments("Field"));
+    }
+}

Robust.Analyzers/DependencyAssignAnalyzer.cs

@@ -0,0 +1,61 @@
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+using Robust.Roslyn.Shared;
+
+namespace Robust.Analyzers;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class DependencyAssignAnalyzer : DiagnosticAnalyzer
+{
+    private const string DependencyAttributeType = "Robust.Shared.IoC.DependencyAttribute";
+
+    private static readonly DiagnosticDescriptor Rule = new (
+        Diagnostics.IdDependencyFieldAssigned,
+        "Assignment to dependency field",
+        "Tried to assign to [Dependency] field '{0}'. Remove [Dependency] or inject it via field injection instead.",
+        "Usage",
+        DiagnosticSeverity.Warning,
+        true);
+
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(Rule);
+
+    public override void Initialize(AnalysisContext context)
+    {
+        context.EnableConcurrentExecution();
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+        context.RegisterOperationAction(CheckAssignment, OperationKind.SimpleAssignment);
+    }
+
+    private static void CheckAssignment(OperationAnalysisContext context)
+    {
+        if (context.Operation is not ISimpleAssignmentOperation assignment)
+            return;
+
+        if (assignment.Target is not IFieldReferenceOperation fieldRef)
+            return;
+
+        var field = fieldRef.Field;
+        var attributes = field.GetAttributes();
+        if (attributes.Length == 0)
+            return;
+
+        var depAttribute = context.Compilation.GetTypeByMetadataName(DependencyAttributeType);
+        if (!HasAttribute(attributes, depAttribute))
+            return;
+
+        context.ReportDiagnostic(Diagnostic.Create(Rule, assignment.Syntax.GetLocation(), field.Name));
+    }
+
+    private static bool HasAttribute(ImmutableArray<AttributeData> attributes, ISymbol symbol)
+    {
+        foreach (var attribute in attributes)
+        {
+            if (SymbolEqualityComparer.Default.Equals(attribute.AttributeClass, symbol))
+                return true;
+        }
+
+        return false;
+    }
+}

Robust.Roslyn.Shared/Diagnostics.cs

@@ -28,6 +28,7 @@ public static class Diagnostics
     public const string IdComponentPauseNoFields = "RA0022";
     public const string IdComponentPauseNoParentAttribute = "RA0023";
     public const string IdComponentPauseWrongTypeAttribute = "RA0024";
+    public const string IdDependencyFieldAssigned = "RA0025";
 
     public static SuppressionDescriptor MeansImplicitAssignment =>
         new SuppressionDescriptor("RADC1000", "CS0649", "Marked as implicitly assigned.");