1 files changed, 107 insertions, 0 deletions
diff --git a/src/SMAPI.ModBuildConfig.Analyzer/ImplicitNetFieldCastAnalyzer.cs b/src/SMAPI.ModBuildConfig.Analyzer/ImplicitNetFieldCastAnalyzer.cs
new file mode 100644
index 00000000..d23bdc2e
--- /dev/null
+++ b/src/SMAPI.ModBuildConfig.Analyzer/ImplicitNetFieldCastAnalyzer.cs
@@ -0,0 +1,107 @@
+using System;
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace StardewModdingAPI.ModBuildConfig.Analyzer
+{
+    /// <summary>Detects implicit conversion from Stardew Valley's <c>Netcode</c> types. These have very unintuitive implicit conversion rules, so mod authors should always explicitly convert the type with appropriate null checks.</summary>
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class ImplicitNetFieldCastAnalyzer : DiagnosticAnalyzer
+    {
+        /*********
+        ** Properties
+        *********/
+        /// <summary>The namespace for Stardew Valley's <c>Netcode</c> types.</summary>
+        private const string NetcodeNamespace = "Netcode";
+
+        /// <summary>Describes the diagnostic rule covered by the analyzer.</summary>
+        private static readonly DiagnosticDescriptor Rule = new DiagnosticDescriptor(
+            id: "SMAPI001",
+            title: "Netcode types shouldn't be implicitly converted",
+            messageFormat: "This implicitly converts '{0}' from {1} to {2}, but {1} has unintuitive implicit conversion rules. Consider comparing against the actual value instead to avoid bugs. See https://smapi.io/buildmsg/SMAPI001 for details.",
+            category: "SMAPI.CommonErrors",
+            defaultSeverity: DiagnosticSeverity.Warning,
+            isEnabledByDefault: true,
+            description: "",
+            helpLinkUri: "https://smapi.io/buildmsg/SMAPI001"
+        );
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>The descriptors for the diagnostics that this analyzer is capable of producing.</summary>
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; }
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        public ImplicitNetFieldCastAnalyzer()
+        {
+            this.SupportedDiagnostics = ImmutableArray.Create(ImplicitNetFieldCastAnalyzer.Rule);
+        }
+
+        /// <summary>Called once at session start to register actions in the analysis context.</summary>
+        /// <param name="context">The analysis context.</param>
+        public override void Initialize(AnalysisContext context)
+        {
+            context.RegisterSyntaxNodeAction(
+                this.Analyse,
+                SyntaxKind.EqualsExpression,
+                SyntaxKind.NotEqualsExpression,
+                SyntaxKind.GreaterThanExpression,
+                SyntaxKind.GreaterThanOrEqualExpression,
+                SyntaxKind.LessThanExpression,
+                SyntaxKind.LessThanOrEqualExpression
+            );
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>Analyse a syntax node and add a diagnostic message if applicable.</summary>
+        /// <param name="context">The analysis context.</param>
+        private void Analyse(SyntaxNodeAnalysisContext context)
+        {
+            try
+            {
+                BinaryExpressionSyntax node = (BinaryExpressionSyntax)context.Node;
+                bool leftHasWarning = this.Analyze(context, node.Left);
+                if (!leftHasWarning)
+                    this.Analyze(context, node.Right);
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException($"Failed processing expression: '{context.Node}'. Exception details: {ex.ToString().Replace('\r', ' ').Replace('\n', ' ')}");
+            }
+        }
+
+        /// <summary>Analyse one operand in a binary expression (like <c>a</c> and <c>b</c> in <c>a == b</c>) and add a diagnostic message if applicable.</summary>
+        /// <param name="context">The analysis context.</param>
+        /// <param name="operand">The operand expression.</param>
+        /// <returns>Returns whether a diagnostic message was raised.</returns>
+        private bool Analyze(SyntaxNodeAnalysisContext context, ExpressionSyntax operand)
+        {
+            const string netcodeNamespace = ImplicitNetFieldCastAnalyzer.NetcodeNamespace;
+
+            TypeInfo operandType = context.SemanticModel.GetTypeInfo(operand);
+            string fromNamespace = operandType.Type?.ContainingNamespace?.Name;
+            string toNamespace = operandType.ConvertedType?.ContainingNamespace?.Name;
+            if (fromNamespace == netcodeNamespace && fromNamespace != toNamespace && toNamespace != null)
+            {
+                string fromTypeName = operandType.Type.Name;
+                string toTypeName = operandType.ConvertedType.Name;
+                context.ReportDiagnostic(Diagnostic.Create(ImplicitNetFieldCastAnalyzer.Rule, context.Node.GetLocation(), operand, fromTypeName, toTypeName));
+                return true;
+            }
+
+            return false;
+        }
+    }
+}