From 6a406b28d1b8f2e27bfa8cb0d8850a62ffbc591c Mon Sep 17 00:00:00 2001
From: Michael Nebel <michaelnebel@github.com>
Date: Tue, 14 Jan 2025 15:57:21 +0100
Subject: [PATCH] C#: Do not insert a synthetic ToString call in interpolation
 expressions, if the type implements IFormattable.

---
 .../CodeAnalysisExtensions/SymbolExtensions.cs        |  6 ++++++
 .../Entities/Expressions/InterpolatedString.cs        | 11 ++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs b/csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs
index bb61efe332c8..cb1f36f8a2de 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs
@@ -532,6 +532,12 @@ attribute.AttributeClass is INamedTypeSymbol nt &&
             return isInline;
         }
 
+        /// <summary>
+        /// Returns true if this type implements `System.IFormattable`.
+        /// </summary>
+        public static bool ImplementsIFormattable(this ITypeSymbol type) =>
+            type.AllInterfaces.Any(i => i.Name == "IFormattable" && i.ContainingNamespace.ToString() == "System");
+
         /// <summary>
         /// Holds if this type is of the form <code>System.ReadOnlySpan&lt;byte&gt;</code>.
         /// </summary>
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/InterpolatedString.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/InterpolatedString.cs
index bdf30bf6618f..6d17d1e7f176 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/InterpolatedString.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/InterpolatedString.cs
@@ -1,4 +1,5 @@
 using System.IO;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Semmle.Extraction.Kinds;
@@ -20,7 +21,15 @@ protected override void PopulateExpression(TextWriter trapFile)
                 {
                     case SyntaxKind.Interpolation:
                         var interpolation = (InterpolationSyntax)c;
-                        ImplicitToString.Create(Context, interpolation.Expression, this, child++);
+                        var exp = interpolation.Expression;
+                        if (Context.GetTypeInfo(exp).Type is ITypeSymbol type && !type.ImplementsIFormattable())
+                        {
+                            ImplicitToString.Create(Context, exp, this, child++);
+                        }
+                        else
+                        {
+                            Create(Context, exp, this, child++);
+                        }
                         break;
                     case SyntaxKind.InterpolatedStringText:
                         // Create a string literal