Handle ILLink origins before first sequence point

src/tools/illink/src/linker/Linker/MessageOrigin.cs

@@ -86,9 +86,14 @@ public MessageOrigin(MessageOrigin other, int ilOffset)
             if (Provider is MethodDefinition method &&
                 method.DebugInformation.HasSequencePoints)
             {
-                var offset = ILOffset == UnsetILOffset ? method.DebugInformation.SequencePoints[0].Offset : ILOffset;
-                SequencePoint? correspondingSequencePoint = method.DebugInformation.SequencePoints
-                    .Where(s => s.Offset <= offset)?.Last();
+                var sequencePoints = method.DebugInformation.SequencePoints;
+                var offset = ILOffset == UnsetILOffset ? sequencePoints[0].Offset : ILOffset;
+                SequencePoint? correspondingSequencePoint = sequencePoints
+                    .Where(s => s.Offset <= offset).LastOrDefault();
+
+                // If the warning comes from IL before the first sequence point
+                // (for example, rewritten IL), report the first available sequence point as a best effort.
+                correspondingSequencePoint ??= sequencePoints.FirstOrDefault();
 
                 // If the warning comes from hidden line (compiler generated code typically)
                 // search for any sequence point with non-hidden line number and report that as a best effort.

src/tools/illink/test/Mono.Linker.Tests/Tests/MessageContainerTests.cs

@@ -1,7 +1,11 @@
 // Copyright (c) .NET Foundation and contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.IO;
+using System.Linq;
 using Xunit;
+using Mono.Cecil;
+using Mono.Cecil.Cil;
 
 namespace Mono.Linker.Tests
 {
@@ -21,5 +25,61 @@ public void MSBuildFormat()
             msg = MessageContainer.CreateInfoMessage("log test");
             Assert.Equal("ILLink: log test", msg.ToMSBuildString());
         }
+
+        [Fact]
+        public void OriginBeforeFirstSequencePointUsesFirstAvailableSequencePoint()
+        {
+            using ModuleDefinition module = ModuleDefinition.CreateModule("test", ModuleKind.Dll);
+            var type = new TypeDefinition("Test", "Type", TypeAttributes.Public, module.TypeSystem.Object);
+            module.Types.Add(type);
+
+            var method = new MethodDefinition("Method", MethodAttributes.Public | MethodAttributes.Static, module.TypeSystem.Void);
+            type.Methods.Add(method);
+
+            var body = method.Body;
+            var firstInstruction = Instruction.Create(OpCodes.Nop);
+            var secondInstruction = Instruction.Create(OpCodes.Ret);
+            body.Instructions.Add(firstInstruction);
+            body.Instructions.Add(secondInstruction);
+
+            method.DebugInformation.SequencePoints.Add(new SequencePoint(secondInstruction, new Document("test.cs"))
+            {
+                StartLine = 10,
+                StartColumn = 5,
+                EndLine = 10,
+                EndColumn = 6
+            });
+
+            using var assemblyStream = new MemoryStream();
+            using var symbolStream = new MemoryStream();
+
+            module.Write(assemblyStream, new WriterParameters
+            {
+                WriteSymbols = true,
+                SymbolStream = symbolStream,
+                SymbolWriterProvider = new PortablePdbWriterProvider()
+            });
+
+            assemblyStream.Position = 0;
+            symbolStream.Position = 0;
+
+            using var assembly = AssemblyDefinition.ReadAssembly(assemblyStream, new ReaderParameters
+            {
+                ReadSymbols = true,
+                SymbolStream = symbolStream,
+                SymbolReaderProvider = new PortablePdbReaderProvider()
+            });
+
+            method = assembly.MainModule.GetType("Test.Type").Methods.Single(m => m.Name == "Method");
+            firstInstruction = method.Body.Instructions[0];
+            Assert.True(firstInstruction.Offset < method.DebugInformation.SequencePoints[0].Offset);
+
+            var msg = MessageContainer.CreateCustomErrorMessage(
+                "message",
+                6001,
+                origin: new MessageOrigin(method, firstInstruction.Offset));
+
+            Assert.Equal("test.cs(10,5): error IL6001: Test.Type.Method(): message", msg.ToMSBuildString());
+        }
     }
 }