Collect custom steps used in --custom-step

Create.cs

@@ -165,6 +165,10 @@ private static int Run(FileInfo binlog, DirectoryInfo? @out, bool force, string?
                         reproArgs.Add(searchDirectory with { Path = CopyDirectoryToInput(searchDirectory.Path) });
                         break;
 
+                    case ILLink.CustomStep customStep:
+                        reproArgs.Add(customStep with { Path = CopyFileToInput(customStep.Path) });
+                        break;
+
                     default:
                         reproArgs.Add(arg);
                         break;

ILLink.cs

@@ -1,4 +1,6 @@
-namespace illinkrepro
+using System.Diagnostics.CodeAnalysis;
+
+namespace illinkrepro
 {
     internal class ILLink
     {
@@ -49,6 +51,11 @@ public record SearchDirectory(string Path) : Argument
             public override string ToString() => $"-d {Path}";
         }
 
+        public record CustomStep(string Prefix, string Path) : Argument
+        {
+            public override string ToString() => $"--custom-step \"{Prefix},{Path}\"";
+        }
+
         readonly string _workingPath;
 
         public string DotnetPath { get; private set; }
@@ -134,6 +141,12 @@ IEnumerable<Argument> Parse(string commandLine)
                         yield return new LinkAttributes(ToPath(lineParts[1])); break;
                     case "-d":
                         yield return new SearchDirectory(ToPath(lineParts[1])); break;
+                    case "--custom-step":
+                        if (ParseCustomStep(lineParts[1], out var customStep))
+                            yield return customStep;
+                        else
+                            goto default;
+                        break;
                     default:
                         yield return new UnknownArgument(string.Join(' ', lineParts)); break;
                 }
@@ -168,6 +181,21 @@ static IEnumerable<string> SplitLine(string line)
                 yield return line[start..];
         }
 
+        bool ParseCustomStep(string input, [NotNullWhen(true)] out CustomStep? customStep)
+        {
+            input = input.Trim('"');
+            int index;
+
+            if ((index = input.LastIndexOf(',')) >= 0)
+            {
+                customStep = new CustomStep(input.Substring(0, index), ToPath(input.Substring(index + 1)));
+                return true;
+            }
+
+            customStep = null;
+            return false;
+        }
+
         string ToPath(string v)
         {
             v = v.Trim('"');