Don't use ref assemblies for NETCoreApp runtime libraries

Solves an issue with structs not being recognized.
This is not a great fix but it should be enough for now.

Author: dubiousconst282

src/DistIL.Cli/Program.cs

@@ -31,9 +31,12 @@ static void RunOptimizer(OptimizerOptions options)
     var logger = new ConsoleLogger() { MinLevel = options.Verbosity };
 
     var resolver = new ModuleResolver(logger);
+    resolver.AddSearchPaths(new[] { Path.GetDirectoryName(Path.GetFullPath(options.InputPath))! });
     resolver.AddSearchPaths(options.ResolverPaths);
-    resolver.AddSearchPaths(new[] { Path.GetDirectoryName(options.InputPath)!, Environment.CurrentDirectory });
-    resolver.AddTrustedSearchPaths();
+
+    if (!options.NoResolverFallback) {
+        resolver.AddTrustedSearchPaths();
+    }
 
     var module = resolver.Load(options.InputPath);
 
@@ -136,6 +139,9 @@ class OptimizerOptions
     [Option('r', HelpText = "Module resolver search paths.")]
     public IEnumerable<string> ResolverPaths { get; set; } = null!;
 
+    [Option("no-resolver-fallback", HelpText = "Don't use fallback search paths for module resolution.")]
+    public bool NoResolverFallback { get; set; } = false;
+
     [Option("filter-unmarked", HelpText = "Only transform methods and classes marked with `OptimizeAttribute`.")]
     public bool FilterUnmarked { get; set; } = false;
 

src/DistIL.OptimizerTask/DistIL.OptimizerTask.targets

@@ -14,7 +14,7 @@
     <PropertyGroup>
       <!-- Trailing slashes are removed to prevent issues with argument splitting. -->
       <JoinedAsmRefPaths>@(AsmRefPaths -> '"%(Identity)"', ' ')</JoinedAsmRefPaths>
-      <ResolverPathsArg>-r $(JoinedAsmRefPaths.Replace('\"', '"'))</ResolverPathsArg>
+      <ResolverPathsArg>-r $(JoinedAsmRefPaths.Replace('\"', '"')) --no-resolver-fallback</ResolverPathsArg>
 
       <FilterUnmarkedArg Condition="'$(DistilAllMethods)' != 'true'">--filter-unmarked</FilterUnmarkedArg>
     </PropertyGroup>

src/DistIL/AsmIO/ModuleResolver.cs

@@ -3,6 +3,7 @@ namespace DistIL.AsmIO;
 using System.IO;
 using System.Reflection;
 using System.Reflection.PortableExecutable;
+using System.Text.RegularExpressions;
 
 public class ModuleResolver
 {
@@ -25,7 +26,27 @@ public ModuleResolver(ICompilationLogger? logger = null)
 
     public void AddSearchPaths(IEnumerable<string> paths)
     {
-        _searchPaths = _searchPaths.Concat(paths).Distinct(StringComparer.OrdinalIgnoreCase).ToArray();
+        _searchPaths = _searchPaths
+            .Concat(paths)
+            .Select(FixRuntimePackRefPath)
+            .Distinct(StringComparer.OrdinalIgnoreCase)
+            .ToArray();
+
+        //Try to change search path for the `NETCore.App.Ref` pack to the actual implementation path.
+        //This is done for a couple reasons:
+        // - We make the assumption that "System.Private.CoreLib" always exist, but it doesn't in ref packs.
+        //   This would lead to multiple defs for e.g. "System.ValueType", which would cause issues.
+        // - We want to depend on _some_ private impl details which are not shipped in ref asms. 
+        //   Notably accessing private `List<T>` fields.
+        static string FixRuntimePackRefPath(string path)
+        {
+            //e.g. "C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\7.0.3\ref\net7.0"
+            //                              shared                     ****      ***********
+            //(I guess this piece will be in the top 3 reasons why I'll be sent to code hell.)
+            string normPath = Path.GetFullPath(path).Replace('\\', '/');
+            string implPath = Regex.Replace(normPath, @"(.+?\/)packs(\/Microsoft\.NETCore\.App)\.Ref(\/.+?)\/.+", "$1shared$2$3");
+            return implPath != normPath && Directory.Exists(implPath) ? implPath : path;
+        }
     }
 
     public void AddTrustedSearchPaths()