Add public APIs for persisted AssemblyBulder

src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

@@ -4289,4 +4289,7 @@
   <data name="Reflection_Disabled" xml:space="preserve">
     <value>This operation is not available because the reflection support was disabled at compile time.</value>
   </data>
-</root>
+  <data name="NotSupported_AssemblySave" xml:space="preserve">
+    <value>This AssemblyBuilder instance doesn't support saving. Use AssemblyBuilder.DefinePersistedAssembly to create an AssemblyBuilder instance that supports saving.</value>
+  </data>
+</root>

src/libraries/System.Private.CoreLib/src/System/Reflection/Emit/AssemblyBuilder.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Runtime.CompilerServices;
@@ -32,8 +33,56 @@ public ModuleBuilder DefineDynamicModule(string name)
             return GetDynamicModuleCore(name);
         }
 
+        /// <summary>
+        /// Defines an <see cref="AssemblyBuilder"/> that can be saved to a file or stream.
+        /// </summary>
+        /// <param name="name">The name of the assembly.</param>
+        /// <param name="coreAssembly">The assembly that denotes the "system assembly" that houses the well-known types such as <see cref="object"/></param>
+        /// <param name="assemblyAttributes">A collection that contains the attributes of the assembly.</param>
+        /// <returns>An <see cref="AssemblyBuilder"/> that can be persisted.</returns>
+        /// <exception cref="ArgumentNullException">The <paramref name="name"/> or <paramref name="name.Name"/> or <paramref name="coreAssembly"/> is null.</exception>
+        /// <remarks>Currently the persisted assembly doesn't support running, need to save it and load back to run.</remarks>
+        public static AssemblyBuilder DefinePersistedAssembly(AssemblyName name, Assembly coreAssembly, IEnumerable<CustomAttributeBuilder>? assemblyAttributes = null)
+        {
+            ArgumentNullException.ThrowIfNull(name);
+            ArgumentException.ThrowIfNullOrEmpty(name.Name, "AssemblyName.Name");
+            ArgumentNullException.ThrowIfNull(coreAssembly);
+
+            Type assemblyType = Type.GetType("System.Reflection.Emit.AssemblyBuilderImpl, System.Reflection.Emit", throwOnError: true)!;
+            ConstructorInfo con = assemblyType.GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, [typeof(AssemblyName), typeof(Assembly), typeof(IEnumerable<CustomAttributeBuilder>)])!;
+            return (AssemblyBuilder)con.Invoke([name, coreAssembly, assemblyAttributes]);
+        }
+
         protected abstract ModuleBuilder? GetDynamicModuleCore(string name);
 
+        /// <summary>
+        /// Serializes the assembly to <see cref="Stream"/>.
+        /// </summary>
+        /// <param name="stream">The <see cref="Stream"/> to which the assembly serialized.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="stream"/> is null.</exception>
+        /// <exception cref="NotSupportedException">The AssemblyBuilder instance doesn't support saving.</exception>
+        public void Save(Stream stream) => SaveCore(stream);
+
+        /// <summary>
+        /// Saves the assembly to disk.
+        /// </summary>
+        /// <param name="assemblyFileName">The file name of the assembly.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="assemblyFileName"/> is null.</exception>
+        /// <exception cref="NotSupportedException">The AssemblyBuilder instance doesn't support saving.</exception>
+        public void Save(string assemblyFileName)
+        {
+            ArgumentNullException.ThrowIfNull(assemblyFileName);
+
+            using var peStream = new FileStream(assemblyFileName, FileMode.Create, FileAccess.Write);
+            SaveCore(peStream);
+        }
+
+        /// <summary>
+        /// When implemented in a derived type, serializes the assembly to a stream.
+        /// </summary>
+        /// <param name="stream">The stream to which the assembly serialized.</param>
+        protected virtual void SaveCore(Stream stream) => throw new NotSupportedException(SR.NotSupported_AssemblySave);
+
         public void SetCustomAttribute(ConstructorInfo con, byte[] binaryAttribute)
         {
             ArgumentNullException.ThrowIfNull(con);

src/libraries/System.Reflection.Emit/ref/System.Reflection.Emit.cs

@@ -24,6 +24,7 @@ protected AssemblyBuilder() { }
         public static System.Reflection.Emit.AssemblyBuilder DefineDynamicAssembly(System.Reflection.AssemblyName name, System.Reflection.Emit.AssemblyBuilderAccess access) { throw null; }
         [System.Diagnostics.CodeAnalysis.RequiresDynamicCode("Defining a dynamic assembly requires dynamic code.")]
         public static System.Reflection.Emit.AssemblyBuilder DefineDynamicAssembly(System.Reflection.AssemblyName name, System.Reflection.Emit.AssemblyBuilderAccess access, System.Collections.Generic.IEnumerable<System.Reflection.Emit.CustomAttributeBuilder>? assemblyAttributes) { throw null; }
+        public static System.Reflection.Emit.AssemblyBuilder DefinePersistedAssembly(System.Reflection.AssemblyName name, System.Reflection.Assembly coreAssembly, System.Collections.Generic.IEnumerable<System.Reflection.Emit.CustomAttributeBuilder>? assemblyAttributes = null) { throw null; }
         public System.Reflection.Emit.ModuleBuilder DefineDynamicModule(string name) { throw null; }
         protected abstract System.Reflection.Emit.ModuleBuilder DefineDynamicModuleCore(string name);
         public override bool Equals(object? obj) { throw null; }
@@ -54,6 +55,9 @@ protected AssemblyBuilder() { }
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("Types might be removed by trimming. If the type name is a string literal, consider using Type.GetType instead.")]
         public override System.Type? GetType(string name, bool throwOnError, bool ignoreCase) { throw null; }
         public override bool IsDefined(System.Type attributeType, bool inherit) { throw null; }
+        public void Save(string assemblyFileName) { throw null; }
+        public void Save(System.IO.Stream stream) { throw null; }
+        protected virtual void SaveCore(System.IO.Stream stream) { }
         public void SetCustomAttribute(System.Reflection.ConstructorInfo con, byte[] binaryAttribute) { }
         public void SetCustomAttribute(System.Reflection.Emit.CustomAttributeBuilder customBuilder) { }
         protected abstract void SetCustomAttributeCore(System.Reflection.ConstructorInfo con, System.ReadOnlySpan<byte> binaryAttribute);

src/libraries/System.Reflection.Emit/src/ILLink/ILLink.Descriptors.LibraryBuild.xml

@@ -1,9 +1,8 @@
 <linker>
   <assembly fullname="System.Reflection.Emit">
     <type fullname="System.Reflection.Emit.AssemblyBuilderImpl">
-      <!-- Internal API used by tests only. -->
-      <method name="DefinePersistedAssembly" />
-      <method name="Save" />
+      <!-- Internal API called through Reflection by another assembly. -->
+      <method name=".ctor" />
     </type>
   </assembly>
 </linker>

src/libraries/System.Reflection.Emit/src/System/Reflection/Emit/AssemblyBuilderImpl.cs

@@ -19,15 +19,9 @@ internal sealed class AssemblyBuilderImpl : AssemblyBuilder
 
         internal List<CustomAttributeWrapper>? _customAttributes;
 
-        internal AssemblyBuilderImpl(AssemblyName name, Assembly coreAssembly, IEnumerable<CustomAttributeBuilder>? assemblyAttributes)
+        internal AssemblyBuilderImpl(AssemblyName name, Assembly coreAssembly, IEnumerable<CustomAttributeBuilder>? assemblyAttributes = null)
         {
-            ArgumentNullException.ThrowIfNull(name);
-
-            name = (AssemblyName)name.Clone();
-
-            ArgumentException.ThrowIfNullOrEmpty(name.Name, "AssemblyName.Name");
-
-            _assemblyName = name;
+            _assemblyName = (AssemblyName)name.Clone();
             _coreAssembly = coreAssembly;
             _metadataBuilder = new MetadataBuilder();
 
@@ -40,10 +34,6 @@ internal AssemblyBuilderImpl(AssemblyName name, Assembly coreAssembly, IEnumerab
             }
         }
 
-        internal static AssemblyBuilderImpl DefinePersistedAssembly(AssemblyName name, Assembly coreAssembly,
-            IEnumerable<CustomAttributeBuilder>? assemblyAttributes)
-                => new AssemblyBuilderImpl(name, coreAssembly, assemblyAttributes);
-
         private void WritePEImage(Stream peStream, BlobBuilder ilBuilder, BlobBuilder fieldData)
         {
             var peHeaderBuilder = new PEHeaderBuilder(
@@ -64,7 +54,7 @@ private void WritePEImage(Stream peStream, BlobBuilder ilBuilder, BlobBuilder fi
             peBlob.WriteContentTo(peStream);
         }
 
-        internal void Save(Stream stream)
+        protected override void SaveCore(Stream stream)
         {
             ArgumentNullException.ThrowIfNull(stream);
 
@@ -103,14 +93,6 @@ internal void Save(Stream stream)
         private static AssemblyFlags AddContentType(AssemblyFlags flags, AssemblyContentType contentType)
             => (AssemblyFlags)((int)contentType << 9) | flags;
 
-        internal void Save(string assemblyFileName)
-        {
-            ArgumentNullException.ThrowIfNull(assemblyFileName);
-
-            using var peStream = new FileStream(assemblyFileName, FileMode.Create, FileAccess.Write);
-            Save(peStream);
-        }
-
         protected override ModuleBuilder DefineDynamicModuleCore(string name)
         {
             if (_module != null)

src/libraries/System.Reflection.Emit/tests/PersistableAssemblyBuilder/AssemblySaveAssemblyBuilder.cs

@@ -34,7 +34,7 @@ public void AssemblyWithDifferentTypes()
                 aName.CultureInfo = new CultureInfo("en");
                 aName.Flags = AssemblyNameFlags.Retargetable;
 
-                AssemblyBuilder ab = AssemblySaveTools.PopulateAssemblyBuilderAndSaveMethod(aName, null, typeof(string), out MethodInfo saveMethod);
+                AssemblyBuilder ab = AssemblySaveTools.PopulateAssemblyBuilder(aName);
 
                 ab.SetCustomAttribute(new CustomAttributeBuilder(typeof(AssemblyDelaySignAttribute).GetConstructor([typeof(bool)]), [true]));
 
@@ -213,10 +213,12 @@ public void AssemblyWithDifferentTypes()
                 eventb.SetRemoveOnMethod(mbRemove);
                 tbEvents.CreateType();
 
-                saveMethod.Invoke(ab, [file.Path]);
+                ab.Save(file.Path);
 
-                Assembly assemblyFromDisk = AssemblySaveTools.LoadAssemblyFromPath(file.Path);
-                CheckAssembly(assemblyFromDisk);
+                using (MetadataLoadContext mlc = new MetadataLoadContext(new CoreMetadataAssemblyResolver()))
+                {
+                    CheckAssembly(mlc.LoadFromAssemblyPath(file.Path));
+                }
             }
         }