Provide an API to get unreferenced assembly identities obects from the...

Provide an API to get unreferenced assembly identities obects from the diagnostics reported about them.

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1523,6 +1523,9 @@ private MethodSymbol FindEntryPoint(CancellationToken cancellationToken, out Imm
             }
         }
 
+        internal override ImmutableArray<int> UnreferencedAssemblyIdentityDiagnosticCodes { get; }
+            = ImmutableArray.Create((int)ErrorCode.ERR_NoTypeDef);
+
         internal class EntryPoint
         {
             public readonly MethodSymbol MethodSymbol;

src/Compilers/Core/Portable/Compilation/Compilation.cs

@@ -2668,5 +2668,44 @@ internal bool IsTypeMissing(WellKnownType type)
         }
         
         internal abstract bool IsIOperationFeatureEnabled();
+
+        /// <summary>
+        /// Given a <see cref="Diagnostic"/> reporting unreferenced <see cref="AssemblyIdentity"/>s, returns
+        /// the actual <see cref="AssemblyIdentity"/> instances that were not referenced.
+        /// </summary>
+        public ImmutableArray<AssemblyIdentity> GetUnreferencedAssemblyIdentities(Diagnostic diagnostic)
+        {
+            if (diagnostic == null)
+            {
+                throw new ArgumentNullException(nameof(diagnostic));
+            }
+
+            if (UnreferencedAssemblyIdentityDiagnosticCodes.Contains(diagnostic.Code))
+            {
+                return ExtractAssemblyIdenties(diagnostic);
+            }
+
+            return ImmutableArray<AssemblyIdentity>.Empty;
+        }
+
+        /// <summary>
+        /// For testing purposes.
+        /// </summary>
+        internal abstract ImmutableArray<int> UnreferencedAssemblyIdentityDiagnosticCodes { get; }
+
+        internal static ImmutableArray<AssemblyIdentity> ExtractAssemblyIdenties(Diagnostic diagnostic)
+        {
+            var builder = ArrayBuilder<AssemblyIdentity>.GetInstance();
+
+            foreach (var argument in diagnostic.Arguments)
+            {
+                if (argument is AssemblyIdentity id)
+                {
+                    builder.Add(id);
+                }
+            }
+
+            return builder.ToImmutableAndFree();
+        }
     }
 }
\ No newline at end of file

src/Compilers/Core/Portable/PublicAPI.Unshipped.txt

@@ -15,6 +15,7 @@ Microsoft.CodeAnalysis.Compilation.CreateTupleTypeSymbol(Microsoft.CodeAnalysis.
 Microsoft.CodeAnalysis.Compilation.CreateTupleTypeSymbol(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.ITypeSymbol> elementTypes, System.Collections.Immutable.ImmutableArray<string> elementNames = default(System.Collections.Immutable.ImmutableArray<string>), System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.Location> elementLocations = default(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.Location>)) -> Microsoft.CodeAnalysis.INamedTypeSymbol
 Microsoft.CodeAnalysis.Compilation.Emit(System.IO.Stream peStream, System.IO.Stream pdbStream = null, System.IO.Stream xmlDocumentationStream = null, System.IO.Stream win32Resources = null, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.ResourceDescription> manifestResources = null, Microsoft.CodeAnalysis.Emit.EmitOptions options = null, Microsoft.CodeAnalysis.IMethodSymbol debugEntryPoint = null, System.IO.Stream sourceLinkStream = null, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.EmbeddedText> embeddedTexts = null, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> Microsoft.CodeAnalysis.Emit.EmitResult
 Microsoft.CodeAnalysis.Compilation.Emit(System.IO.Stream peStream, System.IO.Stream pdbStream, System.IO.Stream xmlDocumentationStream, System.IO.Stream win32Resources, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.ResourceDescription> manifestResources, Microsoft.CodeAnalysis.Emit.EmitOptions options, Microsoft.CodeAnalysis.IMethodSymbol debugEntryPoint, System.Threading.CancellationToken cancellationToken) -> Microsoft.CodeAnalysis.Emit.EmitResult
+Microsoft.CodeAnalysis.Compilation.GetUnreferencedAssemblyIdentities(Microsoft.CodeAnalysis.Diagnostic diagnostic) -> System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.AssemblyIdentity>
 Microsoft.CodeAnalysis.CompilationOptions.WithConcurrentBuild(bool concurrent) -> Microsoft.CodeAnalysis.CompilationOptions
 Microsoft.CodeAnalysis.CompilationOptions.WithCryptoKeyContainer(string cryptoKeyContainer) -> Microsoft.CodeAnalysis.CompilationOptions
 Microsoft.CodeAnalysis.CompilationOptions.WithCryptoKeyFile(string cryptoKeyFile) -> Microsoft.CodeAnalysis.CompilationOptions

src/Compilers/VisualBasic/Portable/Compilation/VisualBasicCompilation.vb

@@ -2702,6 +2702,14 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
             Return If(IOperationFeatureFlag Is Nothing, False, options.Features.ContainsKey(IOperationFeatureFlag))
         End Function
+
+        Friend Overrides ReadOnly Property UnreferencedAssemblyIdentityDiagnosticCodes As ImmutableArray(Of Integer) =
+            ImmutableArray.Create(Of Integer)(
+                ERRID.ERR_UnreferencedAssemblyEvent3,
+                ERRID.ERR_UnreferencedModuleBase3,
+                ERRID.ERR_UnreferencedModuleImplements3,
+                ERRID.ERR_UnreferencedAssembly3)
+
 #End Region
 
         Private Class SymbolSearcher

src/Test/Utilities/Shared/Diagnostics/DiagnosticExtensions.cs

@@ -97,6 +97,19 @@ public static TCompilation VerifyDiagnostics<TCompilation>(this TCompilation c,
         {
             var diagnostics = c.GetDiagnostics();
             diagnostics.Verify(expected);
+
+            // At this point we've verified that the expected and actual diagnostics are the same.
+            var unreferencedAssemblyDiagnosticIds = c.UnreferencedAssemblyIdentityDiagnosticCodes;
+            foreach (var diagnostic in diagnostics)
+            {
+                if (unreferencedAssemblyDiagnosticIds.Contains(diagnostic.Code))
+                {
+                    // Ensure that this diagnostic contains at least one assembly id.
+                    var assemblyIds = c.GetUnreferencedAssemblyIdentities(diagnostic);
+                    Assert.True(assemblyIds.Length > 0);
+                }
+            }
+
             return c;
         }