Remove AttributeTargets.Module from NullableContextAttribute (#37610)

docs/features/nullable-metadata.md

@@ -78,7 +78,6 @@ Each type parameter definition may have an associated `NullableAttribute` with a
 namespace System.Runtime.CompilerServices
 {
     [System.AttributeUsage(
-        AttributeTargets.Module |
         AttributeTargets.Class |
         AttributeTargets.Delegate |
         AttributeTargets.Interface |
@@ -102,7 +101,7 @@ The type declaration is synthesized by the compiler if not already included in t
 
 The `NullableContextAttribute` is optional - nullable annotations can be represented in metadata with full fidelity using `NullableAttribute` only.
 
-`NullableContextAttribute` is valid in metadata at the module level and at type and method declarations.
+`NullableContextAttribute` is valid in metadata on type and method declarations.
 The `byte` value represents the implicit `NullableAttribute` value for type references within that scope
 that do not have an explicit `NullableAttribute` and would not otherwise be represented by an empty `byte[]`.
 The nearest `NullableContextAttribute` in the metadata hierarchy applies.
@@ -121,8 +120,9 @@ and any `NullableContextAttribute` attributes on immediate children.
 If there are no single `byte` values, there are no changes.
 Otherwise, a `NullableContext(value)` attribute is created at that level where `value` is most common
 value (preferring `0` over `1` and preferring `1` over `2`), and all `NullableAttribute` and `NullableContextAttribute` attributes with that value are removed.
-That iterative process continues up to the module level.
-If the common value at the module level is a value other than `0` (the default), a module level `NullableContext(value)` attribute is emitted.
+That iterative process continues up to, and including, the top-level containing type definition.
+If the common value at the top-level type definition is a value other than `0` (the default),
+a `NullableContext(value)` attribute is emitted.
 
 Note that an assembly compiled with C#8 where all reference types are oblivious will have no
 `NullableContextAttribute` and no `NullableAttribute` attributes emitted.

src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedEmbeddedNullableContextAttributeSymbol.cs

@@ -48,7 +48,7 @@ public SynthesizedEmbeddedNullableContextAttributeSymbol(
         internal override AttributeUsageInfo GetAttributeUsageInfo()
         {
             return new AttributeUsageInfo(
-                AttributeTargets.Module | AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Interface | AttributeTargets.Method | AttributeTargets.Struct,
+                AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Interface | AttributeTargets.Method | AttributeTargets.Struct,
                 allowMultiple: false,
                 inherited: false);
         }

src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests_Nullable.cs

@@ -1733,6 +1733,37 @@ System.IAsyncResult BeginInvoke(System.Object?[] o, System.AsyncCallback callbac
             AssertNullableAttributes(comp, expected);
         }
 
+        [Fact]
+        public void EmitAttribute_NestedEnum()
+        {
+            var source =
+@"#nullable enable
+public class Program
+{
+    public enum E
+    {
+        A,
+        B
+    }
+    public object F1;
+    public object F2;
+    public object F3;
+}";
+            var comp = CreateCompilation(source);
+            var expected =
+@"[NullableContext(1)] [Nullable(0)] Program
+    System.Object! F1
+    System.Object! F2
+    System.Object! F3
+    Program()
+    [NullableContext(0)] Program.E
+        A
+        B
+        E()
+";
+            AssertNullableAttributes(comp, expected);
+        }
+
         [Fact]
         public void EmitAttribute_LambdaReturnType()
         {

src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests_NullableContext.cs

@@ -205,7 +205,7 @@ public class Program
                 Assert.False(attributeUsage.Inherited);
                 Assert.False(attributeUsage.AllowMultiple);
                 Assert.True(attributeUsage.HasValidAttributeTargets);
-                var expectedTargets = AttributeTargets.Module | AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Interface | AttributeTargets.Method | AttributeTargets.Struct;
+                var expectedTargets = AttributeTargets.Class | AttributeTargets.Delegate | AttributeTargets.Interface | AttributeTargets.Method | AttributeTargets.Struct;
                 Assert.Equal(expectedTargets, attributeUsage.ValidTargets);
             });
         }
@@ -241,6 +241,53 @@ public class Program
                 Diagnostic(ErrorCode.ERR_MissingPredefinedMember).WithArguments("System.AttributeUsageAttribute", "Inherited").WithLocation(1, 1));
         }
 
+        /// <summary>
+        /// Module-level NullableContextAttribute is ignored.
+        /// </summary>
+        [Fact]
+        public void ExplicitAttribute_Module()
+        {
+            var source0 =
+@"namespace System.Runtime.CompilerServices
+{
+    public sealed class NullableContextAttribute : Attribute
+    {
+        public NullableContextAttribute(byte b) { }
+    }
+}";
+            var comp0 = CreateCompilation(source0);
+            var ref0 = comp0.EmitToImageReference();
+
+            var source1 =
+@"Imports System.Runtime.CompilerServices
+<Module: NullableContext(2)>
+Public Class A
+    Public Shared FA As Object
+End Class
+<NullableContext(2)>
+Public Class B
+    Public Shared FB As Object
+End Class";
+            var comp1 = CreateVisualBasicCompilation(source1, referencedAssemblies: TargetFrameworkUtil.GetReferences(TargetFramework.Standard).Concat(ref0));
+            var ref1 = comp1.EmitToImageReference();
+
+            var source2 =
+@"#nullable enable
+class Program
+{
+    static void Main()
+    {
+        A.FA.ToString();
+        B.FB.ToString(); // warning
+    }
+}";
+            var comp2 = CreateCompilation(source2, references: new[] { ref1 });
+            comp2.VerifyDiagnostics(
+                // (7,9): warning CS8602: Dereference of a possibly null reference.
+                //         B.FB.ToString(); // warning
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "B.FB").WithLocation(7, 9));
+        }
+
         [Fact]
         public void AttributeField()
         {

src/Compilers/Test/Utilities/CSharp/CSharpTestBase.cs

@@ -55,7 +55,6 @@ public NullableAttribute(byte[] transformFlags)
 namespace System.Runtime.CompilerServices
 {
     [System.AttributeUsage(
-        AttributeTargets.Module |
         AttributeTargets.Class |
         AttributeTargets.Delegate |
         AttributeTargets.Interface |