[Version 9.0] Feature support for nullable attributes carried over from V8 (#1221)

* Restore nullable attributes added in 9

These two attributes were mistakenly added in #1191.  They were removed in #1218

This PR adds them back into the `draft-v9` branch, where they are valid.

* fix link

* add missing type names

---------

Co-authored-by: Rex Jaeschke <rex@RexJaeschke.com>

Author: BillWagner

standard/attributes.md

@@ -853,6 +853,8 @@ The code-analysis attributes are declared in namespace `System.Diagnostics.CodeA
 `MaybeNullWhen` ([§23.5.7.7](attributes.md#23577-the-maybenullwhen-attribute))  | A non-nullable argument may be null when the method returns the specified `bool` value.
 `NotNullWhen` ([§23.5.7.10](attributes.md#235710-the-notnullwhen-attribute))  | A nullable argument won’t be null when the method returns the specified `bool` value.
 `NotNullIfNotNull` ([§23.5.7.9](attributes.md#23579-the-notnullifnotnull-attribute))  | A return value isn’t null if the argument for the specified parameter isn’t null.
+`MemberNotNull` (§membernotnull-attribute)  | The listed member won’t be null when the method returns.
+`MemberNotNullWhen` (§membernotnullwhen-attribute)  | The listed member won’t be null when the method returns the specified `bool` value.
 `DoesNotReturn` ([§23.5.7.4](attributes.md#23574-the-doesnotreturn-attribute))  | This method never returns.
 `DoesNotReturnIf` ([§23.5.7.5](attributes.md#23575-the-doesnotreturnif-attribute))  | This method never returns if the associated `bool` parameter has the specified value.
 
@@ -999,6 +1001,47 @@ Specifies that a non-nullable return value may be null.
 
 Specifies that a non-nullable argument may be `null` when the method returns the specified `bool` value. This is similar to the `MaybeNull` attribute ([§23.5.7.6](attributes.md#23576-the-maybenull-attribute)), but includes a parameter for the specified return value.
 
+#### §membernotnull-attribute The MemberNotNull attribute
+
+Specifies that the given member won’t be `null` when the method returns.
+
+> *Example*: A helper method may include the `MemberNotNull` attribute to list any fields that are assigned to a non-null value in that method. A compiler that analyzes constructors to determine whether all non-nullable reference fields have been initialized may then use this attribute to discover which fields have been set by those helper methods. Consider the following example:
+>
+> <!-- Example: {template:"standalone-lib", name:"MemberNotNullAttribute"} -->
+> ```csharp
+> #nullable enable
+> public class Container
+> {
+>     private string _uniqueIdentifier; // must be initialized.
+>     private string? _optionalMessage;
+>
+>     public Container()
+>     {
+>         Helper();
+>     }
+>
+>     public Container(string message)
+>     {
+>         Helper();
+>         _optionalMessage = message;
+>     }
+>
+>     [MemberNotNull(nameof(_uniqueIdentifier))]
+>     private void Helper()
+>     {
+>         _uniqueIdentifier = DateTime.Now.Ticks.ToString();
+>     }
+> }
+> ```
+>
+> Multiple field names may be given as arguments to the attribute’s constructor. *end example*
+
+#### §membernotnullwhen-attribute The MemberNotNullWhen attribute
+
+Specifies that the listed member won’t be `null` when the method returns the specified `bool` value.
+
+> *Example*: This attribute is like `MemberNotNull` (§membernotnull-attribute) except that `MemberNotNullWhen` takes a `bool` argument. `MemberNotNullWhen` is intended for use in situations in which a helper method returns a `bool` indicating whether it initialized fields. *end example*
+
 #### 23.5.7.8 The NotNull attribute
 
 Specifies that a nullable value will never be `null` if the method returns (rather than throwing).

standard/standard-library.md

@@ -735,6 +735,22 @@ namespace System.Diagnostics.CodeAnalysis
         public MaybeNullWhenAttribute(bool returnValue);
     }
 
+    [System.AttributeUsage(System.AttributeTargets.Method | 
+    System.AttributeTargets.Property, AllowMultiple=true, Inherited=false)]
+    public sealed class MemberNotNullAttribute : Attribute
+    {
+        public MemberNotNullAttribute(string member) {}
+        public MemberNotNullAttribute(params string[] members) {}
+    }
+
+    [System.AttributeUsage(System.AttributeTargets.Method | 
+      System.AttributeTargets.Property, AllowMultiple=true, Inherited=false)]
+    public sealed class MemberNotNullWhenAttribute : Attribute
+    {
+        public MemberNotNullWhenAttribute(bool returnValue, string member) {}
+        public MemberNotNullWhenAttribute(bool returnValue, params string[] members) {}
+    }
+
     [System.AttributeUsage(System.AttributeTargets.Field |
       System.AttributeTargets.Parameter | System.AttributeTargets.Property | 
       System.AttributeTargets.ReturnValue, Inherited=false)]
@@ -1356,6 +1372,8 @@ The following library types are referenced in this specification. The full names
 - `global::System.Diagnostics.CodeAnalysis.DoesNotReturnIfAttribute`
 - `global::System.Diagnostics.CodeAnalysis.MaybeNullAttribute`
 - `global::System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute`
+- `global::System.Diagnostics.CodeAnalysis.MemberNotNullAttribute`
+- `global::System.Diagnostics.CodeAnalysis.MemberNotNullWhenAttribute`
 - `global::System.Diagnostics.CodeAnalysis.NotNullAttribute`
 - `global::System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute`
 - `global::System.Diagnostics.CodeAnalysis.NotNullWhenAttribute`