support required members

Author: RexJaeschke

standard/attributes.md

@@ -498,6 +498,7 @@ A number of attributes affect the language in some way. These attributes include
 - `System.Runtime.CompilerServices.EnumeratorCancellationAttribute` ([§23.5.8](attributes.md#2358-the-enumeratorcancellation-attribute)), which is used to specify parameter for the cancellation token in an asynchronous iterator.
 - `System.Runtime.CompilerServices.ModuleInitializer` ([§23.5.9](attributes.md#2359-the-moduleinitializer-attribute)), which is used to mark a method as a module initializer.
 - `System.Runtime.CompilerServices.InterpolatedStringHandlerAttribute` and `System.Runtime.CompilerServices.InterpolatedStringHandlerArgumentAttribute`, which are used to declare a custom interpolated string expression handler ([§23.5.9.1](attributes.md#23591-custom-interpolated-string-expression-handlers)) and to call one of its constructors, respectively.
+- `System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute` (§SetsRequiredMembers) and `System.Runtime.CompilerServices.RequiredMemberAttribute` (§RequiredMember), which are used in required-member contexts ([§15.7.1](classes.md#1571-general)).
 
 The Nullable static analysis attributes ([§23.5.7](attributes.md#2357-code-analysis-attributes)) can improve the correctness of warnings generated for nullabilities and null states ([§8.9.5](types.md#895-nullabilities-and-null-states)).
 
@@ -1428,6 +1429,65 @@ Attribute `InterpolatedStringHandlerArgument` is applied to the handler paramete
 
 If an `out bool` parameter is also declared to allow the handler to be inhibited ([§23.5.9.1.2](attributes.md#235912-inhibiting-a-custom-handler)) that parameter shall be the final one.
 
+### §Required-Member-Attributes Required member attributes
+
+#### §SetsRequiredMembers The SetsRequiredMembers attribute
+
+This attribute indicates that the constructor it decorates sets all required members for the current type, so callers do not need to set any required members themselves. However, the compiler doesn't verify that the constructor actually initializes all required members.
+
+> *Example*:
+>
+> <!-- Example: {template:"standalone-lib", name:"SetsRequiredMembers", expectedErrors:["CS9035","CS9035"]} -->
+> ```csharp
+> public class Person
+> {
+>     public Person() { }
+>
+>     [SetsRequiredMembers]
+>     public Person(string firstName, string lastName) =>
+>         (FirstName, LastName) = (firstName, lastName);
+>
+>     public required string FirstName { get; init; }
+>     public required string LastName { get; init; }
+>
+>     public int? Age { get; set; }
+> }
+>
+> public class Student : Person
+> {
+>     public Student() : base()
+>     {
+>     }
+>
+>     [SetsRequiredMembers]
+>     public Student(string firstName, string lastName) :
+>         base(firstName, lastName)
+>     {
+>     }
+>
+>     public double GPA { get; set; }
+> }
+>
+> public class Test
+> {
+>     public static void M()
+>     {
+>         var p1 = new Student(); // error: doesn't set required members
+>         var p2 = new Student("Jane", "Williams"); // OK
+>     }
+> }
+> ```
+>
+> *end example*
+<!-- markdownlint-disable MD028 -->
+
+<!-- markdownlint-enable MD028 -->
+> *Note*: As the derived-type constructor `Student(string, string)` chains to the base-type constructor `Person(string, string)`, which has this attribute, the derived-type constructor must also have that attribute ([§15.11.1](classes.md#15111-general)). *end note*
+
+#### §RequiredMember The RequiredMember attribute
+
+This attribute indicates that the current type has one or more required members ([§15.7.1](classes.md#1571-general)), or that a specific member of that type is required. However, it is an error for this attribute to be used explicitly. Instead, the presence of the modifier `required` results in the type or member being treated as if it were decorated with this attribute. 
+
 ## 23.6 Attributes for interoperation
 
 For interoperation with other languages, an indexer may be implemented using indexed properties. If no `IndexerName` attribute is present for an indexer, then the name `Item` is used by default. The `IndexerName` attribute enables a developer to override this default and specify a different name.