Add additional diagnostics

A couple warnings have been added regarding field backed properties.

Author: BillWagner

docs/csharp/language-reference/compiler-messages/partial-declarations.md

@@ -47,6 +47,8 @@ f1_keywords:
   - "CS9257"
   - "CS9258"
   - "CS9263"
+  - "CS9264"
+  - "CS9266"
 helpviewer_keywords:
   - "CS0260"
   - "CS0261"
@@ -93,7 +95,8 @@ helpviewer_keywords:
   - "CS9257"
   - "CS9258"
   - "CS9263"
-ms.date: 11/01/2024
+  - "CS9266"
+ms.date: 11/06/2024
 ---
 # Errors and warnings related to `partial` type and `partial` member declarations
 
@@ -148,6 +151,11 @@ That's by design. The text closely matches the text of the compiler error / warn
 - [**CS9258**](#field-backed-properties): *In this language version, the '`field`' keyword binds to a synthesized backing field for the property. To avoid generating a synthesized backing field, and to refer to the existing member, use '`this.field`' or '`@field`' instead.*
 - [**CS9263**](#field-backed-properties): *A partial property cannot have an initializer on both the definition and implementation.*
 
+The following warnings can be generated for field backed properties:
+
+- [**CS9264**](#field-backed-properties): *Non-nullable property must contain a non-null value when exiting constructor. Consider adding the 'required' modifier, or declaring the property as nullable, or adding '`[field: MaybeNull, AllowNull]`' attributes.**
+- [**CS9266**](#field-backed-properties): *One accessor of property  should use '`field`' because the other accessor is using it.*
+
 The following sections explain the cause and fixes for these errors and warnings.
 
 ## Partial types
@@ -245,9 +253,11 @@ public partial int ImplementingDeclaration { get => field; set; }
 
 - **CS9258**: *In this language version, the '`field`' keyword binds to a synthesized backing field for the property. To avoid generating a synthesized backing field, and to refer to the existing member, use '`this.field`' or '`@field`' instead.*
 - **CS9263**: *A partial property cannot have an initializer on both the definition and implementation.*
+- **CS9264**: *Non-nullable property must contain a non-null value when exiting constructor. Consider adding the 'required' modifier, or declaring the property as nullable, or adding '`[field: MaybeNull, AllowNull]`' attributes.**
+- **CS9266**: *One accessor of property  should use '`field`' because the other accessor is using it.*
 
 [!INCLUDE[field-preview](../../includes/field-preview.md)]
 
 Beginning with C# 13, the preview feature, `field` backed properties allows you to access the compiler synthesized backing field for a property. **CS9258** indicates that you have a variable named `field`, which can be hidden by the contextual keyword `field`.
 
-**CS9263** indicates that your declaring declaration includes an implementation. That implementation might be accessing the compiler synthesized backing field for that property.
+**CS9263** indicates that your declaring declaration includes an implementation. That implementation might be accessing the compiler synthesized backing field for that property. **CS9264** indicates that the your use of `field` assumes a non-nullable backing field while the property declaration is nullable. The compiler assumes both the backing field and the property have the same nullability. You need to add the `[field:MaybeNull, AllowNull]` attribute to the property declaration to indicate that the `field` value should be considered nullable. **CS9266** indicates that one of a properties accessors uses the `field` keyword, but the other uses a hand-declared backing field. The warning indicates you may have done that by accident.

docs/csharp/language-reference/toc.yml

@@ -482,7 +482,8 @@ items:
       partial type, partial member, partial method, partial property, partial indexer,
       CS0260, CS0261, CS0262, CS0263, CS0264, CS0265, CS0267, CS0282, CS0501, CS0750, CS0751, CS0754, CS0755, CS0756, CS0757,
       CS0759, CS0761, CS0762, CS0763, CS0764, CS1067, CS8142, CS8663, CS8795, CS8796, CS8797, CS8798, CS8799, CS8800, CS8817,
-      CS8818, CS8863, CS8988, CS9248, CS9249, CS9250, CS9251, CS9252, CS9253, CS9254, CS9255, CS9256, CS9257, CS9258, CS9263
+      CS8818, CS8863, CS8988, CS9248, CS9249, CS9250, CS9251, CS9252, CS9253, CS9254, CS9255, CS9256, CS9257, CS9258, CS9263,
+      CS9264, CS9266
   - name: Params modifier
     href: ./compiler-messages/params-arrays.md
     displayName: >