Championed proposal: dotnet/csharplang#8635
Spec: proposals/field-keyword.md

public string PropertyConstraint
{
    get;
    set => field = value ?? throw new ArgumentNullException();
} = "";

Compiler:

• language version
• field keyword
  • based on language version
  • as primary_expression only
  • escaped with @
  • this.field, base.field, Qualifier.field
  • in simple property: object P => expr;
  • in property accessors: expression body; block body
  • in indexer accessors
  • in event accessors
  • in nested lambdas and local functions
  • in property initializer
  • in attributes on property, accessor, nested function
  • in nested function signature as default value
  • in nameof()
  • in call to [Conditional]
• warning when previously binding to other symbol
  • based on language version
  • member of containing type, base type
  • local, in same or nested function
  • parameter in nested function
  • type parameter, in containing type or nested function
• mixed auto- and manually-implemented accessors (see FieldKeywordTests.ImplicitAccessorBody_*)
  • get, set, init
  • in interface
• field use
  • static and instance properties on class, struct, ref struct, interface, record*
  • get-only, set-only, init-only (see FieldKeywordTests.AutoPropertyMustHaveGetAccessor_*)
  • field references in multiple properties: each get private backing field
  • auto-implemented properties (including field) must override all accessors (see FieldKeywordTests.Override_*, New_*)
  • in nested function, including static field reference in static function (see FieldKeywordTests.Lambda_*, LocalFunction_*)
  • in attribute arguments (see FieldKeywordTests.Attribute_*)
  • ref returning property (see FieldKeywordTests.RefReturning_*)
  • properties with restricted types (see FieldKeywordTests.RestrictedTypes)
  • properties with by-ref-like types (see FieldKeywordTests.ByRefLikeType_*)
  • @field with and without backing field
  • Color Color
  • passing by reference: (see FieldKeywordTests.PassByReference_*)

    struct S
    {
        readonly object P { set { M1(ref field); M2(in field); } }
        static void M1(ref object o) { }
        static void M2(in object o) { }
    }

• synthesized backing field
  • backing field emitted when any auto-implemented accessor or field reference
  • backing field marked [CompilerGenerated] (see FieldKeywordTests.Attribute_03)
  • emitted when used in [Conditional] code only (see FieldKeywordTests.Conditional) (see open LDM question)

        object P { set { Debug.Assert(field is null); } }

  • emitted to ref assembly
• [field:] attributes
  • emitted on synthesized backing field (see FieldKeywordTests.Attribute_03)
  • not emitted on property (see FieldKeywordTests.Attribute_03)
  • error if no auto-implemented accessors and no field references (see FieldKeywordTests.PartialProperty_Attribute_03)
  • [field: Obsolete] (see FieldKeywordTests.ObsoleteAttribute)
• nameof(field)
  • nameof(field) as invocation
• property initializer (see FieldKeywordTests.Initializer_*)
  • allowed when any auto-implemented accessor or field reference
  • static and instance properties on class, struct, ref struct, interface
  • written to field directly
• constructor assignment (see FieldKeywordTests.ConstructorAssignment_*)
  • assigns to setter if available, otherwise getter
  • static and instance properties on class, struct, ref struct, interface
  • assigned in other initializer
  • assigned in nested function
• flow analysis
  • warning CS0414: The field is assigned but its value is never used
  • warning CS0649: Field is never assigned to, and will always have its default value
  • auto-default initialization of struct backing fields and warnings when property is accessed in constructor (see FieldKeywordTests.DefaultInitialization_*)
    • expect no auto-default inserted when a field initializer is used
    • expect an auto-default when assigning in constructor
• readonly properties (see FieldKeywordTests.ReadOnly_*)
  • backing field is initonly when type, property, or accessor is readonly
  • error reporting when modifying field in get, set accessors
• set accessor in readonly context for property using field (see open LDM question) (see FieldKeywordTests.ReadOnly_01)
• partial properties (see FieldKeywordTests.PartialProperty_*)
  • mix of auto- and manually-implemented accessors
  • static and instance properties on class, struct, ref struct, interface, record*
  • field in accessors
  • initializer on definition or implementation
  • initializer on definition and implementation with same or different values
  • initializer with no field or auto-implementation
  • field references on definition or implementation
  • field references on definition and implementation
  • field references on readonly property or accessor with same or different modifiers on parts
  • [field:] attributes concatenated across parts
    • error when the attributes are the same and AllowMultiple=false
  • partial properties where the implementation uses field and one or both parts have field: targeted attribute lists. (See also PartialPropertiesTests.Attributes_Property_BackingField. This issue referenced in source.)
  • Check warning WRN_SequentialOnPartialClass is produced when needed (see also comment)
  • comment from partial properties
  • comment from partial properties
• nullability (initial implementation) (see FieldKeywordTests.Nullability_*) Implement nullable analysis for field keyword #75244
  • field has annotations of containing type
  • [field:MaybeNull] etc. affect field
  • NullableWalker.Scan().getFieldSymbolToBeInitialized(): Is checking IsAutoProperty sufficient or should we check the appropriate accessors are missing or auto-implemented (see comment)?
    • Moving to standalone issue NullableWalker.Scan().getFieldSymbolToBeInitialized(): Is checking IsAutoProperty sufficient or should we check the appropriate accessors are missing or auto-implemented #75245
  • NullableWalker.TryGetReceiverAndMember(): call Binder.AccessingAutoPropertyFromConstructor to match DefiniteAssignment implementation (see comment)?
    • (wontfix) It feels like the churn of reversing this, just to be consistent with definite assignment, is not worth it right now.
• SemanticModel
  • GetSymbolInfo(fieldExpr) returns backing field
  • add field reference in the speculative semantic model Field-backed properties: support field references in speculative SemanticModel #75398
  • IOperation for expression referencing field
• struct fulfills unmanaged constraint and managed type rules (see pointer types) depending on synthesized fields
• ref safety:

  struct S
  {
      [UnscopedRef]
      Span<int> Prop1 => M(ref field); // should not error
  
      // Should error
      Span<int> Prop2 => M(ref field); // should error
    
      static Span<int> M(ref Span<int> span) => default;
  }

• public API review: [API proposal] Syntax changes for field keyword #74937
• update compiler test plan

Productivity:

• Fixer for compat break
• QuickInfo
• Completion
• Colorization
• Conversion?
• F1 help on field
• EnC: EnC: implement support for field keyword in property accessors #77951