Spec: https://github.com/dotnet/csharplang/blob/main/proposals/low-level-struct-improvements.md
Tracking issue in runtime repo: dotnet/runtime#63768
Championed issue: dotnet/csharplang#1147

Guideline for addressing remaining issues:

• Base local testing off of 17.4 branch (verify behavior shipping in .NET 7)
• Base PRs on main (verify behavior in VS 17.5).
• Issues which we plan to address in 17.5, open new standalone issues and check-off in here
• Scenarios which we do not plan to test, either delete the line or add ❌ at start of line at your discretion.

Compiler

• spec: add grammar
• spec: scoped can only be applied to a ref type or a type that is a ref struct
• import of ref field when runtime feature flag is missing: RefFieldTests.RuntimeFeature
• ref assemblies and consumption by old compilers
  • ref fields are emitted as ref for reference assemblies (no change). Note that our ability to control what old compilers actually do with these fields is somewhat limited. We prefer if it just gives a use site error when the ref field is used, but may not be able to make changes if the behavior does not end up being ideal. (RefFieldTests.RefAssembly)
  • ref assemblies which contain ref fields not consumable by old compilers (even if ref field is private) (not a supported scenario)
  • Test metadata compatibility with previous compiler release (C# and VB) (not a supported scenario)
• check internal enumerations of local variable kinds (ensure that all LocalDeclarationKind values which can by by-ref or ref struct are given correct val escape and ref escape, with or without scoped keyword) Support scoped modifier for other local declarations #62039
• spec: OHI
• manual test of C++/CLI (can reference local assemblies with ref fields) (known issue, tracked by C++/CLI team)
• spec
• add to compiler test plan
• -langversion:11
  • for ref field declarations in source (also requires RuntimeFeature.ByRefFields)
  • for use-site of ref fields from metadata or compilation reference (RefFieldTests.LanguageVersionDiagnostics)
• updated escape rules enabled by
  • -langversion:11 or
  • System.Runtime.CompilerServices.RuntimeFeature.ByRefFields Enable updated escape rules if System.Runtime.CompilerServices.RuntimeFeature.ByRefFields exists #62131
• ref and ref readonly fields for ref struct only
  • declaration error for ref field in class, struct, record in source No error for ref fields in non-ref structs #62596
  • declaration error for ref parameter in record in source
  • use-site error for ref field in class or struct from metadata for C# and VB Report use-site error using ref field from type other than ref struct in C# #64682
• field must be declared readonly ref in readonly ref struct
• escape analysis within readonly methods (RefFieldTests.MethodArgumentsMustMatch_13)
• readonly ref emitted as initonly
• ref readonly emitted with System.Runtime.CompilerServices.IsReadOnlyAttribute
• ref struct with a ref field is not considered unmanaged stackalloc should be disallowed for structs with ref fields #63104
• field initializers:
  • ref initializer Syntax error parsing field initializer with ref expression #64720
  • error reported for value initializer Ref field initializer is treated as a value assignment #64725
  • ref struct field initializer safe-to-escape: see #60577 comment
• ref field modifiers
  • static ref fields disallowed
  • const ref fields disallowed
  • volatile ref fields disallowed
  • fixed ref fields disallowed
• ref auto-properties disallowed

  ref T P0 { get; }       // error
  ref T P1 { get; set; }  // error
  ref T P2 { get; init; } // error

• ref in object initializers supported: new S { F = ref t } Support ref field assignment in object initializers #62120
• ref field of the containing type: Allow ref field of the containing type #62098

  ref struct R<T>
  {
      public ref R<T> Next;
  }

• _ = s.F; should throw NullReferenceException if ref field is null - see Record read of ref local even if value is discarded #60910 (comment)
• scoped as type name
  • supported with -langversion:10 and earlier for compat
  • error with -langversion:11
• scoped modifier roundtripped to metadata
• scoped considered in overrides and interface implementation
• conversion errors for mismatch of scoped
  • for delegate conversions
  • for function pointer conversions
  • allow scoped to be added to a ref or in parameter, or to a ref struct parameter Allow adding scoped or removing [UnscopedRef] for override, interface implementation, or delegate conversion #62340
  • allow [UnscopedRef] to be removed from an out parameter or from a ref parameter of a ref struct Allow adding scoped or removing [UnscopedRef] for override, interface implementation, or delegate conversion #62340
• scoped included in inferred delegate types: var f = (R x, scoped R y) => x;
• scoped parameters
  • ref, in, out
  • scoped this in extension method
  • params scoped
• scoped locals
  • ref, ref readonly
  • const
  • foreach iteration variable
  • using variable
  • deconstruction declaration
  • out declaration
  • var and ref var
• local with/without scoped modifier with scoped/unscoped initializer
• ScopedRefAttribute changes scoped default when targeting:
  • ref to ref struct parameters
  • out parameters
• ScopedRefAttribute disallowed in source Report error if ScopedRefAttribute is used directly in source #62124
• UnscopedRefAttribute:
  • member annotated with [UnscopedRef] cannot implement an interface member Report error implementing unannotated interface member with [UnscopedRef] member #64508
• updated escape rules
  • ref-safe-to-escape for ref fields
  • ref reassignment
  • ref-safe-to-escape and safe-to-escape for scoped, scoped ref, ref scoped parameters and locals
  • struct this is implicitly scoped ref T
  • out parameters are implicitly scoped out
  • scoped parameters affect argument contribution for ref-safe-to-escape and safe-to-escape of method invocation
  • scoped parameters affect "method arguments must match" rule (arg "mixing")
• warn when a parameter could be marked scoped Report warning for unannotated parameters that could be marked scoped #64344
• SymbolDisplay
  • ref qualifier and ref custom modifiers
  • scoped and scoped ref for parameters and locals
  • out parameters (should we show the implicit scoped?)
• document breaking changes:
  • cannot return out parameter by reference
  • rvalue from method invocation that returns a ref struct is safe-to-escape from ... the ref-safe-to-escape of all ref arguments
• Test when scoped ref or scoped in cannot be passed as an argument to an ordinary ref/in parameter.
  • when the target method also returns ref struct (RefFieldTests.ReturnRefFieldByValue)
  • when the target method also returns ref/ref readonly (RefFieldTests.ReturnRefFieldByRef_03)
  • when the target method also has a ref parameter whose type is ref struct (RefFieldTests.MethodArgumentsMustMatch_03)
• auto-default: constructor must implicitly ref-assign a null-ref to ref fields which are not explicitly ref-assigned before all usages and returns. Auto-default ref fields with a null ref-assignment #63065
• cannot make an anonymous type with ref members
• disallow void F([UnscopedRef] scoped ref int i) { } (RefFieldTests.UnscopedScoped)
• allow updated escape rules with either -langversion:11 or RuntimeFeature.ByRefFields (i.e. NET 7) (RefFieldTests.UseUpdatedEscapeRules)
• required ref fields (RefFieldTests.RequiredField_01, _02)
• unskip test RefFieldTests.InitRefField_UnsafeNullRef
• unskip tests from RefFieldTests.vb

Public API

• ScopedTypeSyntax
• IFieldSymbol.RefKind and RefCustomModifiers
• IParameterSymbol.ScopedKind
• ILocalSymbol.ScopedKind
• SymbolDisplay*Options Update SymbolDisplay to include scoped for parameters and locals #63208
• ScopedRefAttribute

Productivity

• classification for scoped
• formatting for scoped
• metadata as source for scoped attributes (extracted to MetadataAsSource should show scoped modifiers #64906)
• QuickInfo for out parameters (should we show the implicit scoped?) (extracted to MetadataAsSource should show scoped modifiers #64906)
• EE:
  • Expression using ref field
  • Expansion of instance with ref field Test ref fields in the Expression Evaluator #62156

Open questions

• diagnostic for scoped modifier when ref or ref struct value cannot escape?

  static void F1<T>(scoped ref T t) { ... }   // warning: scoped is unnecessary
  static void F2<T>(scoped Span<T> s) { ... } // warning: scoped is unnecessary

  Answer: don't give such a diagnostic, it will be inconvenient and disruptive to those who use scoped ref on refs which do not escape as a matter of style.
• diagnostic for scoped out since out parameters are implicitly scoped? Answer: No, unless we hear further interest in such behavior.
• should we emit the [ScopedRef] attribute in metadata for an out parameter? The spec says it's implicitly scoped out. Answer: Don't emit ScopedRef when parameters are implicitly scoped.