Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/unsafe-evolution.md
Proposal issue: dotnet/csharplang#9704
Feature branch: https://github.com/dotnet/roslyn/tree/features/UnsafeEvolution

• MemorySafetyRules attribute
  • opt-in (via compilation options)
  • MemorySafetyRulesAttribute synthesized if missing (see Extern_*)
  • Disallow MemorySafetyRulesAttribute on source modules (see RulesAttribute_ReferencedInSource)
  • LangVer (need test like RulesAttribute_Synthesized but with LangVer)
    • LangVer and extern (see Extern_Method)
  • loaded from metadata
• Operations:
  • pointer indirection (see Pointer_Dereference_*)
  • pointer member access (see Pointer_MemberAccess_*)
  • pointer element access (see Pointer_ElementAccess_*)
  • function pointer invocation (see Member_FunctionPointer) and conversion (see Member_Method_ConvertToFunctionPointer)
  • access on fixed-sized buffer (see FixedSizeBuffer_*)
  • stackalloc conversion (see StackAlloc_*)
  • property and method usage:
    • extensions
    • operator, extension operator (see Member_Operator_*)
    • attribute (see Member_Property_Attribute)
    • function types (missing?)
    • object initializers, with expressions
    • nameof (see Member_Method_NameOf)
    • ...
  • indirect usages:
    • variations of Current in foreach (see Member_Current_*),
    • variations of Dispose in using (see Member_Dispose_*),
    • Deconstruct in deconstructino (see Member_Deconstruct),
    • various member in lock (see Member_LockObject),
    • interpolation handlers (see Member_InterpolatedStringHandler)
    • interceptors (see Member_Interceptor, we only check intercepted)
    • patterns
    • ...
  • constructor usage:
    • object creation
    • this/base (see Member_Constructor_This)
    • attribute (see Member_Constructor_Attribute)
• [RequiresUnsafe]
  • round-trip "caller unsafe" flag using RequiresUnsafeAttribute
  • disallowed on lambda (see Member_Lambda), static constructors and finalizers (see RequiresUnsafeAttribute_ReferencedInSource)
  • warn in legacy mode (see RequiresUnsafeAttribute_ReferencedInSource)
  • partial (missing tests?)
  • on extern (see Extern_Method_Explicit)
  • LangVer
• OHI (see *_Override, *_Implementation, *_Hiding, Extern_Method_Override)
• unsafe
  • alias directives (missing warning, see *_UsingAlias_*)
  • using static directives (missing tests?)
• mark public APIS a [Experimental]
• update compiler test plan
• Ping F# and C++/CLI team as FYI
• Add attributes to BCL
  • MemorySafetyRulesAttribute
  • RequiresUnsafeAttribute
• how should the compiler deal with well-known members that are unexpectedly marked as caller-unsafe? (some are ok, such as UnsafeHelpers for inline arrays, but what about others?)
• PROTOTYPE comments
• spec open issues
• breaking changes?
• LangVer tests should include C#Next
• Should reflection APIs be requires-unsafe? Or should there be a runtime check (must use special flag or API to access unsafe APIs)?[/-]
• Should Activator.CreateInstance<T>() be requires-unsafe?[/-]
• Should dynamic be marked as requires-unsafe?