Test Plan: ref readonly

[jaredpar]

Proposal: dotnet/csharplang#6010
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/ref-readonly-parameters.md

Compiler

• Spec
  • Call out indexers
  • Operators (disallow, made sense for in but not for ref readonly)
  • disallow ref readonly in dynamic invocation
• Parse ref readonly
  • permutations and combinations (RefReadonlyTests parsing tests)
  • allowed on parameters
  • in cref
• Bind, emit and roundtrip parameter declarations
  • Methods, lambdas, local functions, delegates, constructors, primary constructor, function pointers, indexers
  • operators (post-merge)
  • LangVer check on parameter modifiers
  • modreq, attributes
  • Emit RequiresLocation
  • Disallow ref readonly in combination with various attributes
    • [IsReadOnly] (generally disallowed in source, see ManuallyAppliedAttributes_IsReadOnly)
    • [In], [Out] (see ManuallyAppliedAttributes_InOut)
    • [RequiresLocation] (disallow in source, see ManuallyAppliedAttribute; hide on symbols)
  • Disallow in expression trees (post-merge)
  • decoding from metadata
    • misplaced [RequiresLocation] (see ReturnParameter)
    • priority order decoding
  • Default parameter values (warn)
  • extension this parameter (see RefReadonlyWithThis_*)
  • dynamic binding (blocked)
• Argument matching
  • LangVer-conditional for ref argument on in parameter
  • method group conversion
  • function type inference (see MethodGroupComparer_*)
  • OHI (warn on overriding/hiding/implementing with different ref-ness)
  • partial methods (must match strictly, see PartialMembers_*)
  • interceptors
• Bind and emit
  • invocations, indexer access (see Invocation_*, RefReadonlyParameter_Indexer_*)
  • operator
  • LangVer check (behave like old compiler, ie. treat as ref)
  • Check argument value kind during argument coercion (ok for lvalue but warn for rvalue)
  • Warn for no modifier (except on this parameter for extension methods)
• Enforce readonliness
  • Mutable struct with a readonly and a non-readonly member - call the readonly member (no silent copy) and vice versa
• Public API:
  • new RefKind
  • IOperation
  • symbol display
  • check GetSymbolInfo and RefKind on symbol
  • SyntaxNormalizer
  • cref
• Synthesized usages of ref parameters? Maybe in interpolated strings?
• (readonly) refs, ref structs, readonly structs, readonly members
• scoped, UnscopedRef

General

• add to compiler test plan
• compile runtime
• compile VS
• Add RequiresLocationAttribute to BCL (tracked by [API Proposal]: RequiresLocationAttribute (supporting ref readonly parameters) runtime#85910)
• VB should be able to consume by-ref, except for virtual (since those involve modreq). See what we did for in. Override in VB should be disallowed
• Let F# know
• Notify managed C++ team for new modopt, but should be fine (they should tolerate/ignore) (Jared)
• go over usages of RefKind.In and check if RefKind.RefReadonlyParameter needs to be handled there as well
• document any compat breaks
• update feature status

IDE

• colorization and formatting
• QuickInfo
• scoped, readonly completion
• F1/Help
• interface implementation and override generation
• cref
• EE/EnC (manual smoketest)