Tracking bug of smaller work items:

• turn this bug into a readme.md document for the feature.

• syntax model parsing

  • more tests for syntax/parsing if necessary
  • operators: should they be able to return by ref readonly? currently parser fails.

• binding right RefKind, some testing

  • in parameters
  • ref readonly parameters
  • ref readonly returns. (make sure RefKind is not lost to become "ref", properties must not have setters though")

• public API and symbol display,

  • default symbol display in error messages
  • symbol display, possibly switch for "ref readonly"/"in"
  • public API for returns finalized

• method body binding for parameters

  • in parameters can be read
  • diagnostics for readonly behavior

• Invoking methods with “in” parameters

  • Assume only identity conversions,
  • LValues only
  • Support for RValues
  • optimize temps
  • Invalid error message on passing in parameters by ref (Invalid error message on passing in parameters by ref #18477)

• More optimal codegen for indirect reading of readonly fields

  • suppress copying when accessing readonly fields in "readonly ref" context
  • suppress copying when accessing readonly fields to invoke readonly struct methods
  • suppress copying when fetching a field off a readonly field typed as ordinary struct
  • introduce a PEVerify-Compat flag for suppressing the above optimizations where possible.

• check the interop story

  • F#
  • VB
  • C# older versions
  • Write tests for native compilers (Write tests that assert modreq breaking support for native csc/vbc #19909)

• additional diagnostics for semantics of in parameters

  • combine with various stuff - paramarrays, out, pointers, this,
  • out/in attributes?
  • use in expressions - recursive use of in parameters, dynamic, vararg, (the table of expr contexts?)
  • enforce no capturing/spilling

• flow analysis

  • regular scenarios
  • region flow analysis

• OHI

  • updating signature comparers to understand new RefKind (enough?)
  • in and none are ambiguous at call sites.

• Operators?

• Ref return binding

  • validate safe to return rule

• Lambda Inference (similar to out ?)

• Metadata

  • finalize the design ( attributes or modifiers?)
  • encoding
  • decoding
  • tests for broken metadata
  • verifiable
  • Enable EnC and EE scenarios for generating metadata (Enable EnC and EE scenarios for generating metadata for IsReadOnlyAttribute #19274)
  • LDM Question: should compiler support importing ref-readonly abstract/virtual signatures from other assemblies that have the attribute, but not the modreq? (Should referencing [IsReadOnly] signatures without an [InAttribute] modreq be allowed? #19792)

• Passing readonly fields as in

  • ensure that it works
  • Object/ValueType methods that are not overridden can be called
  • more readonly variables - iteration/using/query variables

• Readonly structs

  • parsing
  • binding

• Ref extension methods

  • parsing
  • binding
  • unconstrained receiver types (ref readonly allowed?)
  • Elvis operator in combination with ref/in extensions w/ structs/generics. (async too)
  • Should Nullable types (structs) be allowed?

• Elvis operator on “readonly ref”

• IDE support

  • keyword recommender should not interfere with typing
  • Enable simple scenarios, block scenarios that do not work

• Debug EE. Is there any work?

• Mop up

  • go through all uses of RefKind and check if anything needs to be done for readonly
  • Expression trees - make sure we block what needs to be blocked
  • Symbols should construct their own modifiers appropriately before looking into overridden symbols (Symbols should construct their own modifiers appropriately before looking into overridden symbols #20053)
  • Importing C# symbols with a modreq but a missing IsReadOnly attribute should mark it as a use-site error (Importing C# symbols with a modreq but a missing IsReadOnly attribute should mark it as a use-site error #19951)
  • Update compiler test plan (stackalloc, in, etc.)