* Symbol display for readonly members

* Add coverage

* Test malformed properties and events

* Show readonly keyword on non-field members in QuickInfo

* Use IsDeclaredReadOnly for symbol display (WIP)

* Update based on design and fix some test failures

* Add readonly keywords to various IL tests

* Fix more tests, respond to feedback

* Fix compile error

* Don't display readonly keyword for implicit readonly getters from source

* Don't show readonly keyword for members contained within a readonly struct

* Move from extension methods to private statics

* Implement MetadataAsSource for ref and readonly structs

* MetadataAsSource for readonly members

* Fix NotImplementedException errors. Fix some ArrayBuilder leaks.

* Fix a leak. Fix implicit readonly attribute test.

* List passed to GetUpdatedDeclarationAccessibilityModifiers needs to support Remove

* Allow specifying a metadata language version for MetadataAsSourceTests

* Remove unused SyntaxTokenList members. Comment readonly event generation.

* Check VB MetadataAsSource for readonly members

* Fixes from feedback

* Add test for not implicitly readonly struct getter

* Label arguments in call to GetCustomAttributesForToken

* Reference issue about readonly event public API

* Only emit readonly attributes implicitly when readonly members feature is enabled

* Comment about reasoning and use Theory for test

* Check parser behavior for bad expression property declaration

* Make readonly modifiers more strict on accessors to match accessibility behavior

* Fix tests

* Improve implicit copy warning message. Add compound property assignment test.

* Test ErrorCode.ERR_ReadOnlyModMissingAccessor with an indexer

* Simplify error message for 'static readonly' members

* Use RefKind.In for the ThisParameter of a readonly method

* Remove prototype comments

* Add readonly lambda test

* Fix formatting

* Update compiler test plan

* Tweak wording

* Move OverrideBaseMethod test to CodeGen tests file. Add 'fixed' test.

* Test readonly partial methods

* Add readonly explicit interface implementation test

* Expand on ReadOnlyMembers_BaseMethod code gen tests

* Check symbol APIs for mismatched readonly partial method

* Update speclet

* Respond to feedback

* Refactor metadata round trip test

* Add feature doc for readonly members

* Add IsReadOnly attribute to method and property metadata

* Static methods are never effectively readonly. Add more metadata tests.

* Fix test failures

* 'readonly' properties shouldn't have IsReadOnlyAttribute. Other misc feedback items.

* Test readonly local function

* Test static readonly method from metadata

* Fixes from feedback

* readonly members warnings WIP

* Remove dead code

* Readonly members semantic checks. Warn on implicit copy of 'this'.

* Add to readonly ref readonly test

* Warn on implicit copies for get property access

* Don't warn on calls to base methods

* Add test for setter accesses in a readonly member

* Allow readonly setter accesses in readonly methods

* Test increment, compound assignment, and field-like events

* Fixes from feedback

* Add mutating readonly event accessors test

* Expand on readonly base members test

* Check for implicit copies in foreach/using/deconstruction scenarios

* Split base method tests into multiple tests for clarity

* Test remaining cases

* Fix whitespace in test

* Add ReadOnlyMethod_RefLocal

* Fixes from Fred's feedback

* Add MethodSymbol.IsReadOnly API

* Update emitter to copy 'this' for non-readonly method calls in readonly methods

* Add tests for static methods with in parameters

* Rely on HasHome instead of directly checking IsReadOnly and AddressKind

* Move IL-verifying tests to CodeGenReadonlyStructTests.cs

* Fixes from feedback

* A few more test cases and cleanup

This unifies our API naming convention. When standard is in the name
then we are using the netstandard2.0 API set and when it is not then it
is the simple desktop mscorlib46.

Both variants are necessary for CompileAndVerify style functions because
many of our tests that use it can only run on desktop or simply need to
have some of the netstandard2.0 API set missing.

Addressed PR comments.

minor tweaks to cascaded error reporting

* Stack alloc parsed to spans

* PR feedback

* Document the breaking change

- Made readonly fields homeless outside the constructor when flag is set.

Basically addressing a TODO in codegen.
To pass default by reference we need a temp and to make a default value we need a temp (often mapped to same slot)

We could just pass the reference to the second one. No need to copy between temps.
Besides, they are often mapped to the same IL slot resulting in clearly redundant load/store with the same local.

The scenario gets more common with "ref readonly" so here is the fix.