Championed proposal: dotnet/csharplang#8887
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/collection-expression-arguments.md

Compiler:

Errors

• LangVer behavior LanguageVersion_*
• Obsolete members
  • CollectionBuilder CollectionBuilder_ObsoleteBuilderMethod_*
  • constructor CollectionInitializer_ObsoleteConstructor_*
• Compiler feature required members
  • CollectionBuilder CollectionBuilder_UseSiteError_Method
  • constructor Constructor_UseSiteError_Method
• UnmanagedCallersOnly
  • CollectionBuilder CollectionBuilder_UnmanagedCallersOnly
• Type with required properties WithElement_RequiredProperties
• Non-accessible construction
  • CollectionBuilder CollectionBuilder_Private|Internal|ProtectedMethod
  • Constructor WithElement_Private|Protected|etcConstructor*
• File local CollectionBuilder type - CollectionBuilderFileLocalType1
• File local CollectionBuilder parameter type - CollectionBuilderFileLocalType2
• Dynamic
  • Parameters
    • CollectionBuilder - WithElement_CollectionBuilder_DynamicParameters
    • constructor DynamicArguments_04
  • Arguments
    • CollectionBuilder - WithElement_CollectionBuilder_DynamicArguments
    • constructor DynamicArguments_*
• Disallowed on array and span types CollectionExpressionTests_WithElement_ArraysAndSpans
• with in params: M(with(A, B), C, D) (disallowed)
• update BindCollectionExpressionForErrorRecovery. Tested in TestErrorRecovery_XXX in IOperationTests_ICollectionExpressionOperation.
• Expression trees ExpressionTreeInWithElement
• Type parameter constraints
  • CollectionBuilder CollectionBuilder_MultipleBuilderMethods_GenericConstraints*, CollectionBuilder_GenericConstraints
  • constructor WithElement_TypeConstraints*
• Ref safety
  • CollectionBuilder RefSafety_CollectionBuilderArguments
  • constuctor RefSafety_ConstructorArguments
• Errors for unsupported for IList and ICollection parameters IList_With_OptionalParamsAfterCapacity

Behavior

• order of evaluation
  • CollectionBuilder EvaluationOrder_CollectionInitializer*
  • constructor EvaluationOrder_CollectionInitializer
• Parameter modifiers
  • ref
    • CollectionBuilder CollectionBuilder_RefParameter
    • constructor WithElement_RefParameters
  • out
    • CollectionBuilder CollectionBuilder_OutParameter
    • constructor WithElement_OutParameters
  • in
    • CollectionBuilder CollectionBuilder_InParameter
    • constructor WithElement_InParameters
  • ref readonly
    • CollectionBuilder CollectionBuilder_RefReadonlyParameter
    • constructor - WithElement_RefReadonlyParameter
  • params
    • CollectionBuilder lots of tests
    • constructor WithElement_Params
  • scoped
    • CollectionBuilder RefSafety_CollectionBuilderArguments
    • constructor RefSafety_ConstructorArguments
  • UnscopedRef
    • CollectionBuilder - WithElement_CollectionBuilder_UnscopedRef
    • constructor WithElement_RefParameters, WithElement_UnscopedRef
• Default parameters
  • CollectionBuilder CollectionBuilder_OptionalParameter*
  • constructor CollectionInitializer_OptionalParameter*
• nullability analysis - All in CollectionExpressionTests_WithElement_Nullable.cs
  • CollectionBuilder
  • constructor
• __arglist
  • CollectionBuilder - WithElement_CollectionBuilder_ArgList
  • constructor Arglist*
• interpolation handlers
  • CollectionBuilder - InterpolatedStringHandler_CollectionBuilder
  • constructor InterpolatedStringHandler
• If you add a new expression that can be nested inside itself, test deeply nesting it. Potentially cut off direct nesting. (N/A. Can't nest this directly deeply).
• Overload resolution
  • CollectionBuilder CollectionBuilder_MultipleBuilderMethods_GenericConstraints*
  • constructor WithElement_OverloadResolution*, WithElement_NoMatchingConstructor, WithElement_Constructor_UserDefinedConversion*, WithElement_IncorrectConstructorType
• ORPA
  • CollectionBuilder CollectionBuilderOverloadResolutionPriority
  • constructor OverloadResolutionPriority
• Support for IList and ICollection capacity: parameter IList_With_* (covers both IList and ICollection)

Public APIs

• GetSymbolInfo
• GetSpeculativeSymbolInfo - GetSpeculativeSymbolInfo
• GetMemberGroup (currently returns nothing for with(...) and : this(...))
• IOperation
• Control flow graph
• API review - Expose 'With-Element' information in ICollectionExpressionOperation #80998
• update compiler test plan
• Document breaking change
• Alert partners of breaking change.

IDE

• completion - WithKeywordRecommenderTests.TestInCollectionExpressionX
• colorization - TotalClassifierTest.TestWithElement
• QuickInfo - SemanticQuickInfoSourceTests.TestWithElementXXX
• formatting - FormattingTests.FormatWithElementX
• GoToDef on with - CSharpGoToDefinitionTests.TestCSharpGoToWithElementXXX
• signature/parameter info while typing in with - WithElementSignatureHelpProviderTests
• FAR on constructor or Create method used by with - Not planned.