Championed proposal: dotnet/csharplang#7431
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/inline-arrays.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/InlineArrays

Compiler

• language version
• runtime support required *_RuntimeSupport
• update compiler test plan
• as local
• as parameter including ref, in, out
• as return type
• as field or property
  • in class, struct
  • as ref, ref readonly field
• in array
• in span
• as type argument
• inline array type
  • non-generic, generic
  • multiple attributes InlineArrayType_04_MultipleAttributes
  • Length <= 0 InlineArrayType_*_WrongLength
  • no fields InlineArrayType_09_NoFields
  • multiple fields (including from event, property, primary constructor) InlineArrayType_08_MoreThanOneField
  • static fields InlineArrayType_*StaticField*
  • ref field InlineArrayType_*_RefField
  • class, interface, delegate, enum
  • nested struct InlineArrayType_31_Nested, InlineArrayType_*_Generic
  • ref struct
  • retargeting InlineArrayType_18_Retargeting
  • [StructLayout(...)]
  • with explicit indexer
  • [Required] attribute on element field
• inline array element type
  • inline array
  • ref struct (error)
  • pointer InlineArrayType_30_UnsupportedElementType
• conversions Conversion_*
  • implicit conversion to Span<T> for writable variables only
  • implicit conversion to ReadOnlySpan<T> for variables only
  • no conversion for values Conversion_Value_*
  • standard conversion (participates in user-defined conversion) Conversion_Standard_*
• element access: inlineArray[index]
  • arg type:
    • implicitly convertible to int
    • dynamic ElementAccess_Dynamic_Variable_01
    • System.Index
  • index < 0, index >= Length with constants and non-constants, int and System.Index ElementAccess_Bounds_*
  • ref, in, out index (error) RefOutInIndex
  • element access of writable variable ElementAccess_Variable_*
    • int and System.Index
    • as value, ref, ref readonly
    • ref safety
  • element access of readonly variable (including readonly context) ElementAccess_*ReadOnlyContext_*
    • int and System.Index
    • as value, ref, ref readonly
    • ref safety
  • element access of value ElementAccess_Value_*
    • int and System.Index
    • as value, ref, ref readonly
    • ref safety
  • within async methods ElementAccess_Await_*
  • in object initializer ElementAccess_ObjectInitializer_*
  • explicit indexer ignored *_IndexerIsIgnored_*
• slice: inlineArray[range]
  • start < 0, end >= Length with constants and non-constants Slice_Bounds_*
  • slice of writable variable Slice_Variable_*
    • element access as value, ref, ref readonly
    • ref safety
  • slice of readonly variable (including readonly context) Slice_ReadOnlyContext_*
    • element access as value, ref, ref readonly
    • ref safety
  • slice of value (error)
  • within async methods Slice_Await_*
  • omit slice call if range is a constant and start is 0
  • explicit Slice() method ignored *_SliceMethodIsIgnored_*
• indexer: prefer int over Index over Range Access_ArgumentType_*
• as foreach collection Foreach_*
  • writable variables, readonly variables, values
  • foreach ... ref, foreach ... ref readonly
  • await foreach
  • in async method Foreach_InAsync_*
• conditional access *_ConditionalAccess_*
  • inlineArray?[i] where i is int, Index, Range
  • type?InlineArray[i] where type is class, struct, i is int, Index, Range
• list pattern is [..] (error) ListPattern
• collection literals Support inline arrays with collection literals #68927
  • as collection literal target type
  • as collection literal spread element
• in expression tree (error)
  • element access ElementAccess_ExpressionTree_01
  • conversion Conversion_ExpressionTree_01
• definite assignment DefiniteAssignment_*
  • unassigned vs. new()
  • 1 element vs. > 1 element
  • = default;
  • definite assignment errors for element access
  • definite assignment errors for slice
  • definite assignment errors for implicit conversions to spans
  • definite assignment errors within inline array type
• nullable analysis NullableAnalysis_*
• missing types and methods MissingHelper_*
  • Span<T>, ReadOnlySpan<T>
  • System.Index, System.Range
  • MemoryMarshal.*, Unsafe.*
• PrivateImplementationDetails methods created as needed PrivateImplementationDetails_*
  • InlineArrayAsSpan(), ...AsReadOnlySpan()
  • InlineArrayElementRef(), ...RefReadOnly()
• attribute cycles CycleThroughAttributes_*
• Test element access with an int variable as an index
• Test with types and members synthesizing fields

Public API

• API review: Public API changes for InlineArrays feature #68719

IDE

• IOperation
• CFG
• EE Support inline arrays in EE #68926
  • view elements in Watch window
  • evaluate element access, slice
• EnC: Inline Arrays #69245
• Compiler crashes when emitting EnC delta for assembly with synthesized inline array types  #69398