Proposal issue: dotnet/csharplang#7905
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-14.0/first-class-span-types.md
Label: https://github.com/dotnet/roslyn/labels/New%20Feature%20-%20First-class%20Span%20Types
Feature branch: https://github.com/dotnet/roslyn/tree/features/FirstClassSpan

• conversions
  • based on language version
  • conversions:
    • from T[] to Span<U>
    • from T[] to ReadOnlySpan<U>
    • from Span<T> to ReadOnlySpan<U>
    • from ReadOnlySpan<T> to ReadOnlySpan<U>
    • from string to ReadOnlySpan<U>
  • implicit conversions between the above where the conversion from T to U is:
    • no conversion
    • identity conversion
    • reference conversion
    • non-reference conversion
    • user-defined conversion
  • explicit conversions between the above where the conversion from T to U is:
    • see implicit conversions
  • implicit and explicit conversions from the reverse of the above are disallowed (see FirstClassSpanTests.*_Opposite*)
  • span conversions are standard implicit conversions (see FirstClassSpanTests.*_ThroughUserImplicit)
  • implicit conversions for extension method receiver
    • not considered for extension receiver in method group
  • limited co-variance from ReadOnlySpan<T> to ReadOnlySpan<U>
  • conversions to params {ReadOnly}Span<T> (see FirstClassSpanTests.*_Params*)
  • no span conversion with ref, ref readonly, in modifiers
  • no span conversion from multi-dimensional arrays
  • method group parameter and return type conversions (disallowed) (see FirstClassSpanTests.*_MethodGroup*)
• pattern matching ignores span conversions
• nullable analysis of element types
• ref safety of implicit and explicit conversions (see FirstClassSpanTests.*_RefSafety)
• null coalesce expressions (see FirstClassSpanTests.*_Coalesce)
• conversions in expression trees (see FirstClassSpanTests.*_ExpressionTree*)
• Span<T> and ReadOnlySpan<T> recognized by fully-qualified name
  • must be ref struct
  • multiple definitions
  • retargeted span types
• code generation:
  • uses well-known runtime helpers
  • missing well-known runtime helpers
• overload resolution: based on language version
• overload resolution: better conversion from expression (see FirstClassSpanTests.OverloadResolution_*)
  • implicit span conversion and identity conversion
  • implicit span conversion for both:
    • Span<T> and ReadOnlySpan<U>
    • ReadOnlySpan<T> and ReadOnlySpan<U>
  • implicit span conversion and array variance: {ReadOnly}Span<T> and U[]
  • implicit span conversion and implicit reference conversion: {ReadOnly}Span<T> and IEnumerable<U>
  • implicit span conversion and user-defined conversion
  • extension method receiver (see FirstClassSpanTests.OverloadResolution_*_ExtensionMethodReceiver*)
  • params (see FirstClassSpanTests.OverloadResolution_*_Params*)
• type inference: (see FirstClassSpanTests.TypeInference_*)
  • lower-bound inference from T[] arg to {ReadOnly}Span<U>
  • lower-bound inference from Span<T> arg to ReadOnlySpan<U>
  • exact inference from T[] arg to {ReadOnly}Span<U>
  • exact inference from Span<T> arg to ReadOnlySpan<U>
• semantic model:
  • implicit conversion
  • explicit conversion
  • Conversion.IsSpan { get; }
• public API review: API: Conversion.IsSpan #74738
• document breaking changes
• IDE:
  • simplify cast
• build dotnet/runtime
• build Visual Studio
• update test plan