[neuecc]

Reported from the MessagePack for C# v4 work against 1.3.1: on net9+, passing stack memory to IReadBuffer.CopyTo or ArrayPoolListWriteBuffer.WriteTo fails to compile, so MessagePack had to work around it in two places.

Problem

Both methods only copy into destination. Without scoped, though, the compiler must assume that a ref struct receiver could store the span in itself. So passing a stackalloc'd span fails with CS8350 whenever the buffer can outlive the call. This was reproduced for:

• any buffer called through the generic ref TReadBuffer (allows ref struct) path, which checks against the interface member's signature
• a concrete ReadOnlySpanReadBuffer / ReadOnlySequenceReadBuffer local built over heap memory
• ArrayPoolListWriteBuffer.WriteTo on a local with heap scratch, or on a ref parameter

Change

The following now take scoped Span<byte> destination:

• IReadBuffer.CopyTo
• all four implementations, plus their private CopyToSlow helpers (a scoped span cannot be passed on to an unscoped parameter of the same ref struct)
• ArrayPoolListWriteBuffer.WriteTo and CompatibleArrayPoolListWriteBuffer.WriteTo

The doc comments and the README (interface listing and the custom-implementation contract table) note the new rule.

Audit of the rest of the public surface

Parameter	Decision
ReadOnlySpanReadBuffer(ReadOnlySpan), SpanWriteBuffer(Span), ReadOnlySequenceReadBuffer(ReadOnlySpan), the scratch parameters, BufferSegments ctor	Stored in a field, so they must stay unscoped
ReadOnlySequenceReadBuffer(scoped in ReadOnlySequence<byte>, ...)	Already scoped (#7)
TryGetSpan / TryGetNext out parameters	out is implicitly scoped
CompatibleReadOnlySequenceReadBuffer(in ReadOnlySequence<byte>)	Plain struct, cannot capture a reference; scoped would change nothing
GetReference extension	No span arguments, no issue

Compatibility (please review)

• Source: a ref struct that implements IReadBuffer without scoped on CopyTo now fails with CS8987. The fix is to add scoped. Plain struct implementations are not affected (verified). Since custom implementations are a documented extension point, this probably warrants a minor version bump rather than 1.3.2.
• Binary: unchanged. scoped is encoded as ScopedRefAttribute on the parameter, and the IL signature is the same.

Tests

• New ScopedDestinationTest passes stackalloc'd spans to CopyTo on all four read buffers (generic and concrete, including the segment-straddling CopyToSlow path), and to WriteTo on a local and a ref parameter (scratch plus pooled segments).
• With scoped removed, the file fails to compile with CS8350 at all five call shapes (verified locally).
• 158/158 pass locally.

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