Returning a ReadOnlySpan<byte> from a LibraryImport fails

[ThadHouse]

Description

Returning a ReadOnlySpan from a call generated by LibraryImport fails to compile, even though it should. It correctly works with a byte[] as the return type instead of ReadOnlySpan

Reproduction Steps

Compile the following code definition

    [LibraryImport("wpiutil", EntryPoint = "WPI_GetResource_wpilib_128_png")]
    [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
    [return: MarshalUsing(CountElementName = nameof(len))]
    public static unsafe partial ReadOnlySpan<byte> GetResourceWpilib128Png(out nuint len);

Expected behavior

The code should compile

Actual behavior

The code fails with the following error

C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\src\wpiutil\Natives\ImagesNative.cs(13,53): error SYSLIB1051: The specified parameter needs to be marshalled from unmanaged to managed, but the marshaller type 'global::System.Runtime.InteropServices.Marshalling.ReadOnlySpanMarshaller<,>' does not support it. The generated source will not handle marshalling of the return value of method 'GetResourc
eWpilib128Png'. (https://learn.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib1051) [C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\src\wpiutil\wpiutil.csproj]
C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\src\wpiutil\Microsoft.Interop.LibraryImportGenerator\Microsoft.Interop.LibraryImportGenerator\LibraryImports.g.cs(574,68): error CA1420: Managed parameter or return types require runtime marshalling to be enabled (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1420) [C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\ 
src\wpiutil\wpiutil.csproj]

Regression?

No response

Known Workarounds

No response

Configuration

.NET 8

Other information

No response

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Issue Details

---

Description

Returning a ReadOnlySpan from a call generated by LibraryImport fails to compile, even though it should. It correctly works with a byte[] as the return type instead of ReadOnlySpan

Reproduction Steps

Compile the following code definition

    [LibraryImport("wpiutil", EntryPoint = "WPI_GetResource_wpilib_128_png")]
    [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
    [return: MarshalUsing(CountElementName = nameof(len))]
    public static unsafe partial ReadOnlySpan<byte> GetResourceWpilib128Png(out nuint len);

Expected behavior

The code should compile

Actual behavior

The code fails with the following error

C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\src\wpiutil\Natives\ImagesNative.cs(13,53): error SYSLIB1051: The specified parameter needs to be marshalled from unmanaged to managed, but the marshaller type 'global::System.Runtime.InteropServices.Marshalling.ReadOnlySpanMarshaller<,>' does not support it. The generated source will not handle marshalling of the return value of method 'GetResourc
eWpilib128Png'. (https://learn.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib1051) [C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\src\wpiutil\wpiutil.csproj]
C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\src\wpiutil\Microsoft.Interop.LibraryImportGenerator\Microsoft.Interop.LibraryImportGenerator\LibraryImports.g.cs(574,68): error CA1420: Managed parameter or return types require runtime marshalling to be enabled (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1420) [C:\Users\thadh\Documents\GitHub\robotdotnet\WPILib\ 
src\wpiutil\wpiutil.csproj]

Regression?

No response

Known Workarounds

No response

Configuration

.NET 8

Other information

No response

Author:	ThadHouse
Assignees:	-
Labels:	area-System.Runtime.InteropServices
Milestone:	-

[AaronRobinsonMSFT]

@jtschuster Can you take a look at this?

[ThadHouse]

Talking with @jkoritzinsky this is expected behavior, since the ReadOnlySpan can't be written to later in the source generator. So not a bug.

[AaronRobinsonMSFT]

Talking with @jkoritzinsky this is expected behavior, since the ReadOnlySpan can't be written to later in the source generator. So not a bug.

Agreed on it being by-design for now. I'm going to recommend we look at how to make this more natural in this release. The model should have a way to express both a read-only and read-write concept with respect to types. This is a low-priority issue, but definitely will a win for expressing intent for interop scenarios.