Compiling against methods that accept params span with only System.Runtime reference produces compiler error

Description

We have API in System.Runtime that accepts params ReadonlySpan. Normally we'd be able to consume any API in System.Runtime with just a reference to System.Runtime - but calling these new APIs demands System.Runtime.InteropServices.MemoryMarshal.CreateReadOnlySpan

I wonder if we should push this type down, perhaps others in System.Memory as well. It's implementation already lives in System.Private.CoreLib

Reproduction Steps

Here's a repro that demonstrates a customer scenario where they do runtime compilation and specify System.Runtime only as a reference. This isn't too farfetched since we recommend that folks use reference assemblies.

compileSR.zip -> derived from https://learn.microsoft.com/en-us/archive/msdn-magazine/2017/may/net-core-cross-platform-code-generation-with-roslyn-and-net-core#executing-code-the-emit-apis sample

using System.Reflection;
using System.Runtime.Loader;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.Emit;

const string code = """
        public static class C
        {
            public static string M() => string.Concat("Hello", " ", "World", "!", "!", "!");
        }
        """;
var tree = SyntaxFactory.ParseSyntaxTree(code, new CSharpParseOptions(LanguageVersion.Preview));
string fileName="lib.dll";
// add reference to only System.Runtime
var systemRuntimeRefLocation=Path.Combine(AppContext.BaseDirectory, "refs", "System.Runtime.dll");
var systemRuntimeReference = MetadataReference.CreateFromFile(systemRuntimeRefLocation);
var compilation = CSharpCompilation.Create(fileName)
    .WithOptions(
        new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences(systemRuntimeReference)
    .AddSyntaxTrees(tree);
string path = Path.Combine(Directory.GetCurrentDirectory(), fileName);

EmitResult compilationResult = compilation.Emit(path);
if(compilationResult.Success)
{
    // Load the assembly
    Assembly asm = AssemblyLoadContext.Default.LoadFromAssemblyPath(path);
    object? result = asm!.GetType("C")!.GetMethod("M")!.Invoke(null, []);
    Console.WriteLine(result);
}
else
{
    foreach (Diagnostic codeIssue in compilationResult.Diagnostics)
    {
        string issue = $"ID: {codeIssue.Id}, Message: {codeIssue.GetMessage()}, Location: {codeIssue.Location.GetLineSpan()}, Severity: {codeIssue.Severity}";
        Console.WriteLine(issue);
    }
}

Expected behavior

Comple successful and output "Hello World!!!"

Actual behavior

ID: CS0656, Message: Missing compiler required member 'System.Runtime.InteropServices.MemoryMarshal.CreateReadOnlySpan', Location: : (0,0)-(0,0), Severity: Error

Regression?

Yes, the addition of the params ReadOnlySpan overloads introduced this break.

Known Workarounds

Add a reference to System.Memory

Configuration

No response

Other information

See https://github.com/dotnet/sdk/pull/41014 for some background. CC @333fred @jaredpar @Jozkee

dotnet / runtime