Unexpected error when using Span defined with old ref safety rules #80244
Description
We got a report from @brianrob that a new ref safety error is being reported on upgrade at the following location
https://github.com/microsoft/perfview/blob/049ed4f9462f39e2212a500fe80d55f1821dbb66/src/TraceEvent/EventPipe/SpanReader.cs#L45
(CoreCompile target) ->
E:\perfview\src\TraceEvent\EventPipe\SpanReader.cs(52,28): error CS8157: Cannot return 'ret' by reference because it was initialized to a value that cannot be returned by reference [
E:\perfview\src\TraceEvent\TraceEvent.csproj::TargetFramework=netstandard2.0]
Notably the compilation which is failing, has a netstandard2.0 reference to System.Memory and therefore the Span indexer uses the old ref safety rules.
I think the above case may have been exposed by #79447, the error is not reported when replaying the complog before that PR, and is reported afterwards.
However I think the below repro is showing a more general bug which may have already existed for some time.
<-- MyLib/MyLib.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>net6.0</TargetFramework> <!-- Change to net8.0 to remove the error (new ref safety rules) -->
<ImplicitUsings>enable</ImplicitUsings>
<Nullable>enable</Nullable>
</PropertyGroup>
</Project>// MyLib/Class1.cs
namespace MyLib;
public ref struct RS
{
public ref byte this[int i] => throw null!;
}// lab.cs
#:project ./MyLib
#:property TargetFramework=net9.0
using MyLib;
Console.WriteLine();
static ref byte M1(RS rs)
{
ref byte ret = ref rs[1];
return ref ret; // unexpected error when referencing net6.0
}
static ref byte M2(RS rs)
{
return ref rs[1]; // no error with either net6.0 or 8.0
}