Invoking an unmanaged fastcall function pointer under x86 Windows fails at runtime

[frabert]

Description

Exporting a function using UnmanagedCallersOnly and the Fastcall calling convention will work, but calling ((delegate* unmanaged[Fastcall]<int, int>)somePtr)(someArg) will fail with

System.TypeLoadException
  HResult=0x80131522
  Message=Unsupported unmanaged calling convention.

Reproduction Steps

unsafe
{
    int x = ((delegate* unmanaged[Fastcall]<int, int>)IntPtr.Zero)(0);
}

Expected behavior

The function pointer should be called.

Actual behavior

The call fails due to the Fastcall calling convention being unimplemented.

Regression?

Unlikely that this was implemented before and removed afterwards.

Known Workarounds

No response

Configuration

Windows x86

Technically I'm running from a CoreCLR embedded in an application, but it should not matter in this scenario.

Other information

runtime/src/coreclr/vm/dllimport.cpp

Line 4238 in f17c21c

COMPlusThrow(kTypeLoadException, IDS_INVALID_PINVOKE_CALLCONV);

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