[WIP]: MethodImplOption.Async support

[TheAngryByrd]

Description

Fixes #19056

Checklist

• Test cases added

• Performance benchmarks added in case of performance changes

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@TheAngryByrd,

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Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.0.md	No release notes found or release notes format is not correct

[T-Gro]

I assume this will fail when targeting desktop framework, you might need to use TheoryForNETCOREAPPAttribute in the test declaration.

[majocha]

We should make sure ildasm version that the CI uses actually does support async flag

[TheAngryByrd]

We should make sure ildasm version that the CI uses actually does support async flag

Yeah, I had that changed locally to use the latest but I didn't commit it. Definitely part of my verification work.

[TheAngryByrd]

As noted in: https://github.com/dotnet/runtime/blob/main/docs/design/specs/runtime-async.md

Async methods also do not have matching return type conventions as sync methods. For sync methods,
the stack should contain a value convertible to the stated return type before the ret instruction.
For async methods, the stack should be empty in the case of Task or ValueTask, or the type argument > in the case of Task or ValueTask.

For the following F# code we get this IL:

    [<MethodImpl(MethodImplOptions.Async)>]
    let doThing () =
        task {
            do! Task.Yield()
            return 42
        }

  .method public static class [System.Runtime]System.Threading.Tasks.Task`1<int32> 
          doThing() cil managed async
  {
    // Code size       79 (0x4f)
    .maxstack  4
    .locals init (class [IcedTasks]IcedTasks.TaskBase_Net10.TaskBuilderRuntime V_0,
             class [IcedTasks]IcedTasks.TaskBase_Net10.TaskBuilderRuntime V_1,
             class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32>> V_2,
             class [IcedTasks]IcedTasks.TaskBase_Net10.TaskBuilderBaseRuntime V_3,
             valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32> V_4,
             valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32> V_5,
             valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32> V_6,
             valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32> V_7,
             valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32> V_8)
    IL_0000:  call       class [IcedTasks]IcedTasks.TaskBase_Net10.TaskBuilderRuntime [IcedTasks]IcedTasks.Polyfill.TasksRuntime.TaskBuilder::get_task()
    IL_0005:  stloc.0
    IL_0006:  ldloc.0
    IL_0007:  stloc.1
    IL_0008:  ldloc.0
    IL_0009:  stloc.3
    IL_000a:  ldloc.0
    IL_000b:  newobj     instance void ILSpySamples.TaskRuntime/doThing@1::.ctor(class [IcedTasks]IcedTasks.TaskBase_Net10.TaskBuilderRuntime)
    IL_0010:  stloc.2
    IL_0011:  ldloc.2
    IL_0012:  ldnull
    IL_0013:  callvirt   instance !1 class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<class [FSharp.Core]Microsoft.FSharp.Core.Unit,valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32>>::Invoke(!0)
    IL_0018:  stloc.s    V_4
    IL_001a:  ldloc.s    V_4
    IL_001c:  stloc.s    V_5
    IL_001e:  ldloc.s    V_5
    IL_0020:  stloc.s    V_6
    IL_0022:  ldloca.s   V_6
    IL_0024:  call       instance bool valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32>::get_IsCompleted()
    IL_0029:  ldc.i4.0
    IL_002a:  ceq
    IL_002c:  nop
    IL_002d:  brfalse.s  IL_0039

    IL_002f:  ldloc.s    V_4
    IL_0031:  call       void [System.Runtime]System.Runtime.CompilerServices.AsyncHelpers::UnsafeAwaitAwaiter<valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32>>(!!0)
    IL_0036:  nop
    IL_0037:  br.s       IL_003a

    IL_0039:  nop
    IL_003a:  ldloc.s    V_4
    IL_003c:  stloc.s    V_7
    IL_003e:  ldloc.s    V_7
    IL_0040:  stloc.s    V_8
    IL_0042:  ldloca.s   V_8
    IL_0044:  call       instance !0 valuetype [System.Runtime]System.Runtime.CompilerServices.ValueTaskAwaiter`1<int32>::GetResult()
    IL_0049:  call       class [System.Runtime]System.Threading.Tasks.Task`1<!!0> [System.Runtime]System.Threading.Tasks.Task::FromResult<int32>(!!0)
    IL_004e:  ret
  } // end of method TaskRuntime::doThing

---

Let's take a quick look at the C# equivalent:

    <EnablePreviewFeatures>true</EnablePreviewFeatures>
    <Features>$(Features);runtime-async=on</Features>
    <NoWarn>$(NoWarn);SYSLIB5007</NoWarn>

    public async Task<int> DoThingAsync()
    {
        await Task.Yield();
        return 42;
    }

  .method public hidebysig instance class [System.Runtime]System.Threading.Tasks.Task`1<int32> 
          DoThingAsync() cil managed async
  {
    .custom instance void [System.Runtime]System.Runtime.CompilerServices.NullableContextAttribute::.ctor(uint8) = ( 01 00 01 00 00 ) 
    .custom instance void [System.Runtime]System.Diagnostics.DebuggerStepThroughAttribute::.ctor() = ( 01 00 00 00 ) 
    // Code size       46 (0x2e)
    .maxstack  1
    .locals init (valuetype [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable/YieldAwaiter V_0,
             valuetype [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable V_1,
             int32 V_2)
    IL_0000:  nop
    IL_0001:  call       valuetype [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable [System.Runtime]System.Threading.Tasks.Task::Yield()
    IL_0006:  stloc.1
    IL_0007:  ldloca.s   V_1
    IL_0009:  call       instance valuetype [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable/YieldAwaiter [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable::GetAwaiter()
    IL_000e:  stloc.0
    IL_000f:  ldloca.s   V_0
    IL_0011:  call       instance bool [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable/YieldAwaiter::get_IsCompleted()
    IL_0016:  brtrue.s   IL_001f

    IL_0018:  ldloc.0
    IL_0019:  call       void [System.Runtime]System.Runtime.CompilerServices.AsyncHelpers::UnsafeAwaitAwaiter<valuetype [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable/YieldAwaiter>(!!0)
    IL_001e:  nop
    IL_001f:  ldloca.s   V_0
    IL_0021:  call       instance void [System.Runtime]System.Runtime.CompilerServices.YieldAwaitable/YieldAwaiter::GetResult()
    IL_0026:  nop
    IL_0027:  ldc.i4.s   42
    IL_0029:  stloc.2
    IL_002a:  br.s       IL_002c

    IL_002c:  ldloc.2
    IL_002d:  ret
  } // end of method Thingy::DoThingAsync

I had trouble overcoming this limitation, until I remembered I can cheat by using the fun old trick of:

    module internal Unsafe =
        let inline cast<'a, 'b> (a: 'a) : 'b =
            (# "" a : 'b #)

which does seem to make the compiler and IL happy.