Runtime async via intrinsic

[majocha]

Yet another proof of concept (see also #19449)

The experiment here is to explore viability of working CE builders with just a minimal compiler support.

consider

__runtimeAsync: 't -> Task<'t>

intrinsic, which when used as a member or function body, promotes it to managed async.

We can have a fully inlining builder which wraps the code in it's Run method:

member inline _.Run([<InlineIfLambda>] code) = __runtimeAsync (code())

Because resumption is handled by the runtime, the builder is effectively just a sync builder with

member inline _.Bind(task, [<InlineIfLambda>] continuation) = AsyncHelpers.Await task |> continuation

There are more concerns than suspension / resumption that the runtime apparently does not handle and they would have to be covered either by the compiler or the builder implementations:

• awaits in EH blocks (IAsyncDisposable) - this has a PoC in the tests here.

• byref locals and execution context / sync context, see C# docs below.

runtime spec :
Runtime-async specification

interesting docs on C# implementation

• Runtime-async code-generation contract
• Roslyn runtime async design

[github-actions]

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/FSharp.Core`	docs/release-notes/.FSharp.Core/11.0.100.md
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
`src/Compiler/Facilities/LanguageFeatures.fsi`	docs/release-notes/.Language/preview.md