Enable runtime async via compiler intrinsics

docs/index.md

@@ -15,6 +15,7 @@ Welcome to [the F# compiler and tools repository](https://github.com/dotnet/fsha
 * [Memory usage](memory-usage.md)
 * [Optimizations](optimizations.md)
 * [Equality optimizations](optimizations-equality.md)
+* [Runtime async](runtime-async.md)
 * [Project builds](project-builds.md)
 * [Tooling features](tooling-features.md)
 

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -171,6 +171,7 @@
 * Added warning FS3884 when a function or delegate value is used as an interpolated string argument. ([PR #19289](https://github.com/dotnet/fsharp/pull/19289))
 * Symbols: add ObsoleteDiagnosticInfo ([PR #19359](https://github.com/dotnet/fsharp/pull/19359))
 * FCS: add FSharpCheckFileResults.HasErrors ([PR #19892](https://github.com/dotnet/fsharp/pull/19892))
+* Add preview compiler support for runtime-async `Task<'T>` methods through the `__runtimeAsyncReturn` intrinsic, including carrier validation, target-runtime capability checks, diagnostics for suspension calls outside runtime-async methods, diagnostics for byref, byref-like, or pinned values used after suspension, and recursive specialization of inline suspension fragments when optimization is disabled. ([PR #20235](https://github.com/dotnet/fsharp/pull/20235))
 * Add `#version;;` directive to F# Interactive to display version and environment information. ([Issue #13307](https://github.com/dotnet/fsharp/issues/13307), [PR #19332](https://github.com/dotnet/fsharp/pull/19332))
 * Add diagnostic FS3889 when a namespace and a type have the same fully-qualified name in the same assembly, replacing the misleading FS0247 "namespace and a module" error. ([Issue #17827](https://github.com/dotnet/fsharp/issues/17827), [PR #19802](https://github.com/dotnet/fsharp/pull/19802))
 * Debug: rework for expressions stepping ([PR #19894](https://github.com/dotnet/fsharp/pull/19894))
@@ -191,6 +192,7 @@
 * Add internal hot reload baseline reading for recorded EnC state and synthesized-name snapshot PDB data. ([PR #20026](https://github.com/dotnet/fsharp/pull/20026))
 * Support for the `<include>` XML documentation tag: at compile time, documentation is copied from an external XML file selected by an XPath query and emitted into the generated documentation file. `<inheritdoc>` remains unsupported. ([Issue #19175](https://github.com/dotnet/fsharp/issues/19175), [PR #19186](https://github.com/dotnet/fsharp/pull/19186))
 * Expand `<inheritdoc/>` at tooling time. In IDE tooltips, completion, and signature help, documentation is inherited from base classes, interfaces, overridden members, and constructors (matched by parameter signature). The FCS Symbols API (`FSharpSymbol.XmlDoc`) additionally resolves explicit `cref` targets, but does not expand constructor inheritance. The compiler emits the tag verbatim into generated XML documentation files, matching C#; `<include>` is not implemented. ([Issue #19175](https://github.com/dotnet/fsharp/issues/19175), [PR #19188](https://github.com/dotnet/fsharp/pull/19188))
+* Prototype direct runtime-async sequences from statically known inline recipes, with asynchronous cleanup, direct interface dispatch, and preserved runtime call-await fusion.
 * Add symbol and type highlighting to F# diagnostics ([PR #20097](https://github.com/dotnet/fsharp/pull/20097))
 * IL: hold custom attributes in fields rather than a union case ([PR #20287](https://github.com/dotnet/fsharp/pull/20287))
 * IL: reuse the cached ILTypeRef in ILTypeInfo.FromType ([PR #20255](https://github.com/dotnet/fsharp/pull/20255))

docs/release-notes/.FSharp.Core/11.0.100.md

@@ -12,6 +12,7 @@
 
 ### Added
 
+* Add the compiler-recognized `StateMachineHelpers.__runtimeAsyncReturn` intrinsic to the `net10.0` FSharp.Core target for .NET runtime-async methods. ([PR #20235](https://github.com/dotnet/fsharp/pull/20235))
 * Add `Unchecked.withNull`, an interop escape hatch that re-types any `'T` to `'T | null` without the usual `not null`/`not struct` constraints, so unconstrained C# nullable-generic APIs (e.g. `T? M<T>()`) can be implemented and consumed from F#. ([Issue #17734](https://github.com/dotnet/fsharp/issues/17734), [PR #20232](https://github.com/dotnet/fsharp/pull/20232))
 * Added generic `print` and `printn` functions (`'T -> unit`) to `ExtraTopLevelOperators` for simple value printing to stdout. ([RFC FS-1125](https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1125-print-printn-functions.md), [PR #19265](https://github.com/dotnet/fsharp/pull/19265))
 * Ship `FSharp.Core` with an additional `net10.0` target framework (next to `netstandard2.0` and `netstandard2.1`). The `net`-TFM assembly is public-surface-identical to the `netstandard2.1` one; the target version is a pinned, deliberately advanced knob. ([PR #20229](https://github.com/dotnet/fsharp/pull/20229))
@@ -27,6 +28,7 @@
 * Add conversion functions `Task.ofValueTask` and `ValueTask.ofTask`. ([LanguageSuggestion #1466](https://github.com/fsharp/fslang-suggestions/issues/1466), [PR #19844](https://github.com/dotnet/fsharp/pull/19844))
 * Add `parallelLimit` to `Task` and `Async` modules, for bounded-parallelism execution of async/task computations, flowing cancellation and returning the results as an array. ([LanguageSuggestion #1467](https://github.com/fsharp/fslang-suggestions/issues/1467), [PR #20294](https://github.com/dotnet/fsharp/pull/20294))
 * Add `parallelDoLimit` to `Task` and `Async` modules, for bounded-parallelism execution of async/task computations, flowing cancellation. ([LanguageSuggestion #1467](https://github.com/fsharp/fslang-suggestions/issues/1467), [PR #20294](https://github.com/dotnet/fsharp/pull/20294))
+* Prototype a compiler-services entry and reference host for direct runtime-async sequences on the `net10.0` target, reusing the first enumerator instance.
 * Add `sequentialDo` to `Task` and `Async`, running computations one at a time, flowing cancellation, without yielding an egregious `unit[]`. ([LanguageSuggestion #1467](https://github.com/fsharp/fslang-suggestions/issues/1467), [PR #20294](https://github.com/dotnet/fsharp/pull/20294))
 * Add `Task.sequential`, running task computations one at a time, flowing cancellation and returning the results as an array. ([LanguageSuggestion #1467](https://github.com/fsharp/fslang-suggestions/issues/1467), [PR #20294](https://github.com/dotnet/fsharp/pull/20294))
 * Add `Task.startAsyncImmediate`, starting an `Async<'T>` on the current thread and returning a `Task<'T>`, flowing cancellation. ([LanguageSuggestion #1467](https://github.com/fsharp/fslang-suggestions/issues/1467), [PR #20294](https://github.com/dotnet/fsharp/pull/20294))

docs/release-notes/.Language/preview.md

@@ -1,5 +1,6 @@
 ### Added
 
+* Runtime async: `task`/`async`-style computation expressions can be compiled to use the .NET runtime async support (RuntimeAsync preview feature). ([PR #20235](https://github.com/dotnet/fsharp/pull/20235))
 * **Extension members for operators and SRTP constraints** ([RFC FS-1043](https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1043-extension-members-for-operators-and-srtp-constraints.md), [fslang-suggestions#230](https://github.com/fsharp/fslang-suggestions/issues/230), [PR #19602](https://github.com/dotnet/fsharp/pull/19602)): Extension methods now participate in SRTP constraint resolution. This allows defining operators on types you don't own via type extensions:
 
   ```fsharp