Async-runtime-mode

Read the current backend-specific async runtime mode. This API is the narrowest capability probe when you only care whether the current build is running native worker semantics or compatibility behavior.

Interface

pub fn async_runtime_mode() -> AsyncRuntimeMode {}

input

• none - No arguments are required.

output

• AsyncRuntimeMode - Either NativeWorker or Compatibility.

Explanation

Detailed rules explaining key parameters and behaviors

• The return value is determined by the active backend implementation.
• NativeWorker is the expected mode on native-style backends, while Compatibility is the expected mode on targets without native background-worker semantics.
• async_runtime_mode_label(...) converts the enum into a stable string value.
• async_runtime_supports_background_worker() is a narrower boolean probe built on the same idea.
• async_runtime_state() packages this mode together with the current background-worker capability into one AsyncRuntimeState snapshot.
• In the current backend implementations, NativeWorker pairs with background_worker=true and Compatibility pairs with background_worker=false.
• Concretely, the native runtime entrypoint returns native_worker_async_runtime_mode(), while the compatibility stub returns compatibility_async_runtime_mode().
• The enum is therefore selected directly by the active backend helper on each call rather than read back out of a cached runtime snapshot object.
• This API is intentionally small and useful for lightweight branching.
• The mode result describes runtime behavior only; it should not be read as proof that every backend has been equally re-verified in the current release cycle.

How to Use

Here are some specific examples provided.

When Need A Small Capability Branch

When behavior should differ between native worker mode and compatibility mode:

match async_runtime_mode() {
  AsyncRuntimeMode::NativeWorker => println("native worker")
  AsyncRuntimeMode::Compatibility => println("compat")
}

In this example, the branch is explicit and readable.

When Need Stable String Output

When the mode should be included in logs or telemetry labels:

println(async_runtime_mode_label(async_runtime_mode()))

In this example, the output becomes a stable string instead of an enum pattern-match requirement.

Error Case

e.g.:

• This API does not have a normal runtime failure mode; it reflects the compiled backend behavior.

• If callers need the current mode paired with the matching worker-support flag, prefer a fresh async_runtime_state() call instead of combining async_runtime_mode() with an older saved runtime snapshot.

• If you need worker support as a direct boolean, use async_runtime_supports_background_worker() instead.

Notes

1. Use this API for minimal mode branching.

2. Use async_runtime_state() when you also want worker support packaged into one object.

3. Use async_runtime_mode_label(...) when the result should leave enum space and become stable text such as native_worker or compatibility.

4. This mode distinction is about runtime behavior, not whether src-async itself is expected to compile for the target.

5. See target-verification.md for the current verification status of individual targets.