Module ext 

Fluent spawn surface over the object-safe Runtime. Carries the axis combinations the trait’s named methods pre-combine. Fluent surface over Runtime, carrying the combinations the trait’s named methods pre-combine.

Runtime is consumed as Arc<dyn Runtime> (App::offline_runtime, the recording spigot), so it must stay object-safe: no generics, no RPITIT, and every payload boxed. That erasure is real on the trait and needless at the call site — this trait is never used as dyn itself, so it may carry generics, and the boxing happens once inside it. Every method here stays callable through a trait object: no Self: Sized bounds, and the blanket impl is ?Sized, so Arc<dyn Runtime> reaches the whole builder.

Three independent axes the named methods weld together:

axis	choices
placement	current core (default) / chosen core / background pool
payload	future / blocking closure
failure	SpawnError on a chosen core

spawn_on_current_core binds placement and drops the Result; spawn_factory_on_core binds placement and payload in one identifier. The grid has more cells than there are names for, and this is where the rest of them live.

rt.spawn(fut);                       // current core
rt.core(id).spawn(fut)?;             // chosen core, SpawnError surfaced
rt.blocking(work).spawn().await?;    // background pool, typed R back
rt.core([1, 2, 3]).spawn_with(|| work(), |_, e| *e == SpawnError::InboxFull)?;
// -> Ok(CoreId) which core took it, or Err(Exhausted { attempts, last })

text, not a doctest: every line needs a live Runtime. A doctest that cannot run is worse than prose — ignore compiles nothing while cargo test --doc still exits 0, which is how a broken example ships. The executable form of all four lines is in this module’s tests.

Structs

Blocking

Payload bound to thread-occupying work.

Exhausted

Every candidate was tried and none accepted — the sequence’s own termination signal, kept out of SpawnError because that type answers for one core and its implementors produce only its two arms. Following fan_in’s convention: termination lives in the Err channel as a distinct value, not as a domain error reused to mean something else.

OnCore

Placement bound. Every stage is complete on its own — the default it replaces is a real value, not an absent one.

Enums

Placement

Which core, stated as intent rather than as an index the caller computed.

Traits

RuntimeExt

Fluent spawn surface. Blanket-implemented for every Runtime; never used as dyn itself, which is what buys back the generics.