feat(hydro_lang): add embedded runtime helpers and embedded_flow! macro

[shadaj]

Upstreams a downstream app's embedded-mode runtime helpers into hydro_lang behind the
embedded_runtime feature, as the new hydro_lang::embedded module:

• InputBuffer<T> / InputStream<T>: address-stable queue backing a Stream input.
• CallbackSlot<T>: address-stable slot holding a scoped FnMut(T) output callback.
• AliasedBox<T> / OwnedErasedBox: owning allocations without Box's noalias guarantee.
• DfirRunnable: object-safe erasure for the unnameable Dfir<impl TickClosure>.
• embedded_flow!: declares a safe, self-contained holder struct for a generated flow.

Safety/perf fixes relative to the downstream version (validated under Miri/Stacked Borrows):

• fix: Box-move aliasing UB — holder fields were Boxes whose moves invalidate the
  flow's raw pointers into their contents; replaced with new AliasedBox (raw-pointer
  ownership, Deref only), and the cluster self-id is now owned via OwnedErasedBox.
• fix: invoke_with left a dangling callback installed on panic, and nested invoke_with
  cleared (instead of restored) the outer callback; a drop guard now saves/restores the
  previous pointer.
• fix: re-entrant slot invocation could alias the running callback's &mut; the callback
  pointer lives in a Cell and is taken out while it runs.
• fix: push held a borrow of the waker cell while running foreign waker code; wakes are
  now taken-and-consumed (coalescing redundant wakes), poll_next skips waker bookkeeping
  on the ready path and uses Waker::clone_from (will_wake fast path), with a queue
  re-check to avoid lost wakeups.
• fix: InputBuffer was auto-Send, letting safe code race pushes across threads; it is
  now !Send (PhantomData<*mut ()>).
• fix: the holder no longer runs the flow during construction before callbacks can be
  installed (which silently discarded initialization outputs).
• no_std-readiness: core-only primitives with const fn new, Pin-based address
  stability contracts, and alloc-dependent pieces isolated behind embedded::__private
  re-exports so the macro can later switch std -> alloc non-breakingly.

The macro is redesigned from the downstream two-variant form into a single arm with
optional sections (self_id, membership, inputs, network_in, outputs,
network_out), covering shapes the old macro could not (e.g. receive-only cluster
members, membership streams). It generates accessors, run(), and run_with(...)
(which installs all output callbacks via nested invoke_with).

test(hydro_lang): unit tests for the primitives (waker coalescing, FIFO order, move
stability, nested/panicking/re-entrant callback installs, erased drop).

test(hydro_test_embedded): embedded_flow! holder tests against the real generated flows
(capitalize, singleton input, o2o echo wiring two holders together, o2m broadcast with
membership, m2m broadcast), including re-entrant input pushes from output callbacks;
these also pass under Miri.

build(hydro_lang,hydro_std,hydro_test): post-process stageleft_tool::gen_final! output
to gate the generated stageleft registration ctor behind cfg(not(miri)), since the
ctor crate's init_array entries trip Miri's ABI checks; marked TODO for removal once
fixed upstream (hydro-project/stageleft#84).

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View logs

[shadaj]

Marking as draft while waiting on Stageleft release.

[MingweiSamuel]

Is this different from Pin<Box<T>>?

[Copilot]

Pull request overview

This PR adds a new hydro_lang::embedded module (behind the embedded_runtime feature) that provides safe runtime primitives and the embedded_flow! macro for driving embedded-mode generated DFIR flows, plus test coverage for both the primitives and real generated embedded flows. It also introduces a build-script workaround to gate stageleft-generated ctor registration under cfg(not(miri)) to keep Miri runs working until upstream fixes land.

Changes:

• Add hydro_lang::embedded runtime helpers (InputBuffer, CallbackSlot, AliasedBox, OwnedErasedBox, DfirRunnable) and the embedded_flow! macro.
• Add embedded holder tests in hydro_test_embedded exercising real generated flows and re-entrancy patterns.
• Post-process stageleft output in multiple crates’ build.rs to gate generated ctor registration under cfg(not(miri)).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
hydro_test/build.rs	Post-processes stageleft-generated staged_deps.rs to gate ctor registration under cfg(not(miri)) for Miri compatibility.
hydro_test_embedded/src/lib.rs	Adds tests for embedded_flow! holder structs against real generated embedded flows (including network wiring scenarios).
hydro_std/build.rs	Same stageleft staged_deps.rs post-processing for Miri compatibility.
hydro_lang/src/lib.rs	Exposes the new embedded module behind the embedded_runtime feature flag.
hydro_lang/src/embedded.rs	Introduces the embedded runtime primitives, helper macros, and unit tests for the primitives.
hydro_lang/build.rs	Same stageleft staged_deps.rs post-processing for Miri compatibility.

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