Phase 18. Embedded no_std + alloc variant

Field	Value
MEP	MEP-53 §Phases
Status	LANDED
Started	2026-05-29 (GMT+7)
Landed	2026-05-29 (GMT+7)
Tracking issue	#22617
Tracking PR	#22499
Commit	f4c4cb32d3

Gate

TestPhase18Embedded in transpiler3/rust/build/phase18_test.go shells out to cargo check --no-default-features --features embedded against the runtime3/rust/mochi-runtime workspace. The gate is exit-zero from cargo. Skipped in -short mode and when cargo is not on PATH.

Lowering decisions

The runtime crate declares two features:

[features]
default = ["std"]
std = []
embedded = []

Default features include std so non-embedded users see no behavior change. The embedded feature, when selected, disables default (via --no-default-features) and signals to the runtime that std is unavailable.

src/lib.rs is gated:

#![cfg_attr(feature = "embedded", no_std)]

#[cfg(feature = "embedded")]
extern crate alloc;

#[cfg(feature = "std")]
pub mod io { /* ... */ }

pub mod conv {
    #[cfg(feature = "embedded")]
    use alloc::string::{String, ToString};
    // ...
}

pub mod strings {
    #[cfg(feature = "embedded")]
    use alloc::string::{String, ToString};
    // ...
}

#[cfg(feature = "std")]
pub mod chan { /* ... */ }

#[cfg(feature = "std")]
pub mod stream { /* ... */ }

#[cfg(feature = "std")]
pub mod panic { /* ... */ }

#[cfg(feature = "std")]
pub mod fetch { /* ... */ }

#[cfg(feature = "std")]
pub mod json { /* ... */ }

#[cfg(feature = "std")]
pub mod llm { /* ... */ }

#[cfg(feature = "std")]
pub mod check { /* ... */ }

Modules that need only alloc (conv, strings) are always exposed but explicitly use alloc::string::{String, ToString}; when the embedded feature is on (under no_std, String is not in the prelude). Modules that need std (io for println!, chan / stream for Rc<RefCell> indirectly via collections, panic for std::panic::catch_unwind, fetch for std::net::TcpStream, json for std::collections::HashMap, llm for std::env and std::fs, check for the panic dep) are gated behind #[cfg(feature = "std")].

The embedded subset is intentionally narrow: only conv and strings work. A user emitting Mochi → Rust for embedded use would be restricted to scalar arithmetic, integer-to-string conversion, and char-aware string slicing. Collections, channels, streams, panic, fetch, JSON, LLM, and runtime-checked arithmetic are unavailable. This is documented as a "Mochi subset" target.

Files changed

File	Purpose
runtime3/rust/mochi-runtime/src/lib.rs	Add cfg(feature = "std") gates and cfg(feature = "embedded") extern crate alloc
runtime3/rust/mochi-runtime/Cargo.toml	Declare default, std, embedded features
transpiler3/rust/build/phase18_test.go	cargo check --no-default-features --features embedded gate

Test set

• TestPhase18Embedded (single subtest, the cargo check itself).

Closeout notes

The first attempt at the embedded feature failed with 72 errors because std:: paths were referenced everywhere unconditionally (println! in io, std::sync::Once in panic, std::net::TcpStream in fetch, std::collections::HashMap in json, etc). The fix was to gate every heavy module behind #[cfg(feature = "std")] rather than try to make them work under no_std + alloc. conv and strings were the only modules that could be made alloc-only without significant rework, and even those required adding use alloc::string::{String, ToString}; at the top of each module (under embedded, these are not in the implicit no_std prelude).

The gate is cargo check rather than cargo build because building a cdylib or staticlib for a no_std target requires a panic_handler and global_allocator, which are application-level concerns. cargo check verifies the library compiles in isolation, which is enough to assert the embedded feature is well-formed.

Future sub-phase 18.1 could add a real bare-metal target (thumbv7em-none-eabihf for Cortex-M4F) and a tiny no_main test program that links against the embedded subset.