feat(nexus)!: port kernel to no_std (core+alloc) (#279)

.config/hakari.toml

@@ -30,6 +30,9 @@ platforms = [
 # feature) into `workspace-hack`, which propagates into every member that
 # depends on `workspace-hack` regardless of feature selection.
 [final-excludes]
+# `nexus` is the no_std kernel — it must not depend on the (std) workspace-hack.
+# hakari removes the edge; `hakari verify` enforces its absence. (#279)
+workspace-members = ["nexus"]
 third-party = [
   { name = "futures-core" },
 ]

Cargo.toml

@@ -39,7 +39,7 @@ serde_json = "1.0.149"
 sqlx = { version = "0.8", default-features = false, features = ["runtime-tokio", "postgres", "macros"] }
 static_assertions = "1.1.0"
 tempfile = "3"
-thiserror = "2.0.18"
+thiserror = { version = "2.0.18", default-features = false }
 tokio = { version = "1.52.1", features = ["full"] }
 tokio-stream = "0.1"
 

crates/nexus-fjall/Cargo.toml

@@ -24,7 +24,7 @@ fjall = { workspace = true }
 futures.workspace = true
 nexus = { version = "0.1.0", path = "../nexus" }
 nexus-store = { version = "0.1.0", path = "../nexus-store", features = ["subscription"] }
-thiserror = { workspace = true }
+thiserror = { workspace = true, features = ["std"] }
 tokio = { workspace = true, features = ["sync"] }
 workspace-hack = { version = "0.1", path = "../workspace-hack" }
 

crates/nexus-postgres/Cargo.toml

@@ -17,7 +17,7 @@ futures = { workspace = true }
 nexus = { version = "0.1.0", path = "../nexus" }
 nexus-store = { version = "0.1.0", path = "../nexus-store", features = ["subscription"] }
 sqlx = { workspace = true }
-thiserror = { workspace = true }
+thiserror = { workspace = true, features = ["std"] }
 tokio = { workspace = true, features = ["rt", "sync"] }
 workspace-hack = { version = "0.1", path = "../workspace-hack" }
 

crates/nexus-store/Cargo.toml

@@ -59,7 +59,7 @@ parking_lot = { workspace = true, optional = true }
 rkyv = { workspace = true, optional = true }
 serde = { workspace = true, optional = true }
 serde_json = { workspace = true, optional = true }
-thiserror = { workspace = true }
+thiserror = { workspace = true, features = ["std"] }
 tokio = { workspace = true, features = ["sync"], optional = true }
 workspace-hack = { version = "0.1", path = "../workspace-hack" }
 

crates/nexus/Cargo.toml

@@ -15,7 +15,6 @@ categories = ["data-structures"]
 arrayvec = { workspace = true }
 nexus-macros = { version = "0.1.0", path = "../nexus-macros", optional = true }
 thiserror = { workspace = true }
-workspace-hack = { version = "0.1", path = "../workspace-hack" }
 
 [dev-dependencies]
 criterion = { workspace = true }
@@ -31,7 +30,11 @@ thiserror = { workspace = true }
 trybuild = "1"
 
 [features]
-default = []
+default = ["std"]
+# Additive: enabling `std` only ADDS (the std::error::Error bridge via
+# thiserror); disabling it (`--no-default-features`) yields a no_std +
+# core::error::Error kernel. (#279)
+std = ["thiserror/std"]
 derive = ["dep:nexus-macros"]
 testing = []
 

crates/nexus/src/aggregate.rs

@@ -3,11 +3,11 @@ use crate::event::DomainEvent;
 use crate::events::Events;
 use crate::id::Id;
 use crate::version::Version;
-use std::error::Error;
-use std::fmt;
-use std::fmt::Debug;
-use std::mem;
-use std::num::NonZeroUsize;
+use core::error::Error;
+use core::fmt;
+use core::fmt::Debug;
+use core::mem;
+use core::num::NonZeroUsize;
 
 /// State of an event-sourced aggregate. Mutated by applying domain events.
 ///

crates/nexus/src/id.rs

@@ -1,5 +1,5 @@
-use std::fmt::{Debug, Display};
-use std::hash::Hash;
+use core::fmt::{Debug, Display};
+use core::hash::Hash;
 
 use crate::ErrorId;
 

crates/nexus/src/lib.rs

@@ -1,3 +1,10 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
+// The production kernel is pure `core` — no allocator required on-device.
+// `alloc` is pulled in only for the test fixture and unit tests.
+#[cfg(any(test, feature = "testing"))]
+extern crate alloc;
+
 mod aggregate;
 mod error;
 mod error_id;

crates/nexus/src/message.rs

@@ -1,3 +1,3 @@
-use std::fmt::Debug;
+use core::fmt::Debug;
 
 pub trait Message: Send + Sync + Debug + 'static {}

crates/nexus/src/testing.rs

@@ -32,6 +32,7 @@ use crate::event::DomainEvent;
 use crate::events::Events;
 use crate::saga::{React, Saga};
 use crate::version::Version;
+use alloc::vec::Vec;
 use core::fmt::Debug;
 
 /// Entry point. Carries the id used to construct the root for replay.

crates/nexus/src/version.rs

@@ -1,6 +1,6 @@
-use std::fmt;
-use std::iter::FusedIterator;
-use std::num::NonZeroU64;
+use core::fmt;
+use core::iter::FusedIterator;
+use core::num::NonZeroU64;
 
 /// A compile-checked [`Version`] literal.
 ///

crates/workspace-hack/Cargo.toml

@@ -30,6 +30,7 @@ sha2 = { version = "0.10" }
 smallvec = { version = "1", default-features = false, features = ["const_generics", "serde"] }
 sqlx-core = { version = "0.8", features = ["_rt-tokio", "any", "json", "migrate", "offline"] }
 sqlx-postgres = { version = "0.8", default-features = false, features = ["migrate", "offline"] }
+thiserror = { version = "2" }
 tokio = { version = "1", features = ["full"] }
 trybuild = { version = "1", default-features = false, features = ["diff"] }
 zerocopy = { version = "0.8", default-features = false, features = ["derive", "simd"] }
@@ -55,6 +56,7 @@ smallvec = { version = "1", default-features = false, features = ["const_generic
 sqlx-core = { version = "0.8", features = ["_rt-tokio", "any", "json", "migrate", "offline"] }
 sqlx-postgres = { version = "0.8", default-features = false, features = ["migrate", "offline"] }
 syn = { version = "2", features = ["extra-traits", "fold", "full", "visit", "visit-mut"] }
+thiserror = { version = "2" }
 tokio = { version = "1", features = ["full"] }
 trybuild = { version = "1", default-features = false, features = ["diff"] }
 zerocopy = { version = "0.8", default-features = false, features = ["derive", "simd"] }