ADR 0024 — M14 K1: static items (global state)

• Status: Accepted (project owner, 2026-10-29), with static mut added at the owner's request (§3.7). The wording is the agent's.
• Scope: M14 K1, OD-28. Global state for a kernel written in Nova (CosmoOS roadmap 2.0's SA2 and SA3), and for hosted programs.

1. Context

• Today Nova has only compile-time const items (type-system §10). static var count: u64 = 0; and var count: u64 = 0; at top level are N0101. CosmoOS's four Nova ports keep all their state in C, and C7's kernel demonstrations (integrations/cosmoos/c7) get every lock from a function argument or a KernelHeap box made at run time.
• A kernel needs tables and singletons protected by locks (the handle table, the module list, lockdep's graph), counters, and per-CPU data. CosmoOS keeps per-CPU data in arrays indexed by CPU id ([CONFIG_MAX_CPUS], beside struct percpu).
• Kernel locks are C state (R-11, type-system §33). A spinlock may not be zero-initialised: a zeroed spinlock_t's owner is CPU 0. CosmoOS writes a static lock with a C initialiser macro, SPINLOCK_INIT(name), which sets the owner to SPINLOCK_NO_OWNER. C7's SpinLock<T> instead initialises its lock at run time, in place (@placed fn init).
• Thread-safety already has a vocabulary. Share (§38.1) says a value may be used from several threads at once. Lock and atomic types are Share by the library's @threadSafe declaration.

2. Options

	A. Immutable statics of Share types, built by compile-time constructors (proposed)	B. static mut with unsafe access (Rust's)	C. Lazily initialised statics (a run-time constructor on first use)
Safe access	borrow NAME, like any shared reference; mutation only through the type's own synchronisation	every access is unsafe	safe, but needs a once-guard on every access
Data races	impossible: the type is Share	the user's argument	impossible
Initialisation	at compile time, as C static initialisers: no constructor runs, no order to get wrong	the same	at run time, in an order the program must not depend on
Kernel fit	yes: a SpinLock<Table> in a static, built with SPINLOCK_INIT	yes, unsafely	no: a once-guard in interrupt context needs a lock of its own

A is Rust's static with interior mutability, and Linux's DEFINE_SPINLOCK. The owner chose A and B together: B as static mut, every use of which is unsafe (§3.7), for the floor and for state whose synchronisation Nova cannot express. C is not taken.

3. Proposed decision: A

3.1 The item

static TABLE: SpinLock<HandleTable> = SpinLock.constant("handles", HandleTable.empty());
static OPENED: AtomicU64 = AtomicU64.constant(0);

fn openOne() -> u64 => OPENED.fetchAdd(1) + 1;

• static NAME: T = e; at package level, public or not. T must be Share (§38.1), or it is a new error, N0337: "a static is shared by every thread". This also covers a static used only from one thread, because the checker cannot know that.
• A static is never assigned or moved (N0337), and NAME in an expression is a place of type T that only shared borrows and copies (for a Copy T) may use. So all mutation goes through T's own methods taking borrow this: a lock's guard or an atomic's operation.
• Region. borrow NAME has the static region, which outlives every other region. A reference to a static may be stored and returned anywhere a reference may (E1–E4 hold trivially), and passed to another thread when T is Share.
• A static is never dropped. A T with a drop is allowed, and its drop never runs: a kernel never exits, and a hosted program's statics end with the process, as in C.
• Generic statics, and statics inside functions, are not in v1 (N0337).

3.2 Compile-time constructors

e must be a static initialiser, evaluated by the compiler to a C constant initialiser. It may be:

• a constant expression (§10), a string literal (as *CChar), or a struct, enum or array literal of static initialisers (fields private to T are allowed only inside T's own @constant constructor);
• a call of a @constant function: one whose body is a single expression that is a static initialiser of its parameters, the parameters being static initialisers at each call. @constant functions are checked as ordinary functions too, so they can be called at run time;
• a call of a C constant macro: an extern "C" function declared @constant, whose name is a C macro that expands to a constant initialiser (@constant fn SPINLOCK_INIT(name: *CChar) -> spinlock_t;). Nova trusts the declaration, as it trusts @effects on an extern (R-2).

A type whose values are made at their final address (@placed init, §33.3) may be in a static only through a @constant constructor, whose initialiser must leave it as init would. For CosmoOS's locks that is the C macro's job, so SpinLock.constant uses SPINLOCK_INIT and does not call init.

3.3 Per-CPU data is a library type

There is no @percpu in the language. The kernel library (K3) provides PerCpu<T>, an array of T per CPU behind a Share type. PerCpu<T>.local(guard) lends the calling CPU's element, for as long as a guard that disables preemption (or interrupts, for data an interrupt handler shares) is borrowed. Its unsafe core argues the rest: the guard pins the CPU, so only this CPU's code, and not its interrupts when the guard masks them, uses the element. So:

static TICKS: PerCpu<u64> = PerCpu.constant(0);

@context(atomic)
fn tick(preempt: borrow PreemptGuard) { *TICKS.local(preempt) += 1; }

That keeps the language change to statics, and puts the CPU model where CosmoOS's own API is (OD-30).

3.4 C

A static is a C object definition with internal linkage, or external with @export (or across ADR 0022's units), named nvS_<name>. Its initialiser is the evaluated C initialiser: the @constant calls are expanded, and the C macros are written as calls. It is const in C only when T contains no C state, no atomic and no lock, so that its methods can write through the pointers they derive, in their own unsafe code.

3.5 Unsafe access at the floor

unsafe code may still take borrow NAME as *T as *mut T, as for any shared reference (unsafe.md). That is how the library types implement their synchronisation. It is not a static mut.

3.7 static mut

static mut BOOT_INFO: BootInfo = BootInfo.empty();

/// Safety: runs once on the boot CPU, before any other CPU or interrupt can read BOOT_INFO.
unsafe fn recordBoot(info: BootInfo) { BOOT_INFO = info; }

• static mut NAME: T = e; takes the same static initialisers (§3.2). T need not be Share.
• Every use is an unsafe operation: reading, writing, assigning to a field, borrowing, or taking its address. U1 (unsafe.md) gains "use a static mut", so outside unsafe code each use is N0280, and a function using one needs its /// Safety: comment (N0283).
• A borrow of a static mut has the static region, as for a static. Nothing checks that two such borrows do not alias, or that two threads do not race: that is the /// Safety: argument (U3's rule for references made from pointers).
• In C it is a non-const object (nvS_<name>), like a static whose type holds C state.
• The interpreter checks accesses through it as it checks raw pointers: an access out of bounds or after the end of a value is a fault (UnsafeTests).

3.6 Hosted programs

The same static works in hosted programs for atomics (AtomicU64.constant, in lib/std) and any @threadSafe type with a @constant constructor. lib/std's Mutex<T> allocates its platform lock and so has none in v1. A static mutex is a later library change: PTHREAD_MUTEX_INITIALIZER and SRWLOCK_INIT are C constant macros.

4. Consequences

• The language: static and static mut items, the static region, @constant functions and C constant macros. Diagnostics: N0337 for a static that is not Share, is assigned or moved, or is generic or local; N0338 for an initialiser that is not a static initialiser; N0280 for a use of a static mut outside unsafe.
• The interpreter evaluates static initialisers once, before main, into its memory. C macros it cannot evaluate (it has no C) fault, so kernel statics are tested natively.
• Tests (planned):
  • run-pass: atomics and a lock in statics, from two threads (Thread.both) under TSan;
  • check-fail: each N0337/N0338 case;
  • a CosmoOS kernel object table in a static SpinLock, as M14 K1's acceptance says.
• @threadSafe outside Nova's libraries. A static's type must be Share, and kernel lock types are Share only by @threadSafe, which today is accepted only in Nova's own library files (N0290). The kernel library (K3, OD-30) needs the same permission. ADR 0025 decides it with the library's home: for example, a package that declares itself a trusted library in its nova.toml.
• Not decided here: lazily initialised statics, generic statics.

5. Questions for the owner

1. Option A: immutable statics of Share types, built at compile time? (Recommended.)
2. @constant functions and @constant C macros as the way to build them? (Recommended. The alternative is to allow only literals and constants, which cannot build a private-field lock or call SPINLOCK_INIT.)
3. Per-CPU data as the kernel library's PerCpu<T>, not a language attribute? (Recommended.)

6. Implementation (2026-10-29)

Specified in type-system §39, grammar StaticDecl, diagnostics N0337 and N0338.

• Uses need no new borrow rule. A function that uses a static gets a hidden local holding its address, a borrow(static) T (borrow(static) mut T for a static mut) made by HirStaticAddress/NirStaticAddress, which holds no loans. NAME is *local. The existing rules then apply: through a shared reference nothing is changed (N0211, N0248) or moved out (N0251), and returned references need a return type that names static (N0245 otherwise). Diagnostics call the place "static 'NAME'". So assigning or moving a static is N0211 or N0251, not N0337 as §4 planned. N0337 is for a static whose type is not Share, and N0338 for an initialiser.
• One exception to Share: a StringView, the only reference a static may hold, viewing static string data.
• Initialisers are type-checked before the function bodies, so the instances their @constant calls create are checked with them. They are folded after the bodies, by substituting arguments into @constant bodies (TypeChecker.StaticFold). A @constant body is checked once with its parameters symbolic.
• C (CEmitter.EmitStatics): static NV_UNUSED T nvS_NAME = { … };. With --split, it is extern in the header, defined in nova_instances.c and renamed like the functions. A C constant macro gets no prototype and no wrapper. A run-time call of one, in a @constant function called at run time, is a compound literal ((T)MACRO(args)).
• Interpreter: each static is a writable allocation, filled before main and not counted as a heap allocation; a C macro in an initialiser is a fault.
• lib/std's atomics have @constant constructors (AtomicU64.constant, …).
• Tests:
  • run-pass/statics.nova, in every C configuration (split and TSan included) and the interpreter: atomics changed from two threads, constants, struct, array and StringView statics, a @constant constructor setting private fields, static mut, and references returned as borrow(static);
  • check-fail statics.nova, static-moves.nova and static-initializers.nova;
  • parse-pass/statics.nova.
• Not yet: @export statics, enum values in initialisers, and a C constant macro compiled natively. The macro path is checked and emitted, but its C needs a header defining the macro: M14 K3's kernel library, with CosmoOS's SPINLOCK_INIT, is its first real use and test. Per-CPU data (PerCpu<T>) is K3 too.