Rc, dispose, and identity

One memory regime: every non-primitive value is a shared, refcounted heap cell; destruction is deterministic; == is identity for cells.

Reference semantics

Every non-primitive value — struct and class records, str, bytes, Vec, [T], enums, opaque boxes, trait-typed values, ?T boxes, closures’ captured cells — is a heap cell handle:

• assignment, argument passing, and returning copy the handle (retain/release), an O(1) move;
• mutation is visible through every alias;
• writing is gated by the mut-binding law (see Modules and visibility), never by the sharing.

There is no eager copy of any composite. own(x) and make_ptr(v) are removed spellings — bindings share by reference. bytes.clone() is the one copy escape hatch: a one-shot deep copy of a buffer’s octets. Every other type shares on binding; a divergent value of any other type is unreachable — build a new one instead.

Recursive shapes (next: ?Node, trees, lists) are legal: composite fields and elements are pointer-sized handle slots, and the refcount walk sees every handle field via the compile-time field table (see Reified types and layout).

Cleanups — on_drop

Destructors are attached, not implemented. The surface is the builtin

builtin fn on_drop<T>(p: ?T, cleanup: fn(?T)) -> nil;

• cleanup(p) runs when the referenced cell’s refcount reaches zero — deterministic destruction, not a collector callback.
• Fields are released after the cleanup body returns.
• One callback per nullable; a second attach is a compile error.
• The host side of the same law: a host payload’s finalizer runs at cell death, before the payload’s own Rust Drop.

Because a struct shared everywhere has no single death to hook, structs cannot carry destructors — if you need one, write a class whose handle is the ownership, and attach the cleanup where you mint it.

Weak references

Weak<T> (see Builtin generic types) demotes any handle to a non-keeping reference:

use ink::{ Logger };

struct Tile { v: i32; }

pub fn main() {
    let log = Logger.new("t");
    let tile = Tile { v: 7 };
    let w = Weak.new(tile);        // does NOT keep the cell alive
    let got: ?Tile = w.upgrade();  // the live referent, or nil once dead
    log.info(f"{got.v}");
}

7

The referent’s death nulls every weak box before any user code runs. A weak edge closes no cycle: strong cycles (two records holding each other, a container that holds its own observer) still leak to engine shutdown — the memory law is the program’s; break cycles with Weak.

Identity — ==

a == b is a builtin operator: no dispatch, no opting in, no element-wise story. a != b is its negation.

• Identity is the only == sharing can defend: with aliasing everywhere, structural equality of two independently built cells is ambiguous, and identity is O(1) with no deep walk. [1, 2] == [1, 2] is false — two cells.
• Enum members are immortal singletons, so Flavor.Sour == Flavor.Sour is true — the one place identity quietly behaves as value.
• ?T == ?T is slot identity: two nils are equal, a null and a box are not; p == nil derefs the nullable side and compares against the null slot.
• An identity-compare lint flags == between two obviously fresh composites (Vec.from([..]) == Vec.from([..])): “always false — compare fields”. Asserting distinctness is legitimate and suppressible.
• Field-wise comparison is a library trait contract (hash + eq implemented per type) — the mechanism value-keyed maps ride. It is not connected to ==.
• when literal patterns are unaffected: arms match compile-time values, never runtime ==.