compose_error_codes

Constant E0010_UNCAPTURED_VARIABLE

Source 
pub const E0010_UNCAPTURED_VARIABLE: ErrorCode;
Expand description

§E0010: Outer variables used in closure but not captured

In Compose, closures must explicitly declare any variables they use from the outer scope. These are listed in a * capture group* before the closure, such as |x| () => .... Compose does not allow implicit capture.

§Example

let x = 1;
let y = 2;
let f = () => x + y;
// autogenerated

✅ Fix:

# let x = 1;
# let y = 2;
let f = |x, y| () => x + y;

§Capture modifiers

Captured variables can be annotated to control how the closure accesses them:

  • x — capture by copy (or clone-on-write for complex types)
  • mut x — capture as a mutable copy
  • ref x — capture a shared reference to a box
  • ref mut x — capture a mutable reference to a box

📌 Only variables bound to box values may be captured using ref or ref mut. Owned captures cannot refer to boxed values — this prevents shallow copies of references to heap-allocated data.

§Example:
let x = box::new(42);
let f = |ref x| () => print(*x);

This explicit capture model prevents accidental aliasing and ensures closures make their dependencies and access modes clear.