Struct ListContext 

pub struct ListContext {
    pub index: Mutex<usize>,
    pub elements: Arc<[Arc<Thunk>]>,
    pub iter_binding: Mutex<Option<(Arc<str>, Value)>>,
}

Iteration context for &prev / &next / &index references inside a list.

Serves two scenarios:

1. Plain list-literal iteration (&prev / &next / &index) — built by Scope::with_list_context. Each element runs under its own child scope with a fixed index; iter_binding is always None.
2. Comprehension hot loop (for x in xs) — built once by Scope::with_iter_loop, then the body reuses the same Arc<Scope> across all iterations. Scope::set_iter_binding refreshes iter_binding + index in place per element, eliminating the Arc::new(Self {...}) per element flagged by the P1-B diagnostic correction (48 MB / 200 K blocks). iter_binding is Mutex-wrapped so Scope stays Send + Sync without unsafe interior mutability.

P2-1: thunks shared via Arc<[Arc<Thunk>]> so Expr::List’s per-element with_list_context(i, elements) only bumps an Arc instead of cloning the whole Vec<Arc<Thunk>> (O(N²) → O(N)).

Fields

index: Mutex<usize>

elements: Arc<[Arc<Thunk>]>

P2-1: Arc<[Arc<Thunk>]> lets with_list_context share the thunks slice across every element’s scope via a refcount bump, instead of Vec::clone’ing all N entries per element (the prior shape paid O(N²) Arc bumps over an N-element list).

iter_binding: Mutex<Option<(Arc<str>, Value)>>

Current named-iteration binding (name, value). None for plain list iteration. Comprehension hot loops fill this in once per element via a lock + write (no allocation, no frame rebuild).

Semantic invariant: when a Closure value is constructed inside a loop body, the constructor MUST snapshot the current binding into captured_env (see the Expr::Closure branch in eval.rs). Otherwise a later iteration would mutate the binding the closure was meant to capture, breaking lexical-snapshot semantics.

Implementations

impl ListContext

pub fn fixed(index: usize, elements: Arc<[Arc<Thunk>]>) -> Self

Plain &prev / &next / &index scenario: fixed index, no iter binding.

pub fn empty(elements: Arc<[Arc<Thunk>]>) -> Self

Comprehension hot loop entry: starts at index 0 with an empty binding slot. Scope::set_iter_binding fills it per element.

pub fn current_index(&self) -> usize

Read the current iteration index — used by &index / &prev / &next.