lang::mut_ref_params

Local &mut aliases write through; &mut Vec<T> / &mut [T] parameters write through on every tier.

&mut T parameters write through to the caller's source place on every tier for scalar values, strings, vectors, slices, structs, enums, and fixed arrays. &mut references are exclusive lexical views, not copied argument values. Their implicit lifetime ends at the closing brace. Gossamer has no explicit lifetime annotations and does not shorten a borrow at its last use. It rejects a second named &mut to the same root while the first remains in scope, an overlapping temporary borrow, and two &mut arguments rooted at the same binding in one call. Every permitted write is observed through the same source place.

Every container obeys the same rule the rest of the language does: a &mut T parameter writes through to the caller's container, and a T parameter takes a value of its own, so the callee's push or insert leaves the caller's untouched. Map, Set, BTreeMap, BTreeSet, Vec, Deque, Queue, Stack, MinHeap, and MaxHeap all reach their elements through a handle, so the copy is made where the binding or the argument is taken rather than inherited from the representation - but nothing about that is visible in the language. A binding taken from a container (let b = a) is a value of its own for the same reason.

A call never creates &mut implicitly. Pass a mutable place explicitly:

fn clear(value: &mut i64) { *value = 0 }

let mut value = 1
clear(&mut value)

clear(value) is rejected even when value is a mutable binding. If a parameter or local already has type &mut T, pass that reference directly: fn forward(value: &mut i64) { clear(value) }.