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.

A Map, Set, Deque, Queue, or Stack parameter mutates the caller's container whether it is written &mut T, &T, or T: the value is a handle to storage the runtime owns. Write &mut on such a parameter to say the function mutates it; reads through it answer the caller's live contents.

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) }.