ownership-not-clone

Ownership, Not Clone

The borrow checker is a map of where ownership is being fudged, not an obstacle to route around. This skill decides who holds a value and for how long.

The rule

Every clone must be explainable in one sentence that is not "the borrow checker complained." A clone that buys a real thing — a value that must outlive the borrow — is fine; one that only silences an error is a deferred design decision.

Read the error, not the workaround

E0502 (a mutable borrow while an immutable one is still live) and E0499 (two mutable borrows of the same place) name a lifetime conflict; the fix is in the structure the error describes, not in the clone that silences it:

// No clone: `first` is Copy, so the borrow ends at the let.
let first = items[0];
for _ in 0..n {
    items.push(first);
}

Borrow splitting

The borrow checker tracks a whole value through method calls, but fields individually through direct field access. A &mut self method that also needs self.other_field is the classic false conflict — destructure once:

// The two fields are separate lets and borrow independently.
impl Server {
    fn handle(&mut self) {
        let Self { connections, log, .. } = self;
        for conn in connections.iter_mut() {
            log.record(conn.id());
        }
    }
}

split_at_mut hands back two disjoint &mut halves of one slice; when destructuring is not enough, extract a free function that takes the two fields.

Take the cheapest thing that works, in argument position

&str over &String, &[T] over &Vec<T>, impl AsRef<Path> over a PathBuf for a path a function only reads. A signature that takes String when it only reads forces every caller into a clone — the bug is in the signature.

// Forces a clone at every call site that holds a &str.
fn log_message(message: String) { ... }

// Reads only — callers pass a reference and copy nothing.
fn log_message(message: &str) { ... }

Cow for genuine borrow-or-own

Cow<'_, str> earns its keep when the common path returns the input untouched and the rare path allocates.

use std::borrow::Cow;
// The common path borrows; only input containing a bracket allocates.
fn sanitize(input: &str) -> Cow<'_, str> {
    if input.contains(['<', '>']) {
        Cow::Owned(input.replace('<', "&lt;").replace('>', "&gt;"))
    } else {
        Cow::Borrowed(input)
    }
}

When Rc/Arc/RefCell/Mutex express a real requirement

Shared ownership is justified when the runtime structure genuinely has multiple owners with no single one outliving the rest, and not when a single owner is merely inconvenient to name. Run the four-question test in SHARED-STATE.md before adding a RefCell; the decision table is in CLONE-DECISIONS.md; the handle clone that passes the one-sentence test is in SHARED-STATE.md.

mem::take and mem::replace

mem::take and mem::replace move a value out of a &mut by leaving a default in its place — the answer to "I need to own this but I only have a mutable borrow":

let old = mem::take(&mut self.buffer);

Avoid statics

A static holding mutable or lazily-initialised state is a lifetime nobody wrote down and an ownership story nobody can follow. Pass the value, or hold it in the type that owns the work.

Deferrals

Reshaping how correct code is expressed is /idiomatic-rust; modelling invariants into types is /type-driven-design; error types are /rust-errors; shared state across .await points is /async-rust.

Verification

Keeping or removing a clone changes no behaviour, so the claim "this still works" is settled by the suite, at the lint level the repo configures:

cargo clippy --all-targets   # add -- -D warnings only if the repo has no lint config
cargo test

clippy::redundant_clone is nursery and off by default — run it one-off with cargo clippy -- -W clippy::redundant_clone as a proposal generator, never write it into the repo lint config; its findings are candidates, not edits.