Rust New Added 10 September 2026

Rust: cannot borrow `*self` as mutable because it is also borrowed as immutable

The immutable borrow is usually hidden in an argument: self.push(self.len()) borrows self to evaluate the argument while the method call already holds a mutable borrow. Borrows end at their last use under NLL, so the fix is nearly always to finish the read before starting the write rather than to restructure the type.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Rejected: the argument is evaluated while &mut self is live
self.items.push(self.items.len());

// Accepted: read first, then write
let n = self.items.len();
self.items.push(n);

// Borrowing two fields at once is fine, borrowing self twice is not
let Self { input, output } = self;      // split borrow
output.extend(input.iter().copied());

// Inside a loop, collect the reads before mutating
let keys: Vec<_> = self.map.keys().cloned().collect();
for k in keys { self.map.remove(&k); }

// std::mem::take lets you own a field for the duration
let mut buf = std::mem::take(&mut self.buffer);
self.flush(&mut buf);
self.buffer = buf;

How to diagnose Rust errors

Rust's compiler errors are unusually helpful. They are closer to a code review than an error message. Almost all of them come from ownership (a value was moved and then used), borrowing (two mutable borrows, or a borrow outliving its owner), or trait resolution (the required bound is not satisfied). The productive habit is to read the error's notes and help sections in full and run rustc --explain on the code.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Run rustc --explain E0502 (or whichever code) for a worked explanation of the rule you violated.
For borrow conflicts, shorten the borrow's scope first, introducing a block or binding the intermediate result, before reaching for Rc<RefCell<T>>.
For moved-value errors, decide deliberately: clone (cost), borrow (lifetime), or restructure so ownership flows one way. Cloning to silence the compiler is fine early and worth revisiting later.
For trait errors, read which bound is missing and where it is required. cargo tree -d finds duplicate crate versions, which cause "trait not implemented" errors between two identical-looking types.
Use cargo clippy routinely; it catches idiom problems that later become borrow-checker fights.

Tools worth reaching for

rustc --explain
cargo clippy
cargo tree -d
cargo expand
RUST_BACKTRACE=1

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

doc.rust-lang.org

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