Vx

E4005

Error · Borrow/Ownership

E4005 — A returned reference escapes the function borrowing a function-local (dangling return)

What the compiler reports

E4005

The fragments the test suite holds the compiler to. An ellipsis marks text the test does not constrain; the emitted message also carries a source location.

A program that triggers it

// Return-escape analysis (#243, case bc4): returning `&x` for a function-local `x`
// leaves a dangling reference once the frame unwinds. Must be rejected.
fn dangle() -> &i32 {
  let x = 5;
  return &x;
}
fn main() -> i32 {
  return 0;
}

From tests/middle_end/fail/borrow_return_local_ref.vx, which asserts this diagnostic on every commit.

Related

Ownership and borrowingThe chapter covering the rule this code enforces. All diagnosticsEvery code the compiler can emit, grouped by stage. E4004Cannot borrow (already mutably borrowed)