E4001

Error · Borrow/Ownership

E4001 — Use of moved or consumed linear variable

What the compiler reports

Error[E4001]{{.*}}Use of moved or consumed linear variable: p

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

//
// A type that has not implemented `Copy` still moves. This is the other half of
// copy_marker_trait.vx, and the more important half: `Copy` had to be opt-in, and a
// change that made every struct copyable would pass that file while quietly removing
// the rule that the whole placement discipline rests on.
//
// `Plain` here is as simple as a struct gets -- one `i32` field -- so nothing about it
// explains the refusal except that it never said `Copy`. Two `Copy` types are declared
// alongside it so the trait exists and is implemented for something; the move checker
// still has to answer separately for a type that is not in that list.

trait Copy {}

struct Marked {
  a : i32,
}

impl Copy for Marked {}

struct Plain {
  a : i32,
}

fn main() -> i32 {
  let m = Marked {
    a : 1,
  };
  let _also_m = m;
  let _m_again = m;
  let p = Plain {
    a : 2,
  };
  let _moved = p;
  return p.a;
}

From tests/frontend/fail/copy_is_opt_in.vx, which asserts this diagnostic on every commit.

Related