E4006

Error · Borrow/Ownership

E4006 — An assignment through a shared reference: *r = v, r.f = v or r[i] = v where r is a &T. A shared reference only reads; writing needs a &mut T.

What the compiler reports

Error[E4006] at 30:3: this assigns through `p`
Error[E4006] at 37:3: this assigns through `r`
Error[E4006] at 38:3: this assigns through `r`
Error[E4006] at 44:5: this assigns through `h`
Error[E4006] at 47:4: this assigns through `t`
Error[E4006] at 49:3: this assigns through `w`
Error[E4006] at 53:5: this assigns through `x`
7 error(s) emitted

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

//
// Part of the Vx Project, under the Apache License v2.0 with LLVM Exceptions.
// See LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//
// Each assignment writes through a shared reference (`&`), which can only be read through:
// `*r = ..`, `*r += ..`, a field, an element, a `&mut` field reached through a `&`, a
// parameter, and the `&i64` items of `v.iter()`.
import core::iter;
import std::vec;

struct Holder {
  n : i64,
  m : &mut i64,
}

fn set(p : &i64) -> i32 {
  *p = 1i64;
  return 0;
}

fn main() -> i32 {
  let a : i64 = 1i64;
  let r : &i64 = &a;
  *r = 5i64;
  *r += 1i64;
  let mut b : i64 = 2i64;
  let hv = Holder {
    n : 1i64, m : &mut b
  };
  let h : &Holder = &hv;
  h.n = 3i64;
  let arr : Tensor<i64, [2]> = [ 1i64, 2i64 ];
  let t : &Tensor<i64, [2]> = &arr;
  t[0] = 4i64;
  let w : &Holder = &hv;
  *w.m = 9i64;
  let mut v = Vec<i64>::new();
  v.push(40i64);
  for x in v.iter() {
    *x = 7i64;
  }
  return 0;
}

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

Related