src/test/ui/borrowck/borrowck-assign-comp.rs

// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// revisions: ast mir
//[mir]compile-flags: -Z borrowck=mir

struct point { x: isize, y: isize }

fn a() {
    let mut p = point {x: 3, y: 4};
    let q = &p;

    // This assignment is illegal because the field x is not
    // inherently mutable; since `p` was made immutable, `p.x` is now
    // immutable.  Otherwise the type of &_q.x (&isize) would be wrong.
    p.x = 5; //[ast]~ ERROR cannot assign to `p.x`
             //[mir]~^ ERROR cannot assign to `p.x` because it is borrowed
    q.x;
}

fn c() {
    // this is sort of the opposite.  We take a loan to the interior of `p`
    // and then try to overwrite `p` as a whole.

    let mut p = point {x: 3, y: 4};
    let q = &p.y;
    p = point {x: 5, y: 7};//[ast]~ ERROR cannot assign to `p`
                           //[mir]~^ ERROR cannot assign to `p` because it is borrowed
    p.x; // silence warning
    *q; // stretch loan
}

fn d() {
    // just for completeness's sake, the easy case, where we take the
    // address of a subcomponent and then modify that subcomponent:

    let mut p = point {x: 3, y: 4};
    let q = &p.y;
    p.y = 5; //[ast]~ ERROR cannot assign to `p.y`
             //[mir]~^ ERROR cannot assign to `p.y` because it is borrowed
    *q;
}

fn main() {
}