src/test/ui/borrowck/two-phase-allow-access-during-reservation.rs

   1 // revisions: nll_target
   2
   3 // The following revisions are disabled due to missing support for two_phase_beyond_autoref
   4 //[nll_beyond] compile-flags: -Z two_phase_beyond_autoref
   5
   6 // This is the second counter-example from Niko's blog post
   7 // smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/
   8 //
   9 // It is "artificial". It is meant to illustrate directly that we
  10 // should allow an aliasing access during reservation, but *not* while
  11 // the mutable borrow is active.
  12 //
  13 // The convention for the listed revisions: "lxl" means lexical
  14 // lifetimes (which can be easier to reason about). "nll" means
  15 // non-lexical lifetimes. "nll_target" means the initial conservative
  16 // two-phase borrows that only applies to autoref-introduced borrows.
  17 // "nll_beyond" means the generalization of two-phase borrows to all
  18 // `&mut`-borrows (doing so makes it easier to write code for specific
  19 // corner cases).
  20
  21 fn main() {
  22     /*0*/ let mut i = 0;
  23
  24     /*1*/ let p = &mut i; // (reservation of `i` starts here)
  25
  26     /*2*/ let j = i;      // OK: `i` is only reserved here
  27                           //[nll_target]~^  ERROR cannot use `i` because it was mutably borrowed [E0503]
  28
  29     /*3*/ *p += 1;        // (mutable borrow of `i` starts here, since `p` is used)
  30
  31     /*4*/ let k = i;      //[nll_beyond]~  ERROR cannot use `i` because it was mutably borrowed [E0503]
  32                           //[nll_target]~^ ERROR cannot use `i` because it was mutably borrowed [E0503]
  33
  34     /*5*/ *p += 1;
  35
  36     let _ = (j, k, p);
  37 }