Auto merge of #2513 - RalfJung:protected, r=saethlin

slightly improve protector-related error messages

I find the current retag messages confusing, since they sound like the item *was* protected, when it still actively *is* protected (and that is, in fact, the issue).

Example error message:
```
error: Undefined Behavior: not granting access to tag <3095> because incompatible item [Unique for <3099>] is protected by call 943
  --> tests/fail/stacked_borrows/invalidate_against_barrier1.rs:5:25
   |
5  |     let _val = unsafe { *x }; //~ ERROR: protect
   |                         ^^ not granting access to tag <3095> because incompatible item [Unique for <3099>] is protected by call 943
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <3095> was created by a SharedReadWrite retag at offsets [0x0..0x4]
  --> tests/fail/stacked_borrows/invalidate_against_barrier1.rs:10:16
   |
10 |     let xraw = &mut x as *mut _;
   |                ^^^^^^
help: <3095> cannot be used for memory access because that would remove protected tag <3099>, protected by this function call
  --> tests/fail/stacked_borrows/invalidate_against_barrier1.rs:1:1
   |
1  | / fn inner(x: *mut i32, _y: &mut i32) {
2  | |     // If `x` and `y` alias, retagging is fine with this... but we really
3  | |     // shouldn't be allowed to use `x` at all because `y` was assumed to be
4  | |     // unique for the duration of this call.
5  | |     let _val = unsafe { *x }; //~ ERROR: protect
6  | | }
   | |_^
help: <3099> was derived from <3098>, which in turn was created here
  --> tests/fail/stacked_borrows/invalidate_against_barrier1.rs:12:17
   |
12 |     inner(xraw, xref);
   |                 ^^^^
   = note: backtrace:
   = note: inside `inner` at tests/fail/stacked_borrows/invalidate_against_barrier1.rs:5:25
note: inside `main` at tests/fail/stacked_borrows/invalidate_against_barrier1.rs:12:5
  --> tests/fail/stacked_borrows/invalidate_against_barrier1.rs:12:5
   |
12 |     inner(xraw, xref);
   |     ^^^^^^^^^^^^^^^^^
```

r? `@saethlin`

0 files changed, 0 insertions, 0 deletions