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`
fec1c7aa32