Don't retag the PtrMetadata(&raw const *_n) in slice indexing

Author: scottmcm

tests/fail/both_borrows/buggy_as_mut_slice.rs

@@ -13,7 +13,7 @@ fn main() {
     let v1 = safe::as_mut_slice(&v);
     let v2 = safe::as_mut_slice(&v);
     v1[1] = 5;
-    //~[stack]^ ERROR: /trying to retag .+ for SharedReadOnly permission .+ tag does not exist in the borrow stack for this location/
+    //~[stack]^ ERROR: /write access .* tag does not exist in the borrow stack/
     v2[1] = 7;
     //~[tree]^ ERROR: /write access through .* is forbidden/
 }

tests/fail/both_borrows/buggy_as_mut_slice.stack.stderr

@@ -1,11 +1,11 @@
-error: Undefined Behavior: trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   --> tests/fail/both_borrows/buggy_as_mut_slice.rs:LL:CC
    |
 LL |     v1[1] = 5;
-   |     ^^^^^
+   |     ^^^^^^^^^
    |     |
-   |     trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
-   |     this error occurs as part of retag at ALLOC[0x0..0xc]
+   |     attempting a write access using <TAG> at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+   |     this error occurs as part of an access at ALLOC[0x4..0x8]
    |
    = 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